[LICM] Only set AA metadata on hoisted load if it executes.

[fhahn]

#116220 clarified that violations of aliasing metadata are UB.

Only set the AA metadata after hoisting a log, if it is guaranteed to execute in the original loop.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Florian Hahn (fhahn)

Changes

#116220 clarified that violations of aliasing metadata are UB.

Only set the AA metadata after hoisting a log, if it is guaranteed to execute in the original loop.

---

Full diff: https://github.com/llvm/llvm-project/pull/117204.diff

3 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/LICM.cpp (+7-1)
• (modified) llvm/test/Transforms/LICM/hoist-metadata.ll (+2-2)
• (modified) llvm/test/Transforms/LICM/hoisting-preheader-debugloc.ll (+3-1)

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 94bfe44a847a37..0d2fb6e81b211b 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -2030,7 +2030,9 @@ bool llvm::promoteLoopAccessesToScalars(
 
   bool DereferenceableInPH = false;
   bool StoreIsGuanteedToExecute = false;
+  bool LoadIsGuanteedToExecute = false;
   bool FoundLoadToPromote = false;
+
   // Goes from Unknown to either Safe or Unsafe, but can't switch between them.
   enum {
     StoreSafe,
@@ -2088,11 +2090,15 @@ bool llvm::promoteLoopAccessesToScalars(
         FoundLoadToPromote = true;
 
         Align InstAlignment = Load->getAlign();
+        bool GuaranteedToExecute =
+            SafetyInfo->isGuaranteedToExecute(*UI, DT, CurLoop);
+        LoadIsGuanteedToExecute |= GuaranteedToExecute;
 
         // Note that proving a load safe to speculate requires proving
         // sufficient alignment at the target location.  Proving it guaranteed
         // to execute does as well.  Thus we can increase our guaranteed
         // alignment as well.
+
         if (!DereferenceableInPH || (InstAlignment > Alignment))
           if (isSafeToExecuteUnconditionally(
                   *Load, DT, TLI, CurLoop, SafetyInfo, ORE,
@@ -2247,7 +2253,7 @@ bool llvm::promoteLoopAccessesToScalars(
       PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
     PreheaderLoad->setAlignment(Alignment);
     PreheaderLoad->setDebugLoc(DebugLoc());
-    if (AATags)
+    if (AATags && LoadIsGuanteedToExecute)
       PreheaderLoad->setAAMetadata(AATags);
 
     MemoryAccess *PreheaderLoadMemoryAccess = MSSAU.createMemoryAccessInBB(
diff --git a/llvm/test/Transforms/LICM/hoist-metadata.ll b/llvm/test/Transforms/LICM/hoist-metadata.ll
index 60b61944b33ae0..fc66d98725b524 100644
--- a/llvm/test/Transforms/LICM/hoist-metadata.ll
+++ b/llvm/test/Transforms/LICM/hoist-metadata.ll
@@ -77,7 +77,7 @@ define void @noalias_metadata_load_may_not_execute() {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[A:%.*]] = alloca i32, align 16
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i32, ptr [[A]]
-; CHECK-NEXT:    [[GEP_PROMOTED:%.*]] = load i32, ptr [[GEP]], align 4, !tbaa [[TBAA3:![0-9]+]], !noalias [[META7:![0-9]+]]
+; CHECK-NEXT:    [[GEP_PROMOTED:%.*]] = load i32, ptr [[GEP]], align 4
 ; CHECK-NEXT:    br label [[LOOP_HEADER:%.*]]
 ; CHECK:       loop.header:
 ; CHECK-NEXT:    [[ADD1:%.*]] = phi i32 [ [[GEP_PROMOTED]], [[ENTRY:%.*]] ], [ [[ADD:%.*]], [[LOOP_LATCH:%.*]] ]
@@ -92,7 +92,7 @@ define void @noalias_metadata_load_may_not_execute() {
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_HEADER]], label [[EXIT]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    [[ADD2:%.*]] = phi i32 [ [[ADD]], [[LOOP_LATCH]] ], [ [[ADD1]], [[LOOP_HEADER]] ]
-; CHECK-NEXT:    store i32 [[ADD2]], ptr [[GEP]], align 4, !tbaa [[TBAA3]], !noalias [[META7]]
+; CHECK-NEXT:    store i32 [[ADD2]], ptr [[GEP]], align 4, !tbaa [[TBAA3:![0-9]+]], !noalias [[META7:![0-9]+]]
 ; CHECK-NEXT:    ret void
 ;
 entry:
diff --git a/llvm/test/Transforms/LICM/hoisting-preheader-debugloc.ll b/llvm/test/Transforms/LICM/hoisting-preheader-debugloc.ll
index 570f4230c1a90d..61f0eb19a9bd1b 100644
--- a/llvm/test/Transforms/LICM/hoisting-preheader-debugloc.ll
+++ b/llvm/test/Transforms/LICM/hoisting-preheader-debugloc.ll
@@ -1,6 +1,8 @@
 ; RUN: opt -passes=licm %s -S | FileCheck %s
 
-; CHECK: %arrayidx4.promoted = load i32, ptr %arrayidx4, align 4, !tbaa !{{[0-9]+$}}
+; CHECK: %arrayidx4.promoted = load i32, ptr %arrayidx4, align 4
+; CHECK-NOT: !dbg
+; CHECK: br label %for.body
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
 

[nikic]

Suggested change

	bool LoadIsGuanteedToExecute = false;
	bool LoadIsGuaranteedToExecute = false;

[fhahn]

Done. thanks!

[nikic]

I think we also need to check StoreIsGuaranteedToExecute for the LoopPromoter use above. Should only copy AATags if it's a non-speculative store.

[fhahn]

Updated, thanks.

[nikic]

Unnecessary intermediate variable?

[fhahn]

removed, thanks

[nikic]

Rather than passing down the variable, can we pass empty AATags here if !guaranteed?

[fhahn]

Done, thanks!

[nikic]

Maybe wrap this in if (!LoadIsGuaranteedToExecute) to save unnecessary queries.

[fhahn]

Done, thanks!

[nikic]

LGTM