Improve getFallthrough (#1643)

That method looks through tee_locals and other operations that receive a value and let it flow through them, like a block's final value, etc. It just handled a few such operations, with this PR all of them should be handled.

Also refactor it out of the OptimizeInstructions pass as I think it may be useful for propagating returned constants.

4 files changed, 176 insertions, 18 deletions

diff --git a/src/ir/properties.h b/src/ir/properties.h
index 7cbbe73f0..6848e9481 100644
--- a/src/ir/properties.h
+++ b/src/ir/properties.h
@@ -146,6 +146,42 @@ inline Index getZeroExtBits(Expression* curr) {
   return Bits::getMaskedBits(curr->cast<Binary>()->right->cast<Const>()->value.geti32());
 }
 
+// Returns a falling-through value, that is, it looks through a tee_local
+// and other operations that receive a value and let it flow through them.
+inline Expression* getFallthrough(Expression* curr) {
+  // If the current node is unreachable, there is no value
+  // falling through.
+  if (curr->type == unreachable) {
+    return curr;
+  }
+  if (auto* set = curr->dynCast<SetLocal>()) {
+    if (set->isTee()) {
+      return getFallthrough(set->value);
+    }
+  } else if (auto* block = curr->dynCast<Block>()) {
+    // if no name, we can't be broken to, and then can look at the fallthrough
+    if (!block->name.is() && block->list.size() > 0) {
+      return getFallthrough(block->list.back());
+    }
+  } else if (auto* loop = curr->dynCast<Loop>()) {
+    return getFallthrough(loop->body);
+  } else if (auto* iff = curr->dynCast<If>()) {
+    if (iff->ifFalse) {
+      // Perhaps just one of the two actually returns.
+      if (iff->ifTrue->type == unreachable) {
+        return getFallthrough(iff->ifFalse);
+      } else if (iff->ifFalse->type == unreachable) {
+        return getFallthrough(iff->ifTrue);
+      }
+    }
+  } else if (auto* br = curr->dynCast<Break>()) {
+    if (br->condition && br->value) {
+      return getFallthrough(br->value);
+    }
+  }
+  return curr;
+}
+
 } // Properties
 
 } // wasm
diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 55042d96f..73f78611b 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -258,22 +258,6 @@ Index getMaxBits(Expression* curr, LocalInfoProvider* localInfoProvider) {
   }
 }
 
-// looks through fallthrough operations, like tee_local, block fallthrough, etc.
-// too and block fallthroughs, etc.
-Expression* getFallthrough(Expression* curr) {
-  if (auto* set = curr->dynCast<SetLocal>()) {
-    if (set->isTee()) {
-      return getFallthrough(set->value);
-    }
-  } else if (auto* block = curr->dynCast<Block>()) {
-    // if no name, we can't be broken to, and then can look at the fallthrough
-    if (!block->name.is() && block->list.size() > 0) {
-      return getFallthrough(block->list.back());
-    }
-  }
-  return curr;
-}
-
 // Useful information about locals
 struct LocalInfo {
   static const Index kUnknown = Index(-1);
@@ -316,7 +300,7 @@ struct LocalScanner : PostWalker<LocalScanner> {
     auto type = getFunction()->getLocalType(curr->index);
     if (type != i32 && type != i64) return;
     // an integer var, worth processing
-    auto* value = getFallthrough(curr->value);
+    auto* value = Properties::getFallthrough(curr->value);
     auto& info = localInfo[curr->index];
     info.maxBits = std::max(info.maxBits, getMaxBits(value, this));
     auto signExtBits = LocalInfo::kUnknown;
@@ -421,7 +405,7 @@ struct OptimizeInstructions : public WalkerPass<PostWalker<OptimizeInstructions,
         Index extraShifts;
         auto bits = Properties::getAlmostSignExtBits(binary, extraShifts);
         if (extraShifts == 0) {
-          if (auto* load = getFallthrough(ext)->dynCast<Load>()) {
+          if (auto* load = Properties::getFallthrough(ext)->dynCast<Load>()) {
             // pattern match a load of 8 bits and a sign extend using a shl of 24 then shr_s of 24 as well, etc.
             if (LoadUtils::canBeSigned(load) &&
                 ((load->bytes == 1 && bits == 8) || (load->bytes == 2 && bits == 16))) {
diff --git a/test/passes/optimize-instructions.txt b/test/passes/optimize-instructions.txt
index 0b156f803..5a0a45e3d 100644
--- a/test/passes/optimize-instructions.txt
+++ b/test/passes/optimize-instructions.txt
@@ -11,6 +11,7 @@
  (type $9 (func (param i32 i64 f32 f64)))
  (type $10 (func (param i32 i64 f32)))
  (type $11 (func (param i32 i64 f64 i32)))
+ (type $12 (func (result f64)))
  (memory $0 0)
  (export "load-off-2" (func $load-off-2))
  (func $f (; 0 ;) (type $0) (param $i1 i32) (param $i2 i64)
@@ -2906,6 +2907,96 @@
    )
   )
  )
+ (func $getFallthrough (; 71 ;) (type $1)
+  (local $x0 i32)
+  (local $x1 i32)
+  (local $x2 i32)
+  (local $x3 i32)
+  (local $x4 i32)
+  (local $x5 i32)
+  (local $x6 i32)
+  (local $x7 i32)
+  (set_local $x0
+   (i32.const 1)
+  )
+  (drop
+   (get_local $x0)
+  )
+  (set_local $x1
+   (tee_local $x2
+    (i32.const 1)
+   )
+  )
+  (drop
+   (get_local $x1)
+  )
+  (set_local $x3
+   (loop $loop-in (result i32)
+    (i32.const 1)
+   )
+  )
+  (drop
+   (get_local $x3)
+  )
+  (set_local $x4
+   (if (result i32)
+    (i32.const 1)
+    (i32.const 2)
+    (i32.const 3)
+   )
+  )
+  (drop
+   (i32.and
+    (get_local $x4)
+    (i32.const 7)
+   )
+  )
+  (set_local $x5
+   (if (result i32)
+    (i32.const 1)
+    (unreachable)
+    (i32.const 3)
+   )
+  )
+  (drop
+   (get_local $x5)
+  )
+  (set_local $x6
+   (if (result i32)
+    (i32.const 1)
+    (i32.const 3)
+    (unreachable)
+   )
+  )
+  (drop
+   (get_local $x6)
+  )
+  (drop
+   (block $out (result i32)
+    (set_local $x7
+     (br_if $out
+      (i32.const 1)
+      (i32.const 1)
+     )
+    )
+    (drop
+     (get_local $x7)
+    )
+    (unreachable)
+   )
+  )
+ )
+ (func $tee-with-unreachable-value (; 72 ;) (type $12) (result f64)
+  (local $var$0 i32)
+  (block $label$1 (result f64)
+   (tee_local $var$0
+    (br_if $label$1
+     (f64.const 1)
+     (unreachable)
+    )
+   )
+  )
+ )
 )
 (module
  (type $0 (func))
diff --git a/test/passes/optimize-instructions.wast b/test/passes/optimize-instructions.wast
index b65432715..aa979c62a 100644
--- a/test/passes/optimize-instructions.wast
+++ b/test/passes/optimize-instructions.wast
@@ -3479,6 +3479,53 @@
       )
     )
   )
+  (func $getFallthrough ;; unit tests for Properties::getFallthrough
+    (local $x0 i32)
+    (local $x1 i32)
+    (local $x2 i32)
+    (local $x3 i32)
+    (local $x4 i32)
+    (local $x5 i32)
+    (local $x6 i32)
+    (local $x7 i32)
+    ;; the trivial case
+    (set_local $x0 (i32.const 1))
+    (drop (i32.and (get_local $x0) (i32.const 7)))
+    ;; tees
+    (set_local $x1 (tee_local $x2 (i32.const 1)))
+    (drop (i32.and (get_local $x1) (i32.const 7)))
+    ;; loop
+    (set_local $x3 (loop (result i32) (i32.const 1)))
+    (drop (i32.and (get_local $x3) (i32.const 7)))
+    ;; if - two sides, can't
+    (set_local $x4 (if (result i32) (i32.const 1) (i32.const 2) (i32.const 3)))
+    (drop (i32.and (get_local $x4) (i32.const 7)))
+    ;; if - one side, can
+    (set_local $x5 (if (result i32) (i32.const 1) (unreachable) (i32.const 3)))
+    (drop (i32.and (get_local $x5) (i32.const 7)))
+    ;; if - one side, can
+    (set_local $x6 (if (result i32) (i32.const 1) (i32.const 3) (unreachable)))
+    (drop (i32.and (get_local $x6) (i32.const 7)))
+    ;; br_if with value
+    (drop
+      (block $out (result i32)
+        (set_local $x7 (br_if $out (i32.const 1) (i32.const 1)))
+        (drop (i32.and (get_local $x7) (i32.const 7)))
+        (unreachable)
+      )
+    )
+  )
+  (func $tee-with-unreachable-value (result f64)
+   (local $var$0 i32)
+   (block $label$1 (result f64)
+    (tee_local $var$0
+     (br_if $label$1 ;; the f64 does not actually flow through this, it's unreachable (and the type is wrong - but unchecked)
+      (f64.const 1)
+      (unreachable)
+     )
+    )
+   )
+  )
 )
 (module
   (import "env" "memory" (memory $0 (shared 256 256)))