[llvm] 514f5b7 - [ConstantFolding] Fix dropped bits in non-integer-ratio bitcast with undef lane (#202282)

[via llvm-commits]

Author: lijinpei-amd
Date: 2026-06-09T11:51:52+02:00
New Revision: 514f5b766667c4c696b260f783403e8b41bc8798

URL: https://github.com/llvm/llvm-project/commit/514f5b766667c4c696b260f783403e8b41bc8798
DIFF: https://github.com/llvm/llvm-project/commit/514f5b766667c4c696b260f783403e8b41bc8798.diff

LOG: [ConstantFolding] Fix dropped bits in non-integer-ratio bitcast with undef lane (#202282)

When constant-folding a vector bitcast(e.g. <4 x i24> -> <3 x i32>), an
undef source element inserted a DstBitSize-wide zero placeholder into
the bit buffer. This could clobber defined source element, producing a
wrong result on big-endian targets.

Fix by inserting SrcBitSize-wide zero instead.

Alive2 proof:
  before (unsound): https://alive2.llvm.org/ce/z/R_ZQ75
  after  (verified): https://alive2.llvm.org/ce/z/VuV3mz

Added: 
    

Modified: 
    llvm/lib/Analysis/ConstantFolding.cpp
    llvm/test/Transforms/InstSimplify/bitcast-vector-fold.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 959b46f8eff46..5259c6b035188 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -359,7 +359,7 @@ Constant *FoldBitCast(Constant *C, Type *DestTy, const DataLayout &DL) {
         UndefMask.setBits(BitPosition, BitPosition + SrcBitSize);
         if (isa<PoisonValue>(Element))
           PoisonMask.setBits(BitPosition, BitPosition + SrcBitSize);
-        SrcValue = APInt::getZero(DstBitSize);
+        SrcValue = APInt::getZero(SrcBitSize);
       } else {
         auto *Src = dyn_cast<ConstantInt>(Element);
         if (!Src)

diff  --git a/llvm/test/Transforms/InstSimplify/bitcast-vector-fold.ll b/llvm/test/Transforms/InstSimplify/bitcast-vector-fold.ll
index 88f34cabc6188..14789847911a5 100644
--- a/llvm/test/Transforms/InstSimplify/bitcast-vector-fold.ll
+++ b/llvm/test/Transforms/InstSimplify/bitcast-vector-fold.ll
@@ -412,7 +412,7 @@ define <3 x i32> @bitcast_constexpr_3i32_4i24_n1255uu() {
 ; LE-NEXT:    ret <3 x i32> <i32 -1, i32 0, i32 undef>
 ;
 ; BE-LABEL: @bitcast_constexpr_3i32_4i24_n1255uu(
-; BE-NEXT:    ret <3 x i32> <i32 -256, i32 0, i32 undef>
+; BE-NEXT:    ret <3 x i32> <i32 -256, i32 16711680, i32 undef>
 ;
   %cast = bitcast <4 x i24><i24 -1, i24 255, i24 undef, i24 undef> to <3 x i32>
   ret <3 x i32> %cast