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From: Peter Barada <peter@the-baradas.com>
To: sam124@operamail.com
Cc: nathan@codesourcery.com, gcc@gcc.gnu.org
In-reply-to: <20041211162555.C0BE723CE8@ws5-3.us4.outblaze.com>
	(sam124@operamail.com)
Subject: Re: Unomitted frame pointers
References:  <20041211162555.C0BE723CE8@ws5-3.us4.outblaze.com>
Message-Id: <20041212000941.92E8F9842C@baradas.org>
Date: Sun, 12 Dec 2004 00:09:00 -0000
X-SW-Source: 2004-12/txt/msg00406.txt.bz2


>Do you know about macro expansion? CPP would make it seperate strings. So there would be (simplified);
>
>.LC0:
>.string "Hello World!\n"
>.LC1:
>.string "Hello World!\n"
>main:
>; push LC0 into strlen
>push #.LC0, (%eax)
>call strlen
>; push LC1 into write
>push #.LC1, (%ebx)
>...
>
>It automatically happens, and I don't know of any way to avoid cpp from macro-expanding it to that. Though inlining would reduce it to the C code
>
>int main(void)
>{
>       char str1 = "Hello World!\n";
>       char str2 = "Hello World!\n";
>       int str_len = 0;
>       while (*str1++ != "\0")
>              str_len++;
>       write(2, str2, str_len);
>}
>
>embedding strlen inside main. Or even more optimization
>
>int main(void)
>{
>        write(2, "Hello World!\n", 13);
>}

Once I reduced your testcase to:

#include <string.h>

int main(void)
{
       char *str1 = "Hello World!\n";

       write(2, str1, strlen(str1));
}


And compiled it off the current mainline for --target=m68k-elf I get:

	.section	.rodata.str1.1,"aMS",@progbits,1
.LC0:
	.string	"Hello World!\n"
	.text
	.align	2
	.globl	main
	.type	main, @function
main:
	link.w %fp,#0
	pea 13.w        ; <<<< strlen(str1)
	pea .LC0
	pea 2.w
	jbsr write
	lea (12,%sp),%sp
	unlk %fp
	rts


If you notice, the strlen is completely removed and replaced with 'pea
13.w' which does what you hope it does.

Even if I changed the code to:

#include <string.h>

#define STR "Hello World!\n"
int main(void)
{
       char *str1 = STR;

       write(2, str1, strlen(STR));
}

GCC produces the same code.  Even gcc-2.96 on my Linux x86 box
knows to convert the strlen call into a constant.

Use the standard named functions since GCC is smart enough to figure
out(with optimization enabled) that 'strlen(str1)' is 13 since between
the declaration of str1 and the call to strlen, str1 did not change
and GCC knows what strlen of a constant string is, or a pointer to a
string constant that it see doesn't change between its assignment and
the invocation.

Hopefully this helps...

-- 
Peter Barada
peter@the-baradas.com