Subzero: Deterministically sort local var stack offsets.

Currently, TargetLowering::sortVarsByAlignment() uses the variable's required alignment as the primary key, but then std::sort() breaks ties arbitrarily. This can give different results across different STL implementations, for example when building against LLVM trunk versus building in the PNaCl environment. The fix is to use Variable::Number as the secondary key. BUG= none R=kschimpf@google.com Review URL: https://codereview.chromium.org/2295393002 .

Subzero: Deterministically sort local var stack offsets.
53dae72a · Jim Stichnoth · 7c9728fa · 53dae72a
Commit 53dae72a authored Aug 31, 2016 by Jim Stichnoth
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Showing with 5 additions and 2 deletions

IceTargetLowering.cpp src/IceTargetLowering.cpp +5 -2

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--- a/src/IceTargetLowering.cpp
+++ b/src/IceTargetLowering.cpp
@@ -737,8 +737,11 @@ void TargetLowering::sortVarsByAlignment(VarList &Dest,
  // the buckets, if performance is an issue.
  std::sort(Dest.begin(), Dest.end(),
            [this](const Variable *V1, const Variable *V2) {
-              return typeWidthInBytesOnStack(V1->getType()) >
-                     typeWidthInBytesOnStack(V2->getType());
+              const size_t WidthV1 = typeWidthInBytesOnStack(V1->getType());
+              const size_t WidthV2 = typeWidthInBytesOnStack(V2->getType());
+              if (WidthV1 == WidthV2)
+                return V1->getIndex() < V2->getIndex();
+              return WidthV1 > WidthV2;
            });
 }