Hi,
This change introduces a (size) regression for arm-none-eabi:
Testing arm/pr42879.c
doing compile
Executing on host: /build/r17-2833-g32657f29f91871/bin/arm-none-eabi-gcc
/build/gcc_src/gcc/testsuite/gcc.target/arm/pr42879.c -mthumb -march=armv7-m
-mcpu=cortex-m3 -mfloat-abi=soft -mfpu=auto -fdiagnostics-plain-output
-mthumb -Os -ffat-lto-objects -fno-ident -S -o pr42879.s (timeout = 800)
spawn -ignore SIGHUP /build/r17-2833-g32657f29f91871/bin/arm-none-eabi-gcc
/build/gcc_src/gcc/testsuite/gcc.target/arm/pr42879.c -mthumb -march=armv7-m
-mcpu=cortex-m3 -mfloat-abi=soft -mfpu=auto -fdiagnostics-plain-output -mthumb
-Os -ffat-lto-objects -fno-ident -S -o pr42879.s
pid is 180 -180
pid is -1
output is status 0
PASS: gcc.target/arm/pr42879.c (test for excess errors)
FAIL: gcc.target/arm/pr42879.c scan-assembler lsls
$ /build/r17-2412-g2fcbe075ec3b32/bin/arm-none-eabi-gcc
/build/gcc_src/gcc/testsuite/gcc.target/arm/pr42879.c -mthumb -march=armv7-m
-mcpu=cortex-m3 -mfloat-abi=soft -mfpu=auto -Os -fno-ident -S -o -
.cpu cortex-m3
.arch armv7-m
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 4
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "pr42879.c"
.text
.align 1
.global foo
.syntax unified
.thumb
.thumb_func
.type foo, %function
foo:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldrb r3, [r0] @ zero_extendqisi2
lsls r3, r3, #31
bmi .L2
b bar
.L2:
movs r0, #1
bx lr
.size foo, .-foo
$ /build/r17-2413-ge46d96d20bbfa7/bin/arm-none-eabi-gcc
/build/gcc_src/gcc/testsuite/gcc.target/arm/pr42879.c -mthumb -march=armv7-m
-mcpu=cortex-m3 -mfloat-abi=soft -mfpu=auto -Os -fno-ident -S -o -
.cpu cortex-m3
.arch armv7-m
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 4
.eabi_attribute 34, 1
.eabi_attribute 18, 4
.file "pr42879.c"
.text
.align 1
.global foo
.syntax unified
.thumb
.thumb_func
.type foo, %function
foo:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldrb r3, [r0] @ zero_extendqisi2
and r3, r3, #1
cbnz r3, .L2
b bar
.L2:
movs r0, #1
bx lr
.size foo, .-foo
Kind regards,
Torbjörn
On 2026-07-14 11:07, Richard Biener wrote:
On Fri, Jul 10, 2026 at 7:44 PM Roger Sayle <[email protected]> wrote:
Long story... Back in January I proposed the following patch:
https://gcc.gnu.org/pipermail/gcc-patches/2026-January/705292.html
which contained two pieces, one in combine, the other in simplify-rtx.
Both of which were initially approved by Jeff Law here
https://gcc.gnu.org/pipermail/gcc-patches/2026-May/715595.html
but then Richard Sandiford pointed out the combine piece might
cause problems on RISC machines, and suggested improvements.
https://gcc.gnu.org/pipermail/gcc-patches/2026-May/715668.html
In addition to making Richard's recommended changes, I've also
decided to split the patch into two, to enable bisection and
isolate these transformations [in case Richard S's fears come
to pass and the combine transformation needs to be reverted].
This is the "safe" (or less controversial) half. Hopefully,
folks are (still) happy for this bit to be committed?
p.s. the second and third hunks are just micro-optimizations;
we don't need to call simplify_gen_unary (TRUNCATE, ...)
if the operand already has the correct mode. The significant
change is that the modes don't need to match, and the operand
need not be a register [combine can put anything in a SUBREG].
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32},
with no new failures. Ok for mainline?
OK.
Thanks,
Richard.
2026-07-10 Roger Sayle <[email protected]>
gcc/ChangeLog
PR rtl-optimization/123236
* simplify-rtx.cc (simplify_context::simplify_truncation): Handle
cases where a ZERO_EXTRACT or SIGN_EXTRACT has a different mode
to (but at least as wide as) its first operand.
gcc/testsuite/ChangeLog
PR rtl-optimization/123236
* gcc.target/i386/pr123236-1.c: New test case.
Thank again,
Roger
--
