On Sat, Aug 8, 2026 at 5:24 PM Roger Sayle <[email protected]> wrote: > > > My recent patch to improve bitreverse support on cris, posted at > https://gcc.gnu.org/pipermail/gcc-patches/2026-August/726384.html > revealed an optimization opportunity. If you look closely GCC's > expansion of bitreverse (in optabs.cc) generates a left shift > followed by an IOR in several places. In these instances, its > possible to use "any_or_plus", and in fact using PLUS would allow > cris to use its addi instruction, and the x86 to use its lea > instruction. I believe PLUS is always as efficient as IOR for > SImode/wordmode, even on processors that have to use add with carry, > i.e. addc[qh]i3. For other (longer) modes (such as vector modes), > it's better to use IOR, and for shorter modes I'm not sure there's > any advantage to using PLUS. If I'm wrong for some targets, we > can make aop_optab a real optab.
Please note PR108477 [1] where fwprop conversion from '+' to '|' can interfere with your proposed optimization and the discussion about a patch that would generate LEA some more, started from [2]. [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108477 [2] https://gcc.gnu.org/pipermail/gcc-patches/2024-January/642164.html Uros.
