[Turbofan]: Eliminate the check for -0 if it's not possible/observable.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 0cc53ed9b48072796dcc8a44757edd3412ca4660..6fbd3130e6718a615394b1e47742af51b599f400 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -1077,6 +1077,10 @@ class RepresentationSelector {
     return jsgraph_->simplified();
   }
 
+  void ChangeToInt32MulWithOverflowOp(Node* node, CheckForMinusZeroMode kMode) {
+    NodeProperties::ChangeOp(node, simplified()->CheckedInt32Mul(kMode));
+  }
+
   void ChangeToInt32OverflowOp(Node* node) {
     NodeProperties::ChangeOp(node, Int32OverflowOp(node));
   }
@@ -1334,6 +1338,22 @@ class RepresentationSelector {
         // Try to use type feedback.
         BinaryOperationHints::Hint hint = BinaryOperationHintOf(node->op());
 
+        // Determine if one of the inputs is a constant integer > 0.
+        Type* input0_type = NodeProperties::GetType(node->InputAt(0));
+        Type* input1_type = NodeProperties::GetType(node->InputAt(1));

[Jarin, 2016/07/17 18:24:41]

You can use TypeOf here (instead of NodeProperties::GetType).

[mvstanton, 2016/07/18 08:48:58]

Done.

+        bool positive_range = false;
+        if ((input0_type->IsRange() && input0_type->AsRange()->Min() > 0) ||
+            (input1_type->IsRange() && input1_type->AsRange()->Min() > 0)) {

[Jarin, 2016/07/17 16:37:47]

Maybe it is better to not refer to ranges here and just say:

type->Is(Type::OrderedNumber()) && type->Min() > 0

[Benedikt Meurer, 2016/07/17 17:44:07]

Yes, please. And I'd prefer to have this inline below instead of the bool
variable.

[mvstanton, 2016/07/18 08:48:58]

No problemo

[mvstanton, 2016/07/18 08:48:58]

Done.

+          positive_range = true;
+        }
+
+        // If one of the inputs is positive and/or truncation is being applied,
+        // there is no need to return -0.
+        CheckForMinusZeroMode mz_mode =
+            (truncation.TruncatesToWord32() || positive_range)
+                ? CheckForMinusZeroMode::kDontCheckForMinusZero
+                : CheckForMinusZeroMode::kCheckForMinusZero;

[Jarin, 2016/07/17 18:24:41]

Looking at this again, this logic is only used when lowering and only if we are
changing to Int32MulWithOverflow.

Perhaps it would be better to move this into ChangeToInt32MulWithOverflowOp?
(And perhaps rename to LowerToInt32MulWithOverflow.)

WDYT?

[mvstanton, 2016/07/18 08:48:58]

Good idea, done!

+
         // Handle the case when no int32 checks on inputs are necessary
         // (but an overflow check is needed on the output).
         if (BothInputsAre(node, Type::Signed32())) {
@@ -1342,7 +1362,7 @@ class RepresentationSelector {
               hint == BinaryOperationHints::kSigned32) {
             VisitBinop(node, UseInfo::TruncatingWord32(),
                        MachineRepresentation::kWord32, Type::Signed32());
-            if (lower()) ChangeToInt32OverflowOp(node);
+            if (lower()) ChangeToInt32MulWithOverflowOp(node, mz_mode);
             return;
           }
         }
@@ -1351,7 +1371,7 @@ class RepresentationSelector {
             hint == BinaryOperationHints::kSigned32) {
           VisitBinop(node, UseInfo::CheckedSigned32AsWord32(),
                      MachineRepresentation::kWord32, Type::Signed32());
-          if (lower()) ChangeToInt32OverflowOp(node);
+          if (lower()) ChangeToInt32MulWithOverflowOp(node, mz_mode);
           return;
         }