[Bug target/128049] New: [15/16/17 Regression] arm: redundant lsls before lsrs when reading an unsigned bit-field on Thumb-1

[yuki.tanaka at defaultcase dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128049

            Bug ID: 128049
           Summary: [15/16/17 Regression] arm: redundant lsls before lsrs
                    when reading an unsigned bit-field on Thumb-1
           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 15, reading an unsigned bit-field on Thumb-1 (Cortex-M0) at -Os emits
an extra lsls #16 before the shift right. ldrh already zero-extends the loaded
halfword, so a single lsrs #5 is enough.

struct U { unsigned x:5, y:11; };

unsigned get_uy (struct U *p)
{
  return p->y;
}

Options: -mcpu=cortex-m0 -Os -mfloat-abi=soft
Checked on Godbolt.

arm-none-eabi-gcc 15.1: https://godbolt.org/z/czGTvdnfh

get_uy:
        ldrh    r0, [r0]
        lsls    r0, r0, #16
        lsrs    r0, r0, #21
        bx      lr

arm-none-eabi-gcc 14.3: https://godbolt.org/z/9Pha4ssh7

get_uy:
        ldrh    r0, [r0]
        lsrs    r0, r0, #5
        bx      lr

This only happens at -Os and other architectures are seemingly not affected.

Possibly related to r15-1575-gea8061f46a30 ("fwprop: invoke
change_is_worthwhile to judge if a replacement is worthwhile").