https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118608
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Roger Sayle <[email protected]>: https://gcc.gnu.org/g:a9e48eca3a6eef2348c2827ed67b9cbb3f5f82ce commit r16-7265-ga9e48eca3a6eef2348c2827ed67b9cbb3f5f82ce Author: Roger Sayle <[email protected]> Date: Tue Feb 3 17:43:59 2026 +0000 PR middle-end/118608: Fix lack of sign extension on MIPS64 with -Os Here is a refreshed version of my patch from February 2025, for resolving PR middle-end/118608, a wrong code on valid regression where the middle-end is failing to keep values suitably sign-extended (MIPS64 is a rare targetm.mode_rep_extended target, as well as being BYTES_BIG_ENDIAN). This fix requires three independent tweaks, one in each source file. The first tweak is that the logic in my triggering patch for determining whether store_field updates the most significant bit needs to be updated to handle BYTES_BIG_ENDIAN. Of the two insertions in the bugzilla test case, we were generating the sign extension after the wrong one. The second tweak was that this explicit sign-extension was then being eliminated during combine by simplify-rtx that believed the explicit TRUNCATE wasn't required. This patch updates truncated_to_mode to understand that on mode_rep_extended targets, TRUNCATE is used instead of SUBREG because it isn't a no-op. Finally, the third tweak is that the MIPS backend requires a small change to recognize (and split) *extenddi_truncatesi when TARGET_64BIT and !ISA_HAS_EXTS. On mips64-elf with -mabi=64 the following are now generated for prepareNeedle: -O2 sll $5,$5,16 jr $31 or $2,$5,$4 -Os dsll $5,$5,16 or $2,$4,$5 dsll $2,$2,32 jr $31 dsra $2,$2,32 -O2 -march=octeon2 move $2,$0 ins $2,$5,16,16 jr $31 ins $2,$4,0,16 -Os -march=octeon2 move $2,$0 dins $2,$4,0,16 dins $2,$5,16,16 jr $31 sll $2,$2,0 Many thanks to Mateusz Marciniec and Jeff Law for additional testing. gcc/ChangeLog PR middle-end/118608 * expr.cc (store_field_updates_msb_p): New helper function that now also handles BYTES_BIG_ENDIAN targets. (expand_assignment): Use the above function when deciding to emit a required sign/zero extension. * rtlanal.cc (truncated_to_mode): Call targetm.mode_rep_extended to check whether an explicit TRUNCATE is required (i.e. performs an extension) on this target. * config/mips/mips.md (*extenddi_truncate<mode>): Handle all SUBDI modes, not just SHORT modes. gcc/testsuite/ChangeLog PR middle-end/118608 * gcc.target/mips/pr118608-1.c: New test case. * gcc.target/mips/pr118608-2.c: Likewise. * gcc.target/mips/pr118608-3.c: Likewise. * gcc.target/mips/pr118608-4.c: Likewise.
