aqjune added a comment. In D103874#2806519 <https://reviews.llvm.org/D103874#2806519>, @efriedma wrote:
> I noted the cases where it looks like the undef->poison change might actually > impact code using compiler intrinsic functions that have external > specifications. The relevant specifications say the elements in question are > "undefined", without really specifying what that means. > > Currently, for the Intel intrinsics, we treat "undefined" as something more > conservative than LLVM undef; see > https://github.com/llvm/llvm-project/blob/d2012d965d60c3258b3a69d024491698f8aec386/clang/lib/CodeGen/CGBuiltin.cpp#L12483 > . Maybe we should make the cast intrinsics more conservative to match. And > maybe we should do the same for OpenCL. Would need to do some backend work > to make sure we don't regress the generated code, though. Makes sense, the PR (https://llvm.org/PR32176) that is left at the comment says it should be something like `freeze poison` as well. (BTW, this means the current shufflevector lowering is already incorrect as well..) Then, _mm256_castsi128_si256 should be lowered into something like this: %fr = freeze <2 x i64> poison shufflevector <2 x i64> %x, <2 x i64> %fr, <4 x i32> <i32 0, i32 1, i32 2, i32 3> BTW, the Intel intrinsic guide for _mm256_castsi128_si256 ( https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm256_castsi128_si256&expand=628 ) says: This intrinsic is only used for compilation and does not generate any instructions, thus it has zero latency. We should teach the backend to understand the shufflevector+freeze form and lower it into an efficient assembly. But, in practice, the frozen element won't be used in most of the cases; the middle-end's demanded elements analysis will trigger instcombine to almost always remove the freeze. What do you think? If people agree, I'll create a separate patch that lowers this to the new freeze+shufflevector format (since it is already incorrect). Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D103874/new/ https://reviews.llvm.org/D103874 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
