[Bug target/121266] New: [16 regression] (x86) Missed an '-Os' optimization on setting a register value to -1

[Explorer09 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121266

            Bug ID: 121266
           Summary: [16 regression] (x86) Missed an '-Os' optimization on
                    setting a register value to -1
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Explorer09 at gmail dot com
  Target Milestone: ---

I found this when testing the trunk version of GCC in Compiler Explorer
(https://godbolt.org/z/z9oMWKP5P)

There was a little optimization in gcc (15.1.0 and before) enabled in '-Os'
mode that would use 'or' instruction instead of 'mov' when setting a local
variable's value to -1. The trunk version (16.0.0 20250726) currently misses
it.

```c
#include <stdint.h>
int32_t func_32(void) {
    return -1;
}

int64_t func_64(void) {
    return -1;
}
```

x86-64 gcc 15.1 (with '-Os' option):

```assembly
func_32:
        orl     $-1, %eax
        ret
func_64:
        orq     $-1, %rax
        ret
```

x86-64 gcc trunk (with '-Os' option):

```assembly
func_32:
        movl    $-1, %eax
        ret
func_64:
        movq    $-1, %rax
        ret
```

Note, the --version option of gcc in Compiler Explorer outputs this:

```
gcc
(Compiler-Explorer-Build-gcc-11518c841dda4d417573e41ded69bd1469ad6d1f-binutils-2.44)
16.0.0 20250726 (experimental)
Copyright (C) 2025 Free Software Foundation, Inc.
```