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"
