Date: Fri, 05 Mar 2004 18:43:11 -0500 From: Chungwei Hsiung <skuma17@yahoo.com> To: Greg 'groggy' Lehey <grog@FreeBSD.org> Cc: Chungwei Hsiung <skuma17@yahoo.com> Subject: Re: Strange instructions in compiler output Message-ID: <4049108F.5080703@yahoo.com> In-Reply-To: <20040305233209.GO67801@wantadilla.lemis.com> References: <4048CA38.6040203@yahoo.com> <20040305233209.GO67801@wantadilla.lemis.com>
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Greg 'groggy' Lehey wrote:
>On Friday, 5 March 2004 at 13:43:04 -0500, Chungwei Hsiung wrote:
>
>
>>Hello..
>>I am super new to this list, and I have a simple question that I don't
>>know why it does that. I have a simple test program. I compile it, and
>>gdb to disassemble main. I got the following..
>>
>>0x80481f8 <main>: push %ebp
>>0x80481f9 <main+1>: mov %esp,%ebp
>>0x80481fb <main+3>: sub $0x8,%esp
>>0x80481fe <main+6>: and $0xfffffff0,%esp
>>0x8048201 <main+9>: mov $0x0,%eax
>>0x8048206 <main+14>: sub %eax,%esp
>>0x8048208 <main+16>: movl $0x804a6ce,0xfffffff8(%ebp)
>>0x804820f <main+23>: movl $0x0,0xfffffffc(%ebp)
>>0x8048216 <main+30>: sub $0x4,%esp
>>0x8048219 <main+33>: push $0x0
>>0x804821b <main+35>: lea 0xfffffff8(%ebp),%eax
>>0x804821e <main+38>: push %eax
>>0x804821f <main+39>: pushl 0xfffffff8(%ebp)
>>0x8048222 <main+42>: call 0x804823c <execve>
>>0x8048227 <main+47>: add $0x10,%esp
>>0x804822a <main+50>: mov $0x0,%eax
>>0x804822f <main+55>: leave
>>0x8048230 <main+56>: ret
>>
>>I don't know if at line 5, we move zero to %eax. why do we need to sub
>>%eax, %esp? why do we need to substract 0 from the stack pointer??
>>Any help is really appreciated.
>>
>>
>
>This is probably because you didn't optimize the output. You'd be
>surprised how many redundant instructions the compiler puts in under
>these circumstances. Try optimizing and see what the code looks like.
>
>If this *was* done with optimization, let's see the source code.
>
>Greg
>--
>Note: I discard all HTML mail unseen.
>Finger grog@FreeBSD.org for PGP public key.
>See complete headers for address and phone numbers.
>
>
Hello.. thank you very much for the reply
I actually don't know how to use the optimization. I just compile it
with gcc 3.2.2, and use gdb to disassemble main to get this assembly. Is
it possible I can get the non-redundent output?
here is the code I compile..
#include <stdio.h>
int main(void) {
char *name[2];
name[0] = "/bin/sh";
name[1] = NULL;
execve(name[0], name, NULL);
return(0);
}
best regards
Chungwei
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