On Wed, May 21, 2025 at 3:21 PM Jeff Law <jeffreya...@gmail.com> wrote: > > > > On 5/5/25 3:27 PM, Andrew Pinski wrote: > > The problem here is simplify-rtx.cc expects gen_lowpart_no_emit > > to return NULL on failure but combine's hook was returning CLOBBER. > > After r16-160-ge6f89d78c1a7528e93458278, > > gcc.target/i386/avx512bw-pr103750-2.c > > started to fail at -m32 due to this as new simplify code would return > > a RTL with a clobber in it rather than returning NULL. > > To fix this gen_lowpart_no_emit should return NULL when there was an failure > > instead of a clobber. This only changes the gen_lowpart_no_emit hook and > > not the > > generic gen_lowpart hook as parts of combine just pass gen_lowpart result > > directly > > without checking the return value. > > > > Bootstrapped and tested on x86_64-linux-gnu. > > > > PR rtl-optimization/120090 > > gcc/ChangeLog: > > > > * combine.cc (gen_lowpart_for_combine_no_emit): New function. > > (RTL_HOOKS_GEN_LOWPART_NO_EMIT): Set to > > gen_lowpart_for_combine_no_emit. > So that's standard combine behavior from as long as I can remember. So > I'm naturally worried about the various chunks of combine that are > expecting to see valid RTL objects (the CLOBBER) now getting pass a > NULL. If we substitute that value into an insn and try to recognize it > we could well run into a ICE or segfault.
Yes I had worry too which is why I didn't change gen_lowpart_for_combine. and only changed the no_emit hook. Combine itself does not call the no_emit hook directly but rather than the full gen_lowpart which then calls the non no_emit hook version. And it is why I wrote: ``` This only changes the gen_lowpart_no_emit hook and not the generic gen_lowpart hook as parts of combine just pass gen_lowpart result directly without checking the return value. ``` In my original commit message of why we can't just change gen_lowpart hook. Thanks, Andrew > > Jeff > >
