Re: Useless %esp manipulation; omitting "l" suffix in assembly output; getting the address of a label from another function.

[Richard Henderson <rth at redhat dot com>]

On Thu, Apr 10, 2003 at 05:50:00PM -0500, Mark J Roberts wrote:
> 	struct x { int a, b, c, d; };
> 
> 	struct x zook(void)
> 	{
> 		struct x x = {0, 1, 2, 3};
> 		return x;
> 	}
> 
> gcc -S -O2 -fomit-frame-pointer -march=athlon-xp test.c
> 
> 	.file	"test.c"
> 	.text
> 	.p2align 4,,15
> globl zook
> 	.type	zook, @function
> zook:
> 	subl	$28, %esp
> 	movl	32(%esp), %eax
> 	movl	$0, (%eax)
> 	movl	$1, 4(%eax)
> 	movl	$2, 8(%eax)
> 	movl	$3, 12(%eax)
> 	addl	$28, %esp
> 	ret	$4
> 	.size	zook, .-zook
> 	.ident	"GCC: (GNU) 3.4 20030410 (experimental)"
> 
> and you get code that uselessly moves %esp.

This should be fixed by tree-ssa stuff, but can't be done
otherwise.  We committed too early to putting "x" on the
stack.

> 1) Output "mov" instead of "movl", etc, since gas doesn't require
> the suffix and it's easier to read the code when they are omitted.

I disagree about legibility.  And, you've no idea how this
affects various legacy assemblers.

> 2) Given code like
> 
> 	void process(void *addr)
> 	{
> 		goto *addr;
> 	l1:
> 	l2:	something goes here
> 	l3:
> 	l4:
> 	}
> 
> where you're trying to optimize some code by saving the address of a
> label and using it next time the function is called, provide some
> way to get the address of the label from another function (useful
> for initializing addr the first time). Ie,
> 
> 	void setup(struct state *s)
> 	{
> 		s->addr = process.l1;
> 	}
> 
> or whatever.

No thanks.  There are several other solutions you can use here.


r~