On Sat, Jul 22, 2023 at 8:09 AM Jeff Law via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > > > On 7/21/23 11:27, Andrew Pinski via Gcc-patches wrote: > > On Fri, Jul 21, 2023 at 8:09 AM Drew Ross via Gcc-patches > > <gcc-patches@gcc.gnu.org> wrote: > >> > >> Simplifies (x << c) >> c where x is a signed integral type of > >> width >= int and c = precision(type) - 1 into -(x & 1). Tested successfully > >> on x86_64 and x86 targets. > > > > Thinking about this some more, I think this should be handled in > > expand rather than on the gimple level. > > It is very much related to PR 110717 even. We are basically truncating > > to a signed one bit integer and then sign extending that across the > > whole code. > But why defer it to expand? This idiom is going to generate a -1,0 > result which is definitely interesting from a further simplification > standpoint.
It's not 'simpler' so it would be a canonicalization. We talked about providing a SEXT_EXPR at some point (sign-extend from constant bit N). Another canonicalization to existing ops would be (convert (convert:sbool @0)) where sbool is a 1-bit precision signed type. I think that's a better canonicalization than -(x & 1)? For zero-extensions we canonicalize such a conversion sequence to x & const-mask. For sign-extensions there's no single operation representation. Richard. > > Jeff
