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

[Nikita Popov via Phabricator via llvm-commits]

This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG1881711fbb7b: [InstCombine] Remove memset of undef value (authored by nikic).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D124173/new/

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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