[llvm] 6713332 - [LoopVersioningLICM] Fix noalias metadata emission

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2020-10-13T18:58:05+02:00
New Revision: 6713332fddb796f5b14fcb6a7e5d36979676e4ab

URL: https://github.com/llvm/llvm-project/commit/6713332fddb796f5b14fcb6a7e5d36979676e4ab
DIFF: https://github.com/llvm/llvm-project/commit/6713332fddb796f5b14fcb6a7e5d36979676e4ab.diff

LOG: [LoopVersioningLICM] Fix noalias metadata emission

The previous code added the scope on each iteration, so that the
same scope was represented many times in the same !noalias metadata.
That's legal, and semantically equivalent to only storing the scope
once, but it's also wasteful and may pessimize further optimization
if AATags get intersected naively, as done by the AliasSetTracker.

Added: 
    

Modified: 
    llvm/lib/Transforms/Scalar/LoopVersioningLICM.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Scalar/LoopVersioningLICM.cpp b/llvm/lib/Transforms/Scalar/LoopVersioningLICM.cpp
index 06b684ef1e70..06af4da92355 100644
--- a/llvm/lib/Transforms/Scalar/LoopVersioningLICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopVersioningLICM.cpp
@@ -539,8 +539,8 @@ void LoopVersioningLICM::setNoAliasToLoop(Loop *VerLoop) {
   MDBuilder MDB(I->getContext());
   MDNode *NewDomain = MDB.createAnonymousAliasScopeDomain("LVDomain");
   StringRef Name = "LVAliasScope";
-  SmallVector<Metadata *, 4> Scopes, NoAliases;
   MDNode *NewScope = MDB.createAnonymousAliasScope(NewDomain, Name);
+  SmallVector<Metadata *, 4> Scopes{NewScope}, NoAliases{NewScope};
   // Iterate over each instruction of loop.
   // set no-alias for all load & store instructions.
   for (auto *Block : CurLoop->getBlocks()) {
@@ -548,8 +548,6 @@ void LoopVersioningLICM::setNoAliasToLoop(Loop *VerLoop) {
       // Only interested in instruction that may modify or read memory.
       if (!Inst.mayReadFromMemory() && !Inst.mayWriteToMemory())
         continue;
-      Scopes.push_back(NewScope);
-      NoAliases.push_back(NewScope);
       // Set no-alias for current instruction.
       Inst.setMetadata(
           LLVMContext::MD_noalias,