On Fri, 2 Jun 2023, Matthias Kretz wrote:
> > Okay, I see opinions will vary here. I was thinking about our immintrin.h > > which is partially implemented in terms of generic vectors. Imagine we > > extend UBSan to trap on signed overflow for vector types. I expect that > > will blow up on existing code that uses Intel intrinsics. > > _mm_add_epi32 is already implemented via __v4su addition (i.e. unsigned). So > the intrinsic would continue to wrap on signed overflow. Ah, if our intrinsics take care of it, that alleviates my concern. > > I'm not sure what you consider a breaking change here. Is that the implied > > threat to use undefinedness for range deduction and other optimizations? > > Consider the stdx::simd implementation. It currently follows semantics of the > builtin types. So simd<char> can be shifted by 30 without UB. The > implementation of the shift operator depends on the current behavior, even if > it is target-dependent. For PPC the simd implementation adds extra code to > avoid the "UB". With nailing down shifts > sizeof(T) as UB this extra code > now > needs to be added for all targets. What does stdx::simd do on LLVM, where that has always been UB even on x86? Alexander
