On 1/29/2026 5:38 AM, Roger Sayle wrote:
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
This patch has been tested by bootstrapping x86_64-pc-linux-gnu, to
reconfirm
that the middle-end changes to do harm, and by building cc1 for mips64-elf
to confirm that the backend still builds. Very many thanks to Mateusz
Marciniec for confirming that this patch resolves (or used to resolve) the
issue on mips64 hardware. Ok for mainline?
2026-01-29 Roger Sayle <[email protected]>
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.c (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.
No regressions on the crosses or x86 native. The RISC-V natives are
spinning, but won't be done for many many hours.
jeff
