[PATCH] D124173: [InstCombine] Remove memset of undef value

[Nikita Popov via Phabricator via llvm-commits]

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This removes `memset` with undef char. We already do this for stores of undef value.

This comes with the caveat that this optimization is not, strictly speaking, legal for undef values, because we might be overwriting a poison value. However, our entire load/store model currently still operates on undef values, so we need to support undef here as well for internal consistency. Once https://github.com/llvm/llvm-project/issues/52930 is resolved, these and related folds can be limited to poison -- I've added FIXMEs to that effect.


https://reviews.llvm.org/D124173

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
  llvm/test/Transforms/InstCombine/memset.ll
  llvm/test/Transforms/InstCombine/store.ll


Index: llvm/test/Transforms/InstCombine/store.ll
===================================================================
--- llvm/test/Transforms/InstCombine/store.ll
+++ llvm/test/Transforms/InstCombine/store.ll
@@ -1,6 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s
 
+; FIXME: This is technically incorrect because it might overwrite a poison
+; value. Stop folding it once #52930 is resolved.
 define void @store_of_undef(i32* %P) {
 ; CHECK-LABEL: @store_of_undef(
 ; CHECK-NEXT:    ret void
Index: llvm/test/Transforms/InstCombine/memset.ll
===================================================================
--- llvm/test/Transforms/InstCombine/memset.ll
+++ llvm/test/Transforms/InstCombine/memset.ll
@@ -33,9 +33,10 @@
   ret void
 }
 
+; FIXME: This is technically incorrect because it might overwrite a poison
+; value. Stop folding it once #52930 is resolved.
 define void @memset_undef(i8* %p) {
 ; CHECK-LABEL: @memset_undef(
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* noundef nonnull align 1 dereferenceable(8) [[P:%.*]], i8 undef, i32 8, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.memset.p0i8.i32(i8* %p, i8 undef, i32 8, i1 false)
@@ -53,7 +54,6 @@
 
 define void @memset_poison(i8* %p) {
 ; CHECK-LABEL: @memset_poison(
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* noundef nonnull align 1 dereferenceable(8) [[P:%.*]], i8 poison, i32 8, i1 false)
 ; CHECK-NEXT:    ret void
 ;
   call void @llvm.memset.p0i8.i32(i8* %p, i8 poison, i32 8, i1 false)
Index: llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -1435,6 +1435,8 @@
   }
 
   // store undef, Ptr -> noop
+  // FIXME: This is technically incorrect because it might overwrite a poison
+  // value. Change to PoisonValue once #52930 is resolved.
   if (isa<UndefValue>(Val))
     return eraseInstFromFunction(SI);
 
Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -258,6 +258,15 @@
     return MI;
   }
 
+  // Remove memset with an undef value.
+  // FIXME: This is technically incorrect because it might overwrite a poison
+  // value. Change to PoisonValue once #52930 is resolved.
+  if (isa<UndefValue>(MI->getValue())) {
+    // Set the size of the copy to 0, it will be deleted on the next iteration.
+    MI->setLength(Constant::getNullValue(MI->getLength()->getType()));
+    return MI;
+  }
+
   // Extract the length and alignment and fill if they are constant.
   ConstantInt *LenC = dyn_cast<ConstantInt>(MI->getLength());
   ConstantInt *FillC = dyn_cast<ConstantInt>(MI->getValue());


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