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From: CBFalconer <cbfalconer AT yahoo DOT com>
Organization: Ched Research
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Newsgroups: comp.lang.c,comp.os.msdos.djgpp
Subject: Re: Power at compilation time
References: <GpPY9.47960$ZE DOT 1351359 AT twister2 DOT libero DOT it> <kEsZ9.9753$wL1 DOT 5596 AT fe01>
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Date: Wed, 29 Jan 2003 16:42:54 GMT
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NNTP-Posting-Date: Wed, 29 Jan 2003 16:42:54 GMT
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com

Jan Engelhardt wrote:
> Martin Dickop wrote:
> > CBFalconer <cbfalconer AT yahoo DOT com> writes:
> >
> >> It seems to me that a lot of time is spent executing strlen.  I
> >> had expected gcc for the x86 to generate some fast string scan
> >> instructions inline, but every time I look I find calls to the
> >> library.
> 
> Aside from what is shown in earlier reply, try adding the
> -finline-functions flag to GCC. (I use gcc 2.95.3 and I see
> repnz scasb in the GDB and HT disassembler often.)
> 
> > What version of gcc are you using, and what flags? With gcc 3.2
> > on a x86 GNU/Linux system and -O2 optimization, this code
> >
> >    #include <string.h>
> >    size_t foo (const char *s) { return strlen (s); }
> >
> > generates `repnz scasb' as the loop to scan for the '\0'
> > character.

Not here.  Under DJGPP, gcc 3.2.1 with:

    gcc -W -Wall -ansi -pedantic -gstabs+ -O2 -Wa,-alhn=strlgh.s
strlgh.c

gives the following listing with the stabs lines deleted (long
lines, thus pasted as quote):

-minline-all-stringops doesn't bring in any scasb sequences
either. (All this may have more to do with the library than gcc)

>    1              		.file	"strlgh.c"
>    4              		.section .text
>    5              	Ltext0:
>   46              	.globl _s
>   47              	LC0:
>   48 0000 54686973 		.ascii "This is a string\0"
>   48      20697320 
>   48      61207374 
>   48      72696E67 
>   48      00
>   49              		.section .data
>   50              		.p2align 2
>   51              	_s:
>   52 0000 00000000 		.long	LC0
>   53 0004 00000000 		.section .text
>   53      00000000 
>   53      00000000 
>   54              	LC1:
>   55 0011 25642025 		.ascii "%d %s\12\0"
>   55      730A00
>   56 0018 908DB426 		.p2align 4,,15
>   56      00000000 
>   60              	.globl _main
>   61              	_main:
>    1:strlgh.c      **** /* Compile with -Wa,-alhn=strlgh.s to show code generated */
>    2:strlgh.c      **** /*  also try -minline-all-stringops */
>    3:strlgh.c      **** 
>    4:strlgh.c      **** #include <stdio.h>
>    5:strlgh.c      **** #include <string.h>
>    6:strlgh.c      **** 
>    7:strlgh.c      **** char *s = "This is a string";
>    8:strlgh.c      **** 
>    9:strlgh.c      **** int main(int argc, char *argv[])
>   10:strlgh.c      **** {
>   62              	.stabn 68,0,10,LM1-_main
>   63              	LM1:
>   64 0020 55       		pushl	%ebp
>   11:strlgh.c      ****    int lgh;
>   12:strlgh.c      **** 
>   13:strlgh.c      ****    lgh = strlen(s);
>   66              	LM2:
>   67              	LBB2:
>   68 0021 8B0D9000 		movl	_s, %ecx
>   68      0000
>   70              	LM3:
>   71 0027 89E5     		movl	%esp, %ebp
>   72 0029 56       		pushl	%esi
>   73 002a 53       		pushl	%ebx
>   74 002b 83E4F0   		andl	$-16, %esp
>   75 002e 8B750C   		movl	12(%ebp), %esi
>   77              	LM4:
>   78 0031 83EC0C   		subl	$12, %esp
>   80              	LM5:
>   81 0034 8B5D08   		movl	8(%ebp), %ebx
>   83              	LM6:
>   84 0037 51       		pushl	%ecx
>   85 0038 E8C3FFFF 		call	_strlen
>   85      FF
>   14:strlgh.c      ****    printf("%d %s\n", lgh, s);
>   87              	LM7:
>   88 003d 8B159000 		movl	_s, %edx
>   88      0000
>   89 0043 83C40C   		addl	$12, %esp
>   90 0046 52       		pushl	%edx
>   91 0047 50       		pushl	%eax
>   92 0048 68110000 		pushl	$LC1
>   92      00
>   93 004d E8AEFFFF 		call	_printf
>   93      FF
>   15:strlgh.c      ****    if (argc)
>   95              	LM8:
>   96 0052 83C410   		addl	$16, %esp
>   97 0055 85DB     		testl	%ebx, %ebx
>   98 0057 7403     		je	L2
>   16:strlgh.c      ****       while (--argc) {
>  100              	LM9:
>  101 0059 4B       		decl	%ebx
>  102 005a 7509     		jne	L6
>  103              	L2:
>   17:strlgh.c      ****          lgh = strlen(argv[argc]);
>   18:strlgh.c      ****          printf("%d %s\n", lgh, argv[argc]);
>   19:strlgh.c      ****       }
>   20:strlgh.c      ****    return 0;
>   21:strlgh.c      **** }
>  105              	LM10:
>  106              	LBE2:
>  107 005c 8D65F8   		leal	-8(%ebp), %esp
>  108 005f 31C0     		xorl	%eax, %eax
>  109 0061 5B       		popl	%ebx
>  110 0062 5E       		popl	%esi
>  111 0063 5D       		popl	%ebp
>  112 0064 C3       		ret
>  113              		.p2align 4,,7
>  114              	L6:
>  116              	LM11:
>  117              	LBB3:
>  118 0065 83EC0C   		subl	$12, %esp
>  119 0068 8B0C9E   		movl	(%esi,%ebx,4), %ecx
>  120 006b 51       		pushl	%ecx
>  121 006c E88FFFFF 		call	_strlen
>  121      FF
>  123              	LM12:
>  124 0071 8B149E   		movl	(%esi,%ebx,4), %edx
>  125 0074 83C40C   		addl	$12, %esp
>  126 0077 52       		pushl	%edx
>  127 0078 50       		pushl	%eax
>  128 0079 68110000 		pushl	$LC1
>  128      00
>  129 007e E87DFFFF 		call	_printf
>  129      FF
>  130 0083 83C410   		addl	$16, %esp
>  131 0086 4B       		decl	%ebx
>  132 0087 75DC     		jne	L6
>  133 0089 EBD1     		jmp	L2
>  134              	LBE3:
>  143              	Lscope0:
>  146              		.text
>  148              	Letext:
>  149 008b 90909090 		.ident	"GCC: (GNU) 3.2.1"
>  149      90

-- 
Chuck F (cbfalconer AT yahoo DOT com) (cbfalconer AT worldnet DOT att DOT net)
   Available for consulting/temporary embedded and systems.
   <http://cbfalconer.home.att.net>  USE worldnet address!