[PATCH] D93397: [VectorCombine] loosen alignment constraint for load transform

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, RKSimon.
Herald added subscribers: hiraditya, mcrosier.
spatel requested review of this revision.
Herald added a project: LLVM.

As discussed in D93229 <https://reviews.llvm.org/D93229>, we only need a minimal alignment constraint when querying whether a hypothetical vector load is safe. We still pass/use the potentially stronger alignment attribute when checking costs and creating the new load.

There's already a test that changes with the minimum code change, so splitting this off as a preliminary proposal independent of any gep/offset enhancements.


https://reviews.llvm.org/D93397

Files:
  llvm/lib/Transforms/Vectorize/VectorCombine.cpp
  llvm/test/Transforms/VectorCombine/X86/load.ll


Index: llvm/test/Transforms/VectorCombine/X86/load.ll
===================================================================
--- llvm/test/Transforms/VectorCombine/X86/load.ll
+++ llvm/test/Transforms/VectorCombine/X86/load.ll
@@ -403,12 +403,14 @@
   ret <4 x float> %r
 }
 
-; Negative test? - pointer is not as aligned as load.
+; Pointer is not as aligned as load, but that's ok.
+; TODO: What does it mean when alignment attributes are in conflict?
 
 define <4 x float> @load_f32_insert_v4f32_align(float* align 1 dereferenceable(16) %p) {
 ; CHECK-LABEL: @load_f32_insert_v4f32_align(
-; CHECK-NEXT:    [[S:%.*]] = load float, float* [[P:%.*]], align 4
-; CHECK-NEXT:    [[R:%.*]] = insertelement <4 x float> undef, float [[S]], i32 0
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float* [[P:%.*]] to <4 x float>*
+; CHECK-NEXT:    [[TMP2:%.*]] = load <4 x float>, <4 x float>* [[TMP1]], align 4
+; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x float> [[TMP2]], <4 x float> undef, <4 x i32> <i32 0, i32 undef, i32 undef, i32 undef>
 ; CHECK-NEXT:    ret <4 x float> [[R]]
 ;
   %s = load float, float* %p, align 4
Index: llvm/lib/Transforms/Vectorize/VectorCombine.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/VectorCombine.cpp
+++ llvm/lib/Transforms/Vectorize/VectorCombine.cpp
@@ -136,11 +136,11 @@
   // Check safety of replacing the scalar load with a larger vector load.
   unsigned MinVecNumElts = MinVectorSize / ScalarSize;
   auto *MinVecTy = VectorType::get(ScalarTy, MinVecNumElts, false);
-  Align Alignment = Load->getAlign();
-  if (!isSafeToLoadUnconditionally(SrcPtr, MinVecTy, Alignment, DL, Load, &DT))
+  if (!isSafeToLoadUnconditionally(SrcPtr, MinVecTy, Align(1), DL, Load, &DT))
     return false;
 
   // Original pattern: insertelt undef, load [free casts of] PtrOp, 0
+  Align Alignment = Load->getAlign();
   Type *LoadTy = Load->getType();
   int OldCost = TTI.getMemoryOpCost(Instruction::Load, LoadTy, Alignment, AS);
   APInt DemandedElts = APInt::getOneBitSet(MinVecNumElts, 0);


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