Cost analysis: Remove "Unacceptable" hack (#6782)

We marked various expressions as having cost "Unacceptable", fixed at 100, to
ensure we never moved them out from an If arm, etc. Giving them such a high
cost avoids that problem - the cost is higher than the limit we have for moving
code from conditional to unconditional execution - but it also means the total
cost is unrealistic. For example, a function with one such instruction + an add
(cost 1) would end up with cost 101, and removing the add would look
insignificant, which causes issues for things that want to compare costs
(like Monomorphization).

To fix this, adjust some costs. The main change here is to give casts a cost of 5.
I measured this in depth, see the attached benchmark scripts, and it looks
clear that in both V8 and SpiderMonkey the cost of a cast is high enough to
make it not worth turning an if with ref.test arm into a select (which would
always execute the test).

Other costs adjusted here matter a lot less, because they are on operations
that have side effects and so the optimizer will anyhow not move them from
conditional to unconditional execution, but I tried to make them a bit more
realistic while I was removing "Unacceptable":

*   Give most atomic operations the 10 cost we've been using for atomic loads/
    stores. Perhaps wait and notify should be slower, however, but it seems like
    assuming fast switching might be more relevant.
*   Give growth operations a cost of 20, and throw operations a cost of 10. These
    numbers are entirely made up as I am not even sure how to measure them in
    a useful way (but, again, this should not matter much as they have side
    effects).

1 files changed, 29 insertions, 25 deletions

diff --git a/src/ir/cost.h b/src/ir/cost.h
index 6622561b8..38c74a203 100644
--- a/src/ir/cost.h
+++ b/src/ir/cost.h
@@ -31,11 +31,17 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
 
   CostType cost;
 
-  // A cost that is extremely high, something that is far, far more expensive
-  // than a fast operation like an add. This cost is so high it is unacceptable
-  // to add any more of it, say by an If=>Select operation (which would execute
-  // both arms; if either arm contains an unacceptable cost, we do not do it).
-  static const CostType Unacceptable = 100;
+  // The cost of a "slow" atomic operation like RMW.
+  static const CostType AtomicCost = 10;
+
+  // The cost of throwing a wasm exception. This does not include the cost of
+  // catching it (which might be in another function than the one we are
+  // considering).
+  static const CostType ThrowCost = 10;
+
+  // The cost of a cast. This can vary depending on the engine and on the type,
+  // but usually requires some loads and comparisons.
+  static const CostType CastCost = 5;
 
   CostType maybeVisit(Expression* curr) { return curr ? visit(curr) : 0; }
 
@@ -85,26 +91,27 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
   CostType visitGlobalGet(GlobalGet* curr) { return 1; }
   CostType visitGlobalSet(GlobalSet* curr) { return 2 + visit(curr->value); }
   CostType visitLoad(Load* curr) {
-    return 1 + visit(curr->ptr) + 10 * curr->isAtomic;
+    return 1 + visit(curr->ptr) + AtomicCost * curr->isAtomic;
   }
   CostType visitStore(Store* curr) {
-    return 2 + visit(curr->ptr) + visit(curr->value) + 10 * curr->isAtomic;
+    return 2 + visit(curr->ptr) + visit(curr->value) +
+           AtomicCost * curr->isAtomic;
   }
   CostType visitAtomicRMW(AtomicRMW* curr) {
-    return Unacceptable + visit(curr->ptr) + visit(curr->value);
+    return AtomicCost + visit(curr->ptr) + visit(curr->value);
   }
   CostType visitAtomicCmpxchg(AtomicCmpxchg* curr) {
-    return Unacceptable + visit(curr->ptr) + visit(curr->expected) +
+    return AtomicCost + visit(curr->ptr) + visit(curr->expected) +
            visit(curr->replacement);
   }
   CostType visitAtomicWait(AtomicWait* curr) {
-    return Unacceptable + visit(curr->ptr) + visit(curr->expected) +
+    return AtomicCost + visit(curr->ptr) + visit(curr->expected) +
            visit(curr->timeout);
   }
   CostType visitAtomicNotify(AtomicNotify* curr) {
-    return Unacceptable + visit(curr->ptr) + visit(curr->notifyCount);
+    return AtomicCost + visit(curr->ptr) + visit(curr->notifyCount);
   }
-  CostType visitAtomicFence(AtomicFence* curr) { return Unacceptable; }
+  CostType visitAtomicFence(AtomicFence* curr) { return AtomicCost; }
   CostType visitConst(Const* curr) { return 1; }
   CostType visitUnary(Unary* curr) {
     CostType ret = 0;
@@ -516,7 +523,8 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
   CostType visitReturn(Return* curr) { return maybeVisit(curr->value); }
   CostType visitMemorySize(MemorySize* curr) { return 1; }
   CostType visitMemoryGrow(MemoryGrow* curr) {
-    return Unacceptable + visit(curr->delta);
+    // TODO: This should perhaps be higher for shared memories.
+    return 20 + visit(curr->delta);
   }
   CostType visitMemoryInit(MemoryInit* curr) {
     return 6 + visit(curr->dest) + visit(curr->offset) + visit(curr->size);
@@ -572,7 +580,7 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
   }
   CostType visitTableSize(TableSize* curr) { return 1; }
   CostType visitTableGrow(TableGrow* curr) {
-    return Unacceptable + visit(curr->value) + visit(curr->delta);
+    return 20 + visit(curr->value) + visit(curr->delta);
   }
   CostType visitTableFill(TableFill* curr) {
     return 6 + visit(curr->dest) + visit(curr->value) + visit(curr->size);
@@ -589,14 +597,14 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
     return visit(curr->body);
   }
   CostType visitThrow(Throw* curr) {
-    CostType ret = Unacceptable;
+    CostType ret = ThrowCost;
     for (auto* child : curr->operands) {
       ret += visit(child);
     }
     return ret;
   }
-  CostType visitRethrow(Rethrow* curr) { return Unacceptable; }
-  CostType visitThrowRef(ThrowRef* curr) { return Unacceptable; }
+  CostType visitRethrow(Rethrow* curr) { return ThrowCost; }
+  CostType visitThrowRef(ThrowRef* curr) { return ThrowCost; }
   CostType visitTupleMake(TupleMake* curr) {
     CostType ret = 0;
     for (auto* child : curr->operands) {
@@ -612,19 +620,15 @@ struct CostAnalyzer : public OverriddenVisitor<CostAnalyzer, CostType> {
   CostType visitRefI31(RefI31* curr) { return 3 + visit(curr->value); }
   CostType visitI31Get(I31Get* curr) { return 2 + visit(curr->i31); }
   CostType visitRefTest(RefTest* curr) {
-    // Casts have a very high cost because in the VM they end up implemented as
-    // a combination of loads and branches. Given they contain branches, we do
-    // not want to add any more such work.
-    return Unacceptable + nullCheckCost(curr->ref) + visit(curr->ref);
+    return CastCost + nullCheckCost(curr->ref) + visit(curr->ref);
   }
   CostType visitRefCast(RefCast* curr) {
-    return Unacceptable + nullCheckCost(curr->ref) + visit(curr->ref);
+    return CastCost + nullCheckCost(curr->ref) + visit(curr->ref);
   }
   CostType visitBrOn(BrOn* curr) {
-    // BrOn of a null can be fairly fast, but anything else is a cast check
-    // basically, and an unacceptable cost.
+    // BrOn of a null can be fairly fast, but anything else is a cast check.
     CostType base =
-      curr->op == BrOnNull || curr->op == BrOnNonNull ? 2 : Unacceptable;
+      curr->op == BrOnNull || curr->op == BrOnNonNull ? 2 : CastCost;
     return base + nullCheckCost(curr->ref) + maybeVisit(curr->ref);
   }
   CostType visitStructNew(StructNew* curr) {