Walter Bright:
> I didn't know gcc had pure functions. It doesn't have immutable data. If
> it does optimize with it, you can try it and see!
It seems to work:
// test1
int bar(int);
int foo(int i) {
return bar(i) + bar(i);
}
Asm with no optimization and default CPU, GCC 4.2.1:
.file "test1.c"
.text
.globl _foo
.def _foo; .scl 2; .type 32; .endef
_foo:
pushl %ebp
movl %esp, %ebp
pushl %ebx
subl $4, %esp
movl 8(%ebp), %eax
movl %eax, (%esp)
call _bar
movl %eax, %ebx
movl 8(%ebp), %eax
movl %eax, (%esp)
call _bar
leal (%ebx,%eax), %eax
addl $4, %esp
popl %ebx
popl %ebp
ret
.def _bar; .scl 2; .type 32; .endef
With -O2:
.file "test1.c"
.text
.p2align 4,,15
.globl _foo
.def _foo; .scl 2; .type 32; .endef
_foo:
pushl %ebp
movl %esp, %ebp
subl $24, %esp
movl %ebx, -8(%ebp)
movl 8(%ebp), %ebx
movl %esi, -4(%ebp)
movl %ebx, (%esp)
call _bar
movl %ebx, (%esp)
movl %eax, %esi
call _bar
movl -8(%ebp), %ebx
addl %esi, %eax
movl -4(%ebp), %esi
movl %ebp, %esp
popl %ebp
ret
.def _bar; .scl 2; .type 32; .endef
// test2
int bar(int) __attribute__ ((pure));
int foo(int i) {
return bar(i) + bar(i);
}
Asm with no optimization:
.file "test2.c"
.text
.globl _foo
.def _foo; .scl 2; .type 32; .endef
_foo:
pushl %ebp
movl %esp, %ebp
pushl %ebx
subl $4, %esp
movl 8(%ebp), %eax
movl %eax, (%esp)
call _bar
movl %eax, %ebx
movl 8(%ebp), %eax
movl %eax, (%esp)
call _bar
leal (%ebx,%eax), %eax
addl $4, %esp
popl %ebx
popl %ebp
ret
.def _bar; .scl 2; .type 32; .endef
With -O2:
.file "test2.c"
.text
.p2align 4,,15
.globl _foo
.def _foo; .scl 2; .type 32; .endef
_foo:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
movl 8(%ebp), %eax
movl %eax, (%esp)
call _bar
leave
addl %eax, %eax
ret
.def _bar; .scl 2; .type 32; .endef
As you can see now there's just one call to bar.
Bye,
bearophile
