https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112523
--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:aad65285a1c681feb9fc5b041c86d841b24c3d2a commit r14-5442-gaad65285a1c681feb9fc5b041c86d841b24c3d2a Author: Jakub Jelinek <ja...@redhat.com> Date: Tue Nov 14 13:19:48 2023 +0100 i386: Fix up <insn><dwi>3_doubleword_lowpart [PR112523] On Sun, Nov 12, 2023 at 09:03:42PM -0000, Roger Sayle wrote: > This patch improves register pressure during reload, inspired by PR 97756. > Normally, a double-word right-shift by a constant produces a double-word > result, the highpart of which is dead when followed by a truncation. > The dead code calculating the high part gets cleaned up post-reload, so > the issue isn't normally visible, except for the increased register > pressure during reload, sometimes leading to odd register assignments. > Providing a post-reload splitter, which clobbers a single wordmode > result register instead of a doubleword result register, helps (a bit). Unfortunately this broke bootstrap on i686-linux, broke all ACATS tests on x86_64-linux as well as miscompiled e.g. __floattisf in libgcc there as well. The bug is that shrd{l,q} instruction expects the low part of the input to be the same register as the output, rather than the high part as the patch implemented. split_double_mode (<DWI>mode, &operands[1], 1, &operands[1], &operands[3]); sets operands[1] to the lo_half and operands[3] to the hi_half, so if operands[0] is not the same register as operands[1] (rather than [3]) after RA, we should during splitting move operands[1] into operands[0]. Your testcase: > #define MASK60 ((1ul << 60) - 1) > unsigned long foo (__uint128_t n) > { > unsigned long a = n & MASK60; > unsigned long b = (n >> 60); > b = b & MASK60; > unsigned long c = (n >> 120); > return a+b+c; > } still has the same number of instructions. Bootstrapped/regtested on x86_64-linux (where it e.g. turns === acats Summary === -# of unexpected failures 2328 +# of expected passes 2328 +# of unexpected failures 0 and fixes gcc.dg/torture/fp-int-convert-*timode.c FAILs as well) and i686-linux (where it previously didn't bootstrap, but compared to Friday evening's bootstrap the testresults are ok). 2023-11-14 Jakub Jelinek <ja...@redhat.com> PR target/112523 PR ada/112514 * config/i386/i386.md (<insn><dwi>3_doubleword_lowpart): Move operands[1] aka low part of input rather than operands[3] aka high part of input to output if not the same register.
