https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103393
--- Comment #17 from Richard Earnshaw <rearnsha at gcc dot gnu.org> --- (In reply to Jakub Jelinek from comment #16) > (In reply to Richard Earnshaw from comment #15) > > It seems perverse to me that you have a standard named pattern in the x86 > > backend that is enabled, but then you somehow expect the generic parts of > > the compiler to know that it shouldn't be used. > > They should be used, but only if the user code asks for it explicitly. > So, say a 32-byte generic vector in user code, or the <x86intrin.h> > intrinsics that need 32-byte vectors are just fine. > The option just asks that the compiler tries hard not to introduce those on > its own (e.g. vectorization but this string ops expansion is similar to > that). > > With those selected ISAs, such instructions are available, but on some CPUs > use of those is not really performance beneficial and using smaller vectors > might get better results. So the intrinsic should use a non-standard name to implement the expansion. Then the well-known name can be disabled and the mid-end will not use it. We have cases like that in the neon handling of intrinsics, for example to handle that the intrinsics need to work even without fast-math.
