[PATCH] D26634: Make block placement determinisatic

[Rong Xu via llvm-commits]

xur created this revision.
xur added reviewers: davidxl, chandlerc.
xur added a subscriber: llvm-commits.

We fail to produce bit-to-bit matching stage2 and stage3 compiler in PGO bootstrap build. The reason is because LoopBlockSet is of SmallPtrSet type whose iterating order depends on the pointer value.

This patch fixes this issue.


https://reviews.llvm.org/D26634

Files:
  lib/CodeGen/MachineBlockPlacement.cpp


Index: lib/CodeGen/MachineBlockPlacement.cpp
===================================================================
--- lib/CodeGen/MachineBlockPlacement.cpp
+++ lib/CodeGen/MachineBlockPlacement.cpp
@@ -1491,8 +1491,13 @@
   assert(LoopChain.UnscheduledPredecessors == 0);
   UpdatedPreds.insert(&LoopChain);
 
-  for (MachineBasicBlock *LoopBB : LoopBlockSet)
-    fillWorkLists(LoopBB, UpdatedPreds, &LoopBlockSet);
+   // Add the basic blocks to LoopBlockSet. Note that we are NOT using the
+   // LoopBlockSet iterator here because its order depends on the pointer
+   // values. This nondeterministic order will fail to produce a bit-to-bit
+   // matching clang binary in bootstrap build.
+   for (MachineBasicBlock &LoopBB : *F)
+     if (LoopBlockSet.count(&LoopBB))
+       fillWorkLists(&LoopBB, UpdatedPreds, &LoopBlockSet);
 
   buildChain(LoopTop, LoopChain, &LoopBlockSet);
 


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