[x86] these aren't the undefs you're looking for (PR32176)
x86 has undef SSE/AVX intrinsics that should represent a bogus register operand.
This is not the same as LLVM's undef value which can take on multiple bit patterns.
There are better solutions / follow-ups to this discussed here:
https://bugs.llvm.org/show_bug.cgi?id=32176
...but this should prevent miscompiles with a one-line code change.
Differential Revision: https://reviews.llvm.org/D30834
llvm-svn: 297588
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index 85ed2b8..23b3a80 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -7381,7 +7381,12 @@
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: