Improve handling of implicit traps (#898)

* add --ignore-implicit-traps option, and by default do not ignore them, to properly preserve semantics

* implicit traps can be reordered, but are side effects and should not be removed

* add testing for --ignore-implicit-traps

22 files changed, 480 insertions, 164 deletions

diff --git a/auto_update_tests.py b/auto_update_tests.py
index 7a14f6521..b74d07ba9 100755
--- a/auto_update_tests.py
+++ b/auto_update_tests.py
@@ -13,7 +13,7 @@ for asm in sorted(os.listdir('test')):
         cmd = [os.path.join('bin', 'asm2wasm'), os.path.join('test', asm)]
         wasm = asm.replace('.asm.js', '.fromasm')
         if not precise:
-          cmd += ['--imprecise']
+          cmd += ['--imprecise', '--ignore-implicit-traps']
           wasm += '.imprecise'
         if not opts:
           wasm += '.no-opts'
diff --git a/check.py b/check.py
index 7be44e63b..e3c7e0c15 100755
--- a/check.py
+++ b/check.py
@@ -99,7 +99,7 @@ for asm in tests:
         cmd = ASM2WASM + [os.path.join(options.binaryen_test, asm)]
         wasm = asm.replace('.asm.js', '.fromasm')
         if not precise:
-          cmd += ['--imprecise']
+          cmd += ['--imprecise', '--ignore-implicit-traps']
           wasm += '.imprecise'
         if not opts:
           wasm += '.no-opts'
diff --git a/src/ast_utils.h b/src/ast_utils.h
index 2a151d8f7..35ad96987 100644
--- a/src/ast_utils.h
+++ b/src/ast_utils.h
@@ -71,11 +71,13 @@ struct BreakSeeker : public PostWalker<BreakSeeker, Visitor<BreakSeeker>> {
 // TODO: optimize
 
 struct EffectAnalyzer : public PostWalker<EffectAnalyzer, Visitor<EffectAnalyzer>> {
-  EffectAnalyzer() {}
-  EffectAnalyzer(Expression *ast) {
-    analyze(ast);
+  EffectAnalyzer(PassOptions& passOptions, Expression *ast = nullptr) {
+    ignoreImplicitTraps = passOptions.ignoreImplicitTraps;
+    if (ast) analyze(ast);
   }
 
+  bool ignoreImplicitTraps;
+
   void analyze(Expression *ast) {
     breakNames.clear();
     walk(ast);
@@ -91,12 +93,18 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer, Visitor<EffectAnalyzer
   std::set<Name> globalsWritten;
   bool readsMemory = false;
   bool writesMemory = false;
+  bool implicitTrap = false; // a load or div/rem, which may trap. we ignore trap
+                             // differences, so it is ok to reorder these, and we
+                             // also allow reordering them with other effects
+                             // (so a trap may occur later or earlier, if it is
+                             // going to occur anyhow), but we can't remove them,
+                             // they count as side effects
 
   bool accessesLocal() { return localsRead.size() + localsWritten.size() > 0; }
   bool accessesGlobal() { return globalsRead.size() + globalsWritten.size() > 0; }
   bool accessesMemory() { return calls || readsMemory || writesMemory; }
-  bool hasSideEffects() { return calls || localsWritten.size() > 0 || writesMemory || branches || globalsWritten.size() > 0; }
-  bool hasAnything() { return branches || calls || accessesLocal() || readsMemory || writesMemory || accessesGlobal(); }
+  bool hasSideEffects() { return calls || localsWritten.size() > 0 || writesMemory || branches || globalsWritten.size() > 0 || implicitTrap; }
+  bool hasAnything() { return branches || calls || accessesLocal() || readsMemory || writesMemory || accessesGlobal() || implicitTrap; }
 
   // checks if these effects would invalidate another set (e.g., if we write, we invalidate someone that reads, they can't be moved past us)
   bool invalidates(EffectAnalyzer& other) {
@@ -124,6 +132,10 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer, Visitor<EffectAnalyzer
     for (auto global : globalsRead) {
       if (other.globalsWritten.count(global)) return true;
     }
+    // we are ok to reorder implicit traps, but not conditionalize them
+    if ((implicitTrap && other.branches) || (other.implicitTrap && branches)) {
+      return true;
+    }
     return false;
   }
 
@@ -189,8 +201,48 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer, Visitor<EffectAnalyzer
   void visitSetGlobal(SetGlobal *curr) {
     globalsWritten.insert(curr->name);
   }
-  void visitLoad(Load *curr) { readsMemory = true; }
-  void visitStore(Store *curr) { writesMemory = true; }
+  void visitLoad(Load *curr) {
+    readsMemory = true;
+    if (!ignoreImplicitTraps) implicitTrap = true;
+  }
+  void visitStore(Store *curr) {
+    writesMemory = true;
+    if (!ignoreImplicitTraps) implicitTrap = true;
+  }
+  void visitUnary(Unary *curr) {
+    if (!ignoreImplicitTraps) {
+      switch (curr->op) {
+        case TruncSFloat32ToInt32:
+        case TruncSFloat32ToInt64:
+        case TruncUFloat32ToInt32:
+        case TruncUFloat32ToInt64:
+        case TruncSFloat64ToInt32:
+        case TruncSFloat64ToInt64:
+        case TruncUFloat64ToInt32:
+        case TruncUFloat64ToInt64: {
+          implicitTrap = true;
+        }
+        default: {}
+      }
+    }
+  }
+  void visitBinary(Binary *curr) {
+    if (!ignoreImplicitTraps) {
+      switch (curr->op) {
+        case DivSInt32:
+        case DivUInt32:
+        case RemSInt32:
+        case RemUInt32:
+        case DivSInt64:
+        case DivUInt64:
+        case RemSInt64:
+        case RemUInt64: {
+          implicitTrap = true;
+        }
+        default: {}
+      }
+    }
+  }
   void visitReturn(Return *curr) { branches = true; }
   void visitHost(Host *curr) { calls = true; }
   void visitUnreachable(Unreachable *curr) { branches = true; }
diff --git a/src/pass.h b/src/pass.h
index dc9159aa0..74579e162 100644
--- a/src/pass.h
+++ b/src/pass.h
@@ -200,6 +200,10 @@ public:
     return runner;
   }
 
+  PassOptions& getPassOptions() {
+    return runner->options;
+  }
+
   void setPassRunner(PassRunner* runner_) {
     runner = runner_;
   }
diff --git a/src/passes/CodePushing.cpp b/src/passes/CodePushing.cpp
index f4beff1a8..55b15ac9d 100644
--- a/src/passes/CodePushing.cpp
+++ b/src/passes/CodePushing.cpp
@@ -83,9 +83,10 @@ class Pusher {
   ExpressionList& list;
   LocalAnalyzer& analyzer;
   std::vector<Index>& numGetsSoFar;
+  PassOptions& passOptions;
 
 public:
-  Pusher(Block* block, LocalAnalyzer& analyzer, std::vector<Index>& numGetsSoFar) : list(block->list), analyzer(analyzer), numGetsSoFar(numGetsSoFar) {
+  Pusher(Block* block, LocalAnalyzer& analyzer, std::vector<Index>& numGetsSoFar, PassOptions& passOptions) : list(block->list), analyzer(analyzer), numGetsSoFar(numGetsSoFar), passOptions(passOptions) {
     // Find an optimization segment: from the first pushable thing, to the first
     // point past which we want to push. We then push in that range before
     // continuing forward.
@@ -119,7 +120,7 @@ private:
     // but also have no side effects, as it may not execute if pushed.
     if (analyzer.isSFA(index) &&
         numGetsSoFar[index] == analyzer.getNumGets(index) &&
-        !EffectAnalyzer(set->value).hasSideEffects()) {
+        !EffectAnalyzer(passOptions, set->value).hasSideEffects()) {
       return set;
     }
     return nullptr;
@@ -146,8 +147,8 @@ private:
     // of earlier ones. Once we know all we can push, we push it all
     // in one pass, keeping the order of the pushables intact.
     assert(firstPushable != Index(-1) && pushPoint != Index(-1) && firstPushable < pushPoint);
-    EffectAnalyzer cumulativeEffects; // everything that matters if you want
-                                      // to be pushed past the pushPoint
+    EffectAnalyzer cumulativeEffects(passOptions); // everything that matters if you want
+                                                   // to be pushed past the pushPoint
     cumulativeEffects.analyze(list[pushPoint]);
     cumulativeEffects.branches = false; // it is ok to ignore the branching here,
                                         // that is the crucial point of this opt
@@ -158,9 +159,13 @@ private:
       if (pushable) {
         auto iter = pushableEffects.find(pushable);
         if (iter == pushableEffects.end()) {
-          pushableEffects.emplace(pushable, pushable);
+          iter = pushableEffects.emplace(
+            std::piecewise_construct,
+            std::forward_as_tuple(pushable),
+            std::forward_as_tuple(passOptions, pushable)
+          ).first;
         }
-        auto& effects = pushableEffects[pushable];
+        auto& effects = iter->second;
         if (cumulativeEffects.invalidates(effects)) {
           // we can't push this, so further pushables must pass it
           cumulativeEffects.mergeIn(effects);
@@ -248,7 +253,7 @@ struct CodePushing : public WalkerPass<PostWalker<CodePushing, Visitor<CodePushi
     // ordering invalidation issue, since if this isn't a loop, it's fine (we're not
     // used outside), and if it is, we hit the assign before any use (as we can't
     // push it past a use).
-    Pusher pusher(curr, analyzer, numGetsSoFar);
+    Pusher pusher(curr, analyzer, numGetsSoFar, getPassOptions());
   }
 };
 
diff --git a/src/passes/MergeBlocks.cpp b/src/passes/MergeBlocks.cpp
index 383837beb..f8d044212 100644
--- a/src/passes/MergeBlocks.cpp
+++ b/src/passes/MergeBlocks.cpp
@@ -216,9 +216,9 @@ struct MergeBlocks : public WalkerPass<PostWalker<MergeBlocks, Visitor<MergeBloc
     if (!child) return outer;
     if ((dependency1 && *dependency1) || (dependency2 && *dependency2)) {
       // there are dependencies, things we must be reordered through. make sure no problems there
-      EffectAnalyzer childEffects(child);
-      if (dependency1 && *dependency1 && EffectAnalyzer(*dependency1).invalidates(childEffects)) return outer;
-      if (dependency2 && *dependency2 && EffectAnalyzer(*dependency2).invalidates(childEffects)) return outer;
+      EffectAnalyzer childEffects(getPassOptions(), child);
+      if (dependency1 && *dependency1 && EffectAnalyzer(getPassOptions(), *dependency1).invalidates(childEffects)) return outer;
+      if (dependency2 && *dependency2 && EffectAnalyzer(getPassOptions(), *dependency2).invalidates(childEffects)) return outer;
     }
     if (auto* block = child->dynCast<Block>()) {
       if (!block->name.is() && block->list.size() >= 2) {
@@ -278,7 +278,7 @@ struct MergeBlocks : public WalkerPass<PostWalker<MergeBlocks, Visitor<MergeBloc
   void handleCall(T* curr, Block* outer = nullptr) {
     for (Index i = 0; i < curr->operands.size(); i++) {
       outer = optimize(curr, curr->operands[i], outer);
-      if (EffectAnalyzer(curr->operands[i]).hasSideEffects()) return;
+      if (EffectAnalyzer(getPassOptions(), curr->operands[i]).hasSideEffects()) return;
     }
   }
 
@@ -292,7 +292,7 @@ struct MergeBlocks : public WalkerPass<PostWalker<MergeBlocks, Visitor<MergeBloc
 
   void visitCallIndirect(CallIndirect* curr) {
     auto* outer = optimize(curr, curr->target);
-    if (EffectAnalyzer(curr->target).hasSideEffects()) return;
+    if (EffectAnalyzer(getPassOptions(), curr->target).hasSideEffects()) return;
     handleCall(curr, outer);
   }
 };
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 5c1aa5fc3..f94285930 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -317,8 +317,8 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
       auto* condition = select->condition->dynCast<Unary>();
       if (condition && condition->op == EqZInt32) {
         // flip select to remove eqz, if we can reorder
-        EffectAnalyzer ifTrue(select->ifTrue);
-        EffectAnalyzer ifFalse(select->ifFalse);
+        EffectAnalyzer ifTrue(getPassOptions(), select->ifTrue);
+        EffectAnalyzer ifFalse(getPassOptions(), select->ifFalse);
         if (!ifTrue.invalidates(ifFalse)) {
           select->condition = condition->value;
           std::swap(select->ifTrue, select->ifFalse);
@@ -408,8 +408,8 @@ private:
     auto* left = binary->left;
     auto* right = binary->right;
     if (!Properties::emitsBoolean(left) || !Properties::emitsBoolean(right)) return nullptr;
-    auto leftEffects = EffectAnalyzer(left).hasSideEffects();
-    auto rightEffects = EffectAnalyzer(right).hasSideEffects();
+    auto leftEffects = EffectAnalyzer(getPassOptions(), left).hasSideEffects();
+    auto rightEffects = EffectAnalyzer(getPassOptions(), right).hasSideEffects();
     if (leftEffects && rightEffects) return nullptr; // both must execute
    // canonicalize with side effects, if any, happening on the left
     if (rightEffects) {
diff --git a/src/passes/RemoveUnusedBrs.cpp b/src/passes/RemoveUnusedBrs.cpp
index 69a7c4ff1..b4b188416 100644
--- a/src/passes/RemoveUnusedBrs.cpp
+++ b/src/passes/RemoveUnusedBrs.cpp
@@ -28,11 +28,11 @@ namespace wasm {
 // to turn an if into a br-if, we must be able to reorder the
 // condition and possible value, and the possible value must
 // not have side effects (as they would run unconditionally)
-static bool canTurnIfIntoBrIf(Expression* ifCondition, Expression* brValue) {
+static bool canTurnIfIntoBrIf(Expression* ifCondition, Expression* brValue, PassOptions& options) {
   if (!brValue) return true;
-  EffectAnalyzer value(brValue);
+  EffectAnalyzer value(options, brValue);
   if (value.hasSideEffects()) return false;
-  return !EffectAnalyzer(ifCondition).invalidates(value);
+  return !EffectAnalyzer(options, ifCondition).invalidates(value);
 }
 
 struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<RemoveUnusedBrs>>> {
@@ -146,7 +146,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
       // if without an else. try to reduce   if (condition) br  =>  br_if (condition)
       Break* br = curr->ifTrue->dynCast<Break>();
       if (br && !br->condition) { // TODO: if there is a condition, join them
-        if (canTurnIfIntoBrIf(curr->condition, br->value)) {
+        if (canTurnIfIntoBrIf(curr->condition, br->value, getPassOptions())) {
           br->condition = curr->condition;
           br->finalize();
           replaceCurrent(Builder(*getModule()).dropIfConcretelyTyped(br));
@@ -263,7 +263,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
         return false;
       }
       // if there is control flow, we must stop looking
-      if (EffectAnalyzer(curr).branches) {
+      if (EffectAnalyzer(getPassOptions(), curr).branches) {
         return false;
       }
       if (i == 0) return false;
@@ -394,6 +394,9 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
     // perform some final optimizations
     struct FinalOptimizer : public PostWalker<FinalOptimizer, Visitor<FinalOptimizer>> {
       bool selectify;
+      PassOptions& passOptions;
+
+      FinalOptimizer(PassOptions& passOptions) : passOptions(passOptions) {}
 
       void visitBlock(Block* curr) {
         // if a block has an if br else br, we can un-conditionalize the latter, allowing
@@ -406,7 +409,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
           auto* iff = list[i]->dynCast<If>();
           if (!iff || !iff->ifFalse || isConcreteWasmType(iff->type)) continue; // if it lacked an if-false, it would already be a br_if, as that's the easy case
           auto* ifTrueBreak = iff->ifTrue->dynCast<Break>();
-          if (ifTrueBreak && !ifTrueBreak->condition && canTurnIfIntoBrIf(iff->condition, ifTrueBreak->value)) {
+          if (ifTrueBreak && !ifTrueBreak->condition && canTurnIfIntoBrIf(iff->condition, ifTrueBreak->value, passOptions)) {
             // we are an if-else where the ifTrue is a break without a condition, so we can do this
             ifTrueBreak->condition = iff->condition;
             ifTrueBreak->finalize();
@@ -416,7 +419,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
           }
           // otherwise, perhaps we can flip the if
           auto* ifFalseBreak = iff->ifFalse->dynCast<Break>();
-          if (ifFalseBreak && !ifFalseBreak->condition && canTurnIfIntoBrIf(iff->condition, ifFalseBreak->value)) {
+          if (ifFalseBreak && !ifFalseBreak->condition && canTurnIfIntoBrIf(iff->condition, ifFalseBreak->value, passOptions)) {
             ifFalseBreak->condition = Builder(*getModule()).makeUnary(EqZInt32, iff->condition);
             ifFalseBreak->finalize();
             list[i] = Builder(*getModule()).dropIfConcretelyTyped(ifFalseBreak);
@@ -435,7 +438,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
               if (!br2 || !br2->condition) continue;
               if (br1->name == br2->name) {
                 assert(!br1->value && !br2->value);
-                if (!EffectAnalyzer(br2->condition).hasSideEffects()) {
+                if (!EffectAnalyzer(passOptions, br2->condition).hasSideEffects()) {
                   // it's ok to execute them both, do it
                   Builder builder(*getModule());
                   br1->condition = builder.makeBinary(OrInt32, br1->condition, br2->condition);
@@ -480,11 +483,11 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
         if (curr->ifFalse && isConcreteWasmType(curr->ifTrue->type) && isConcreteWasmType(curr->ifFalse->type)) {
           // if with else, consider turning it into a select if there is no control flow
           // TODO: estimate cost
-          EffectAnalyzer condition(curr->condition);
+          EffectAnalyzer condition(passOptions, curr->condition);
           if (!condition.hasSideEffects()) {
-            EffectAnalyzer ifTrue(curr->ifTrue);
+            EffectAnalyzer ifTrue(passOptions, curr->ifTrue);
             if (!ifTrue.hasSideEffects()) {
-              EffectAnalyzer ifFalse(curr->ifFalse);
+              EffectAnalyzer ifFalse(passOptions, curr->ifFalse);
               if (!ifFalse.hasSideEffects()) {
                 auto* select = getModule()->allocator.alloc<Select>();
                 select->condition = curr->condition;
@@ -498,7 +501,7 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs, Visitor<R
         }
       }
     };
-    FinalOptimizer finalOptimizer;
+    FinalOptimizer finalOptimizer(getPassOptions());
     finalOptimizer.setModule(getModule());
     finalOptimizer.selectify = getPassRunner()->options.shrinkLevel > 0;
     finalOptimizer.walkFunction(func);
diff --git a/src/passes/SimplifyLocals.cpp b/src/passes/SimplifyLocals.cpp
index 6509c9da5..8590fbe96 100644
--- a/src/passes/SimplifyLocals.cpp
+++ b/src/passes/SimplifyLocals.cpp
@@ -84,9 +84,7 @@ struct SimplifyLocals : public WalkerPass<LinearExecutionWalker<SimplifyLocals,
     Expression** item;
     EffectAnalyzer effects;
 
-    SinkableInfo(Expression** item) : item(item) {
-      effects.walk(*item);
-    }
+    SinkableInfo(Expression** item, PassOptions& passOptions) : item(item), effects(passOptions, *item) {}
   };
 
   // a list of sinkables in a linear execution trace
@@ -246,7 +244,7 @@ struct SimplifyLocals : public WalkerPass<LinearExecutionWalker<SimplifyLocals,
   static void visitPre(SimplifyLocals* self, Expression** currp) {
     Expression* curr = *currp;
 
-    EffectAnalyzer effects;
+    EffectAnalyzer effects(self->getPassOptions());
     if (effects.checkPre(curr)) {
       self->checkInvalidations(effects);
     }
@@ -274,7 +272,7 @@ struct SimplifyLocals : public WalkerPass<LinearExecutionWalker<SimplifyLocals,
       }
     }
 
-    EffectAnalyzer effects;
+    EffectAnalyzer effects(self->getPassOptions());
     if (effects.checkPost(*currp)) {
       self->checkInvalidations(effects);
     }
@@ -282,7 +280,7 @@ struct SimplifyLocals : public WalkerPass<LinearExecutionWalker<SimplifyLocals,
     if (set && self->canSink(set)) {
       Index index = set->index;
       assert(self->sinkables.count(index) == 0);
-      self->sinkables.emplace(std::make_pair(index, SinkableInfo(currp)));
+      self->sinkables.emplace(std::make_pair(index, SinkableInfo(currp, self->getPassOptions())));
     }
 
     self->expressionStack.pop_back();
diff --git a/src/passes/Vacuum.cpp b/src/passes/Vacuum.cpp
index 8d9bb421c..38e607c24 100644
--- a/src/passes/Vacuum.cpp
+++ b/src/passes/Vacuum.cpp
@@ -74,22 +74,22 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum, Visitor<Vacuum>>
           }
           // for unary, binary, and select, we need to check their arguments for side effects
           if (auto* unary = curr->dynCast<Unary>()) {
-            if (EffectAnalyzer(unary->value).hasSideEffects()) {
+            if (EffectAnalyzer(getPassOptions(), unary->value).hasSideEffects()) {
               curr = unary->value;
               continue;
             } else {
               return nullptr;
             }
           } else if (auto* binary = curr->dynCast<Binary>()) {
-            if (EffectAnalyzer(binary->left).hasSideEffects()) {
-              if (EffectAnalyzer(binary->right).hasSideEffects()) {
+            if (EffectAnalyzer(getPassOptions(), binary->left).hasSideEffects()) {
+              if (EffectAnalyzer(getPassOptions(), binary->right).hasSideEffects()) {
                 return curr; // leave them
               } else {
                 curr = binary->left;
                 continue;
               }
             } else {
-              if (EffectAnalyzer(binary->right).hasSideEffects()) {
+              if (EffectAnalyzer(getPassOptions(), binary->right).hasSideEffects()) {
                 curr = binary->right;
                 continue;
               } else {
@@ -99,11 +99,11 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum, Visitor<Vacuum>>
           } else {
             // TODO: if two have side effects, we could replace the select with say an add?
             auto* select = curr->cast<Select>();
-            if (EffectAnalyzer(select->ifTrue).hasSideEffects()) {
-              if (EffectAnalyzer(select->ifFalse).hasSideEffects()) {
+            if (EffectAnalyzer(getPassOptions(), select->ifTrue).hasSideEffects()) {
+              if (EffectAnalyzer(getPassOptions(), select->ifFalse).hasSideEffects()) {
                 return curr; // leave them
               } else {
-                if (EffectAnalyzer(select->condition).hasSideEffects()) {
+                if (EffectAnalyzer(getPassOptions(), select->condition).hasSideEffects()) {
                   return curr; // leave them
                 } else {
                   curr = select->ifTrue;
@@ -111,15 +111,15 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum, Visitor<Vacuum>>
                 }
               }
             } else {
-              if (EffectAnalyzer(select->ifFalse).hasSideEffects()) {
-                if (EffectAnalyzer(select->condition).hasSideEffects()) {
+              if (EffectAnalyzer(getPassOptions(), select->ifFalse).hasSideEffects()) {
+                if (EffectAnalyzer(getPassOptions(), select->condition).hasSideEffects()) {
                   return curr; // leave them
                 } else {
                   curr = select->ifFalse;
                   continue;
                 }
               } else {
-                if (EffectAnalyzer(select->condition).hasSideEffects()) {
+                if (EffectAnalyzer(getPassOptions(), select->condition).hasSideEffects()) {
                   curr = select->condition;
                   continue;
                 } else {
@@ -174,7 +174,7 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum, Visitor<Vacuum>>
     if (!curr->name.is()) {
       if (list.size() == 1) {
         // just one element. replace the block, either with it or with a nop if it's not needed
-        if (isConcreteWasmType(curr->type) || EffectAnalyzer(list[0]).hasSideEffects()) {
+        if (isConcreteWasmType(curr->type) || EffectAnalyzer(getPassOptions(), list[0]).hasSideEffects()) {
           replaceCurrent(list[0]);
         } else {
           if (curr->type == unreachable) {
@@ -307,7 +307,7 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum, Visitor<Vacuum>>
     } else {
       ExpressionManipulator::nop(curr->body);
     }
-    if (curr->result == none && !EffectAnalyzer(curr->body).hasSideEffects()) {
+    if (curr->result == none && !EffectAnalyzer(getPassOptions(), curr->body).hasSideEffects()) {
       ExpressionManipulator::nop(curr->body);
     }
   }
diff --git a/src/tools/optimization-options.h b/src/tools/optimization-options.h
index 7f3936d35..368e319a7 100644
--- a/src/tools/optimization-options.h
+++ b/src/tools/optimization-options.h
@@ -23,7 +23,6 @@
            [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 2;
              passOptions.shrinkLevel = 1;
-             passOptions.ignoreImplicitTraps = true;
              runOptimizationPasses = true;
            })
       .add("", "-O0", "execute no optimization passes",
@@ -31,14 +30,12 @@
            [&passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 0;
              passOptions.shrinkLevel = 0;
-             passOptions.ignoreImplicitTraps = false;
            })
       .add("", "-O1", "execute -O1 optimization passes",
            Options::Arguments::Zero,
            [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 1;
              passOptions.shrinkLevel = 0;
-             passOptions.ignoreImplicitTraps = true;
              runOptimizationPasses = true;
            })
       .add("", "-O2", "execute -O2 optimization passes",
@@ -46,7 +43,6 @@
            [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 2;
              passOptions.shrinkLevel = 0;
-             passOptions.ignoreImplicitTraps = true;
              runOptimizationPasses = true;
            })
       .add("", "-O3", "execute -O3 optimization passes",
@@ -54,7 +50,6 @@
            [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 3;
              passOptions.shrinkLevel = 0;
-             passOptions.ignoreImplicitTraps = true;
              runOptimizationPasses = true;
            })
       .add("", "-Os", "execute default optimization passes, focusing on code size",
@@ -62,7 +57,6 @@
            [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 2;
              passOptions.shrinkLevel = 1;
-             passOptions.ignoreImplicitTraps = true;
              runOptimizationPasses = true;
            })
       .add("", "-Oz", "execute default optimization passes, super-focusing on code size",
@@ -70,7 +64,6 @@
            [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
              passOptions.optimizeLevel = 2;
              passOptions.shrinkLevel = 2;
-             passOptions.ignoreImplicitTraps = true;
              runOptimizationPasses = true;
            })
       .add("--optimize-level", "-ol", "How much to focus on optimizing code",
@@ -83,3 +76,9 @@
            [&passOptions](Options* o, const std::string& argument) {
              passOptions.shrinkLevel = atoi(argument.c_str());
            })
+      .add("--ignore-implicit-traps", "-iit", "Optimize under the helpful assumption that no surprising traps occur (from load, div/mod, etc.)",
+           Options::Arguments::Zero,
+           [&runOptimizationPasses, &passOptions](Options*, const std::string&) {
+             passOptions.ignoreImplicitTraps = true;
+           })
+
diff --git a/test/emcc_O2_hello_world.fromasm b/test/emcc_O2_hello_world.fromasm
index 5ac3b57bb..1cc6427ef 100644
--- a/test/emcc_O2_hello_world.fromasm
+++ b/test/emcc_O2_hello_world.fromasm
@@ -1935,7 +1935,8 @@
                   )
                 )
                 (if
-                  (select
+                  (if i32
+                    (get_local $12)
                     (i32.lt_u
                       (get_local $3)
                       (i32.sub
@@ -1946,7 +1947,6 @@
                       )
                     )
                     (i32.const 0)
-                    (get_local $12)
                   )
                   (block
                     (if
@@ -3668,19 +3668,12 @@
                 )
               )
               (if
-                (select
-                  (i32.and
-                    (i32.lt_u
-                      (get_local $13)
-                      (get_local $20)
-                    )
-                    (i32.ge_u
-                      (get_local $13)
-                      (get_local $46)
+                (if i32
+                  (if i32
+                    (i32.eq
+                      (get_local $7)
+                      (i32.const 203)
                     )
-                  )
-                  (i32.const 0)
-                  (select
                     (i32.eqz
                       (i32.and
                         (i32.load offset=12
@@ -3690,11 +3683,18 @@
                       )
                     )
                     (i32.const 0)
-                    (i32.eq
-                      (get_local $7)
-                      (i32.const 203)
+                  )
+                  (i32.and
+                    (i32.lt_u
+                      (get_local $13)
+                      (get_local $20)
+                    )
+                    (i32.ge_u
+                      (get_local $13)
+                      (get_local $46)
                     )
                   )
+                  (i32.const 0)
                 )
                 (block
                   (i32.store
@@ -8117,7 +8117,11 @@
             )
           )
           (set_local $15
-            (select
+            (if i32
+              (i32.eq
+                (get_local $17)
+                (i32.const 2)
+              )
               (i32.const 0)
               (i32.sub
                 (get_local $2)
@@ -8125,10 +8129,6 @@
                   (get_local $16)
                 )
               )
-              (i32.eq
-                (get_local $17)
-                (i32.const 2)
-              )
             )
           )
         )
diff --git a/test/emcc_hello_world.fromasm b/test/emcc_hello_world.fromasm
index 690c869ca..7dbe75147 100644
--- a/test/emcc_hello_world.fromasm
+++ b/test/emcc_hello_world.fromasm
@@ -1065,7 +1065,11 @@
         )
       )
       (set_local $2
-        (select
+        (if i32
+          (i32.eq
+            (get_local $4)
+            (i32.const 2)
+          )
           (i32.const 0)
           (i32.sub
             (get_local $2)
@@ -1073,10 +1077,6 @@
               (get_local $1)
             )
           )
-          (i32.eq
-            (get_local $4)
-            (i32.const 2)
-          )
         )
       )
     )
@@ -4198,7 +4198,13 @@
                                                   )
                                                 )
                                               )
-                                              (select
+                                              (if f64
+                                                (i32.eq
+                                                  (i32.load8_s
+                                                    (get_local $9)
+                                                  )
+                                                  (i32.const 45)
+                                                )
                                                 (f64.neg
                                                   (f64.add
                                                     (get_local $15)
@@ -4217,12 +4223,6 @@
                                                   )
                                                   (get_local $15)
                                                 )
-                                                (i32.eq
-                                                  (i32.load8_s
-                                                    (get_local $9)
-                                                  )
-                                                  (i32.const 45)
-                                                )
                                               )
                                             )
                                           )
@@ -6220,20 +6220,20 @@
                                                         )
                                                       )
                                                       (br_if $do-once115
-                                                        (i32.or
-                                                          (i32.and
-                                                            (get_local $17)
-                                                            (i32.lt_s
-                                                              (get_local $7)
-                                                              (i32.const 1)
-                                                            )
+                                                        (i32.and
+                                                          (get_local $17)
+                                                          (i32.lt_s
+                                                            (get_local $7)
+                                                            (i32.const 1)
                                                           )
-                                                          (i32.and
-                                                            (i32.load
-                                                              (get_local $0)
-                                                            )
-                                                            (i32.const 32)
+                                                        )
+                                                      )
+                                                      (br_if $do-once115
+                                                        (i32.and
+                                                          (i32.load
+                                                            (get_local $0)
                                                           )
+                                                          (i32.const 32)
                                                         )
                                                       )
                                                       (drop
@@ -13480,7 +13480,7 @@
       (i32.eq
         (tee_local $5
           (i32.and
-            (tee_local $8
+            (tee_local $7
               (i32.load
                 (i32.add
                   (get_local $0)
@@ -13495,12 +13495,12 @@
       )
       (call $_abort)
     )
-    (set_local $7
+    (set_local $8
       (i32.add
         (get_local $1)
         (tee_local $0
           (i32.and
-            (get_local $8)
+            (get_local $7)
             (i32.const -8)
           )
         )
@@ -13509,7 +13509,7 @@
     (block $do-once
       (if
         (i32.and
-          (get_local $8)
+          (get_local $7)
           (i32.const 1)
         )
         (block
@@ -13521,6 +13521,11 @@
           )
         )
         (block
+          (set_local $7
+            (i32.load
+              (get_local $1)
+            )
+          )
           (if
             (i32.eqz
               (get_local $5)
@@ -13534,11 +13539,7 @@
                   (get_local $1)
                   (i32.sub
                     (i32.const 0)
-                    (tee_local $8
-                      (i32.load
-                        (get_local $1)
-                      )
-                    )
+                    (get_local $7)
                   )
                 )
               )
@@ -13548,7 +13549,7 @@
           )
           (set_local $0
             (i32.add
-              (get_local $8)
+              (get_local $7)
               (get_local $0)
             )
           )
@@ -13567,7 +13568,7 @@
                       (i32.load
                         (tee_local $2
                           (i32.add
-                            (get_local $7)
+                            (get_local $8)
                             (i32.const 4)
                           )
                         )
@@ -13617,13 +13618,13 @@
           )
           (set_local $5
             (i32.shr_u
-              (get_local $8)
+              (get_local $7)
               (i32.const 3)
             )
           )
           (if
             (i32.lt_u
-              (get_local $8)
+              (get_local $7)
               (i32.const 256)
             )
             (block
@@ -13778,7 +13779,7 @@
                       (i32.load
                         (tee_local $4
                           (i32.add
-                            (tee_local $8
+                            (tee_local $7
                               (i32.add
                                 (get_local $1)
                                 (i32.const 16)
@@ -13793,11 +13794,11 @@
                   (if
                     (tee_local $5
                       (i32.load
-                        (get_local $8)
+                        (get_local $7)
                       )
                     )
                     (set_local $4
-                      (get_local $8)
+                      (get_local $7)
                     )
                     (block
                       (set_local $6
@@ -13809,7 +13810,7 @@
                 )
                 (loop $while-in
                   (if
-                    (tee_local $8
+                    (tee_local $7
                       (i32.load
                         (tee_local $10
                           (i32.add
@@ -13821,7 +13822,7 @@
                     )
                     (block
                       (set_local $5
-                        (get_local $8)
+                        (get_local $7)
                       )
                       (set_local $4
                         (get_local $10)
@@ -13830,7 +13831,7 @@
                     )
                   )
                   (if
-                    (tee_local $8
+                    (tee_local $7
                       (i32.load
                         (tee_local $10
                           (i32.add
@@ -13842,7 +13843,7 @@
                     )
                     (block
                       (set_local $5
-                        (get_local $8)
+                        (get_local $7)
                       )
                       (set_local $4
                         (get_local $10)
@@ -13883,7 +13884,7 @@
                 (if
                   (i32.ne
                     (i32.load
-                      (tee_local $8
+                      (tee_local $7
                         (i32.add
                           (get_local $10)
                           (i32.const 12)
@@ -13908,7 +13909,7 @@
                   )
                   (block
                     (i32.store
-                      (get_local $8)
+                      (get_local $7)
                       (get_local $4)
                     )
                     (i32.store
@@ -14044,7 +14045,7 @@
                 (get_local $12)
               )
               (if
-                (tee_local $8
+                (tee_local $7
                   (i32.load
                     (tee_local $4
                       (i32.add
@@ -14056,17 +14057,17 @@
                 )
                 (if
                   (i32.lt_u
-                    (get_local $8)
+                    (get_local $7)
                     (get_local $5)
                   )
                   (call $_abort)
                   (block
                     (i32.store offset=16
                       (get_local $6)
-                      (get_local $8)
+                      (get_local $7)
                     )
                     (i32.store offset=24
-                      (get_local $8)
+                      (get_local $7)
                       (get_local $6)
                     )
                   )
@@ -14128,7 +14129,7 @@
     (if
       (i32.ge_u
         (get_local $2)
-        (get_local $7)
+        (get_local $8)
       )
       (call $_abort)
     )
@@ -14139,7 +14140,7 @@
             (i32.load
               (tee_local $0
                 (i32.add
-                  (get_local $7)
+                  (get_local $8)
                   (i32.const 4)
                 )
               )
@@ -14181,7 +14182,7 @@
       (block
         (if
           (i32.eq
-            (get_local $7)
+            (get_local $8)
             (i32.load
               (i32.const 200)
             )
@@ -14231,7 +14232,7 @@
         )
         (if
           (i32.eq
-            (get_local $7)
+            (get_local $8)
             (i32.load
               (i32.const 196)
             )
@@ -14293,14 +14294,14 @@
             (block
               (set_local $4
                 (i32.load offset=12
-                  (get_local $7)
+                  (get_local $8)
                 )
               )
               (if
                 (i32.ne
                   (tee_local $1
                     (i32.load offset=8
-                      (get_local $7)
+                      (get_local $8)
                     )
                   )
                   (tee_local $0
@@ -14331,7 +14332,7 @@
                       (i32.load offset=12
                         (get_local $1)
                       )
-                      (get_local $7)
+                      (get_local $8)
                     )
                     (call $_abort)
                   )
@@ -14392,7 +14393,7 @@
                           )
                         )
                       )
-                      (get_local $7)
+                      (get_local $8)
                     )
                     (set_local $14
                       (get_local $0)
@@ -14413,7 +14414,7 @@
             (block
               (set_local $6
                 (i32.load offset=24
-                  (get_local $7)
+                  (get_local $8)
                 )
               )
               (block $do-once6
@@ -14421,10 +14422,10 @@
                   (i32.eq
                     (tee_local $0
                       (i32.load offset=12
-                        (get_local $7)
+                        (get_local $8)
                       )
                     )
-                    (get_local $7)
+                    (get_local $8)
                   )
                   (block
                     (if
@@ -14435,7 +14436,7 @@
                               (i32.add
                                 (tee_local $1
                                   (i32.add
-                                    (get_local $7)
+                                    (get_local $8)
                                     (i32.const 16)
                                   )
                                 )
@@ -14530,7 +14531,7 @@
                       (i32.lt_u
                         (tee_local $4
                           (i32.load offset=8
-                            (get_local $7)
+                            (get_local $8)
                           )
                         )
                         (i32.load
@@ -14549,7 +14550,7 @@
                             )
                           )
                         )
-                        (get_local $7)
+                        (get_local $8)
                       )
                       (call $_abort)
                     )
@@ -14563,7 +14564,7 @@
                             )
                           )
                         )
-                        (get_local $7)
+                        (get_local $8)
                       )
                       (block
                         (i32.store
@@ -14588,14 +14589,14 @@
                 (block
                   (if
                     (i32.eq
-                      (get_local $7)
+                      (get_local $8)
                       (i32.load
                         (tee_local $0
                           (i32.add
                             (i32.shl
                               (tee_local $3
                                 (i32.load offset=28
-                                  (get_local $7)
+                                  (get_local $8)
                                 )
                               )
                               (i32.const 2)
@@ -14654,7 +14655,7 @@
                               )
                             )
                           )
-                          (get_local $7)
+                          (get_local $8)
                         )
                         (i32.store
                           (get_local $0)
@@ -14692,7 +14693,7 @@
                       (i32.load
                         (tee_local $0
                           (i32.add
-                            (get_local $7)
+                            (get_local $8)
                             (i32.const 16)
                           )
                         )
diff --git a/test/memorygrowth.fromasm b/test/memorygrowth.fromasm
index d6800684d..e33525578 100644
--- a/test/memorygrowth.fromasm
+++ b/test/memorygrowth.fromasm
@@ -8170,7 +8170,11 @@
             )
           )
           (set_local $15
-            (select
+            (if i32
+              (i32.eq
+                (get_local $17)
+                (i32.const 2)
+              )
               (i32.const 0)
               (i32.sub
                 (get_local $2)
@@ -8178,10 +8182,6 @@
                   (get_local $16)
                 )
               )
-              (i32.eq
-                (get_local $17)
-                (i32.const 2)
-              )
             )
           )
         )
diff --git a/test/passes/code-pushing.txt b/test/passes/code-pushing_ignore-implicit-traps.txt
index f09d0dfb5..f09d0dfb5 100644
--- a/test/passes/code-pushing.txt
+++ b/test/passes/code-pushing_ignore-implicit-traps.txt
diff --git a/test/passes/code-pushing.wast b/test/passes/code-pushing_ignore-implicit-traps.wast
index e1cb30f4a..e1cb30f4a 100644
--- a/test/passes/code-pushing.wast
+++ b/test/passes/code-pushing_ignore-implicit-traps.wast
diff --git a/test/passes/optimize-instructions_optimize-level=2.txt b/test/passes/optimize-instructions_optimize-level=2_ignore-implicit-traps.txt
index 243421e8c..243421e8c 100644
--- a/test/passes/optimize-instructions_optimize-level=2.txt
+++ b/test/passes/optimize-instructions_optimize-level=2_ignore-implicit-traps.txt
diff --git a/test/passes/optimize-instructions_optimize-level=2.wast b/test/passes/optimize-instructions_optimize-level=2_ignore-implicit-traps.wast
index 7874907f3..7874907f3 100644
--- a/test/passes/optimize-instructions_optimize-level=2.wast
+++ b/test/passes/optimize-instructions_optimize-level=2_ignore-implicit-traps.wast
diff --git a/test/passes/remove-unused-brs_shrink-level=1.txt b/test/passes/remove-unused-brs_shrink-level=1.txt
index 68015f73f..b81c12abe 100644
--- a/test/passes/remove-unused-brs_shrink-level=1.txt
+++ b/test/passes/remove-unused-brs_shrink-level=1.txt
@@ -4,7 +4,7 @@
   (type $2 (func (result i32)))
   (memory $0 256 256)
   (func $b14 (type $2) (result i32)
-    (block $topmost i32
+    (drop
       (select
         (block $block1 i32
           (i32.const 12)
@@ -15,6 +15,34 @@
         (i32.const 1)
       )
     )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.load
+          (i32.const 10)
+        )
+        (i32.const 27)
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.rem_s
+          (i32.const 11)
+          (i32.const 12)
+        )
+        (i32.const 27)
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.trunc_u/f64
+          (f64.const 12.34)
+        )
+        (i32.const 27)
+      )
+    )
   )
   (func $join-br_ifs (type $1)
     (block $out
@@ -85,11 +113,11 @@
         (nop)
       )
     )
-    (block $out80
-      (br_if $out80
+    (block $out83
+      (br_if $out83
         (i32.const 1)
       )
-      (br_if $out80
+      (br_if $out83
         (call $b14)
       )
     )
diff --git a/test/passes/remove-unused-brs_shrink-level=1.wast b/test/passes/remove-unused-brs_shrink-level=1.wast
index 5a2234de6..e48ced178 100644
--- a/test/passes/remove-unused-brs_shrink-level=1.wast
+++ b/test/passes/remove-unused-brs_shrink-level=1.wast
@@ -3,8 +3,8 @@
   (type $0 (func (param i32)))
   (type $1 (func))
   (type $2 (func (result i32)))
-  (func $b14 (type $2) (result i32)
-    (block $topmost i32
+  (func $b14 (type $2)
+    (drop
       (if i32 ;; with shrinking, this can become a select
         (i32.const 1)
         (block $block1 i32
@@ -15,6 +15,27 @@
         )
       )
     )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.load (i32.const 10)) ;; load may have side effects, unless ignored
+        (i32.const 27)
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.rem_s (i32.const 11) (i32.const 12)) ;; rem may have side effects, unless ignored
+        (i32.const 27)
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.trunc_u/f64 (f64.const 12.34)) ;; float to int may have side effects, unless ignored
+        (i32.const 27)
+      )
+    )
   )
   (func $join-br_ifs
     (block $out
diff --git a/test/passes/remove-unused-brs_shrink-level=1_ignore-implicit-traps.txt b/test/passes/remove-unused-brs_shrink-level=1_ignore-implicit-traps.txt
new file mode 100644
index 000000000..35cfd53c8
--- /dev/null
+++ b/test/passes/remove-unused-brs_shrink-level=1_ignore-implicit-traps.txt
@@ -0,0 +1,125 @@
+(module
+  (type $0 (func (param i32)))
+  (type $1 (func))
+  (type $2 (func (result i32)))
+  (memory $0 256 256)
+  (func $b14 (type $2) (result i32)
+    (drop
+      (select
+        (block $block1 i32
+          (i32.const 12)
+        )
+        (block $block3 i32
+          (i32.const 27)
+        )
+        (i32.const 1)
+      )
+    )
+    (drop
+      (select
+        (i32.load
+          (i32.const 10)
+        )
+        (i32.const 27)
+        (i32.const 1)
+      )
+    )
+    (drop
+      (select
+        (i32.rem_s
+          (i32.const 11)
+          (i32.const 12)
+        )
+        (i32.const 27)
+        (i32.const 1)
+      )
+    )
+    (drop
+      (select
+        (i32.trunc_u/f64
+          (f64.const 12.34)
+        )
+        (i32.const 27)
+        (i32.const 1)
+      )
+    )
+  )
+  (func $join-br_ifs (type $1)
+    (block $out
+      (br_if $out
+        (i32.or
+          (i32.const 1)
+          (i32.const 2)
+        )
+      )
+      (nop)
+      (br_if $out
+        (i32.const 3)
+      )
+    )
+    (block $out2
+      (block $out3
+        (br_if $out2
+          (i32.const 1)
+        )
+        (br_if $out3
+          (i32.const 2)
+        )
+        (br_if $out2
+          (i32.const 3)
+        )
+      )
+      (unreachable)
+    )
+    (block $out4
+      (block $out5
+        (br_if $out4
+          (i32.const 1)
+        )
+        (br_if $out5
+          (i32.or
+            (i32.const 2)
+            (i32.const 3)
+          )
+        )
+        (nop)
+      )
+      (unreachable)
+    )
+    (block $out6
+      (block $out7
+        (br_if $out6
+          (i32.or
+            (i32.const 1)
+            (i32.const 2)
+          )
+        )
+        (nop)
+        (br_if $out7
+          (i32.const 3)
+        )
+      )
+      (unreachable)
+    )
+    (if
+      (i32.eqz
+        (i32.or
+          (call $b14)
+          (i32.const 0)
+        )
+      )
+      (block
+        (nop)
+        (nop)
+      )
+    )
+    (block $out83
+      (br_if $out83
+        (i32.const 1)
+      )
+      (br_if $out83
+        (call $b14)
+      )
+    )
+  )
+)
diff --git a/test/passes/remove-unused-brs_shrink-level=1_ignore-implicit-traps.wast b/test/passes/remove-unused-brs_shrink-level=1_ignore-implicit-traps.wast
new file mode 100644
index 000000000..e48ced178
--- /dev/null
+++ b/test/passes/remove-unused-brs_shrink-level=1_ignore-implicit-traps.wast
@@ -0,0 +1,80 @@
+(module
+  (memory 256 256)
+  (type $0 (func (param i32)))
+  (type $1 (func))
+  (type $2 (func (result i32)))
+  (func $b14 (type $2)
+    (drop
+      (if i32 ;; with shrinking, this can become a select
+        (i32.const 1)
+        (block $block1 i32
+          (i32.const 12)
+        )
+        (block $block3 i32
+          (i32.const 27)
+        )
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.load (i32.const 10)) ;; load may have side effects, unless ignored
+        (i32.const 27)
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.rem_s (i32.const 11) (i32.const 12)) ;; rem may have side effects, unless ignored
+        (i32.const 27)
+      )
+    )
+    (drop
+      (if i32
+        (i32.const 1)
+        (i32.trunc_u/f64 (f64.const 12.34)) ;; float to int may have side effects, unless ignored
+        (i32.const 27)
+      )
+    )
+  )
+  (func $join-br_ifs
+    (block $out
+      (br_if $out (i32.const 1))
+      (br_if $out (i32.const 2))
+      (br_if $out (i32.const 3))
+    )
+    (block $out2
+      (block $out3
+        (br_if $out2 (i32.const 1))
+        (br_if $out3 (i32.const 2))
+        (br_if $out2 (i32.const 3))
+      )
+      (unreachable)
+    )
+    (block $out4
+      (block $out5
+        (br_if $out4 (i32.const 1))
+        (br_if $out5 (i32.const 2))
+        (br_if $out5 (i32.const 3))
+      )
+      (unreachable)
+    )
+    (block $out6
+      (block $out7
+        (br_if $out6 (i32.const 1))
+        (br_if $out6 (i32.const 2))
+        (br_if $out7 (i32.const 3))
+      )
+      (unreachable)
+    )
+    (block $out8
+      (br_if $out8 (call $b14)) ;; side effect
+      (br_if $out8 (i32.const 0))
+    )
+    (block $out8
+      (br_if $out8 (i32.const 1))
+      (br_if $out8 (call $b14)) ;; side effect
+    )
+  )
+)
+