commit 53dae72a05ab4e1c2f241fd6027a7ad1050843bd
author Jim Stichnoth <stichnot@chromium.org> Wed Aug 31 15:32:32 2016 -0700
committer Jim Stichnoth <stichnot@chromium.org> Wed Aug 31 15:32:32 2016 -0700
tree d30df71e67996d0cf88687488bd73a41d1f02551
parent 7c9728faead1368150bcaa8313bc460df29ea1b7

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 .

src/IceTargetLowering.cpp

diff --git a/src/IceTargetLowering.cpp b/src/IceTargetLowering.cpp
index ff93e70..0ac6e8b 100644
--- a/src/IceTargetLowering.cpp
+++ b/src/IceTargetLowering.cpp
@@ -737,8 +737,11 @@
   // 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;
             });
 }