[llvm] 3ea0173 - [InstCombine] Don't fold `store undef` at non-byte-sized widths (#230955)

[via llvm-commits]

Author: Mitch Briles
Date: 2026-10-12T01:33:57+08:00
New Revision: 3ea01739ccc7a042c17ddd2ea012430ee5bd6d96

URL: https://github.com/llvm/llvm-project/commit/3ea01739ccc7a042c17ddd2ea012430ee5bd6d96
DIFF: https://github.com/llvm/llvm-project/commit/3ea01739ccc7a042c17ddd2ea012430ee5bd6d96.diff

LOG: [InstCombine] Don't fold `store undef` at non-byte-sized widths (#230955)

It's well known that we can't remove `store i1 undef` unless we can
prove the byte is non-`poison`. However, since
https://github.com/llvm/llvm-project/pull/180739 it's doubly wrong
unless we can prove that bits 1..7 in memory are already zero, since the
stored value is zero-extended to the next byte boundary.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
    llvm/test/Transforms/InstCombine/store.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 527ba917da3cb..0fa61d56cce20 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -16,6 +16,7 @@
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/Loads.h"
 #include "llvm/Analysis/VectorUtils.h"
+#include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
@@ -1586,9 +1587,11 @@ Instruction *InstCombinerImpl::visitStoreInst(StoreInst &SI) {
   }
 
   // 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))
+  // Non-byte-sized stores of undef (e.g. i12) are padded with zeros, so we
+  // can't remove the instruction.
+  // FIXME: It's incorrect to overwrite poison with undef
+  if (isa<PoisonValue>(Val) ||
+      (isa<UndefValue>(Val) && DL.typeSizeEqualsStoreSize(Val->getType())))
     return eraseInstFromFunction(SI);
 
   // Replace byte constants with integer constants in stores.

diff  --git a/llvm/test/Transforms/InstCombine/store.ll b/llvm/test/Transforms/InstCombine/store.ll
index 8b801a4a3c946..43a4e633650ff 100644
--- a/llvm/test/Transforms/InstCombine/store.ll
+++ b/llvm/test/Transforms/InstCombine/store.ll
@@ -11,6 +11,17 @@ define void @store_of_undef(ptr %P) {
   ret void
 }
 
+; Wrong to fold for two reasons: possible poison overwrite, and can't guarantee
+; that the high bit of this byte in memory will be zero.
+define void @store_of_nonbyte_undef(ptr %P) {
+; CHECK-LABEL: @store_of_nonbyte_undef(
+; CHECK-NEXT:    store i7 undef, ptr [[P:%.*]], align 1
+; CHECK-NEXT:    ret void
+;
+  store i7 undef, ptr %P
+  ret void
+}
+
 define void @store_of_poison(ptr %P) {
 ; CHECK-LABEL: @store_of_poison(
 ; CHECK-NEXT:    ret void