[Bug c/126734] New: Continuing regression in quality of generated x86 machine code for a simple bit rotation

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126734

            Bug ID: 126734
           Summary: Continuing regression in quality of generated x86
                    machine code for a simple bit rotation
           Product: gcc
           Version: 16.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: contact at ivanchev dot net
  Target Milestone: ---

I have the following bit-rotation sample in a set of tools for stress-testing
compilers:

#include <stdint.h>

uint32_t func(uint32_t a) {
    uint64_t b = ((uint64_t) a << 32) | a;
    return (uint32_t) (b >> 16);
}

The quality of the generated machine code with "-O2 -m32" has been dropping
continuously since GCC 12.5 as is evident here:

https://godbolt.org/z/cajcb5Gz4

GCC 16.2 generates useless push / pop sequence and the trunk is straight up
suspicious whereas clang correctly identifies the pattern and emits a ROR
instruction.