https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128048
Bug ID: 128048
Summary: [17 Regression] arm: redundant sxth after sbfx when
reading a signed bit-field
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: yuki.tanaka at defaultcase dot de
Target Milestone: ---
Since GCC 17, reading a signed bit-field on Thumb-2 (Cortex-M3) emits an extra
sxth after the sbfx. The sbfx already sign-extends the field, so the sxth is
redundant.
struct i { int x : 5; int y : 11; };
int get_iy (struct i *p) { return p->y; }
Options: -mcpu=cortex-m3 -O2 -mthumb
Checked on Godbolt.
arm-none-eabi-gcc trunk: https://godbolt.org/z/5PjxYqGTT
get_iy:
ldrh r0, [r0]
sbfx r0, r0, #5, #11
sxth r0, r0
bx lr
arm-none-eabi-gcc 16.1: https://godbolt.org/z/PEf7Pqq56
get_iy:
ldrh r0, [r0]
sbfx r0, r0, #5, #11
bx lr
Possibly caused by r17-2413-ge46d96d20bbf ("PR middle-end/123236: Simplify
(int)((long long)x >> 4)").
