Optimizing Immutable and Purity

[bearophile]

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