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-rw-r--r--clang/lib/CodeGen/CGBuiltin.cpp7
1 files changed, 6 insertions, 1 deletions
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 85ed2b86777..23b3a80180c 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -7381,7 +7381,12 @@ Value *CodeGenFunction::EmitX86BuiltinExpr(unsigned BuiltinID,
case X86::BI__builtin_ia32_undef128:
case X86::BI__builtin_ia32_undef256:
case X86::BI__builtin_ia32_undef512:
- return UndefValue::get(ConvertType(E->getType()));
+ // The x86 definition of "undef" is not the same as the LLVM definition
+ // (PR32176). We leave optimizing away an unnecessary zero constant to the
+ // IR optimizer and backend.
+ // TODO: If we had a "freeze" IR instruction to generate a fixed undef
+ // value, we should use that here instead of a zero.
+ return llvm::Constant::getNullValue(ConvertType(E->getType()));
case X86::BI__builtin_ia32_vec_init_v8qi:
case X86::BI__builtin_ia32_vec_init_v4hi:
case X86::BI__builtin_ia32_vec_init_v2si:
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