[Bug c/110567] New: GCC fails using a register to initialize multiple variables with a constant

[vincent.riviere at freesbee dot fr via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110567

            Bug ID: 110567
           Summary: GCC fails using a register to initialize multiple
                    variables with a constant
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent.riviere at freesbee dot fr
  Target Milestone: ---
            Target: m68k-elf

This is about a missed optimization for an obvious case.
GCC fails to use a register to initialize several global variables with the
same constant value.

$ cat foo.c
extern long init;
extern long g1, g2, g3, g4, g5;

void f(void)
{
    long val = 12345678;

    g1 = val;
    g2 = val;
    g3 = val;
    g4 = val;
    g5 = val;
}

$ m68k-elf-gcc -S foo.c -o - -Os
#NO_APP
        .file   "foo.c"
        .text
        .align  2
        .globl  f
        .type   f, @function
f:
        move.l #12345678,g1
        move.l #12345678,g2
        move.l #12345678,g3
        move.l #12345678,g4
        move.l #12345678,g5
        rts
        .size   f, .-f
        .ident  "GCC: (GNU) 13.1.0"

We can see that 12345678 is used many times. This is a waste of space and time.

On the other hand, if I replace 12345678 by "init" (an external global
variable) then GCC produces the expected optimized output:

$ m68k-elf-gcc -S foo.c -o - -Os
#NO_APP
        .file   "foo.c"
        .text
        .align  2
        .globl  f
        .type   f, @function
f:
        move.l init,%d0
        move.l %d0,g1
        move.l %d0,g2
        move.l %d0,g3
        move.l %d0,g4
        move.l %d0,g5
        rts
        .size   f, .-f
        .ident  "GCC: (GNU) 13.1.0"

GCC should be able to optimize a constant initializer, just as it does with a
variable initializer.

I can reproduce the same wrong behaviour with GCC 11.4.0 for x86_64-pc-cygwin
host with -m32:

$ gcc -m32 -S foo.c -o - -Os
        .file   "foo.c"
        .text
        .globl  _f
        .def    _f;     .scl    2;      .type   32;     .endef
_f:
        movl    $12345678, _g1
        movl    $12345678, _g2
        movl    $12345678, _g3
        movl    $12345678, _g4
        movl    $12345678, _g5
        ret
        .ident  "GCC: (GNU) 11.4.0"