Date: Fri, 05 Mar 2004 19:53:42 -0500 From: Chungwei Hsiung <skuma17@yahoo.com> To: Greg 'groggy' Lehey <grog@FreeBSD.org> Cc: freebsd-hackers@freebsd.org Subject: Re: Strange instructions in compiler output Message-ID: <40492116.80607@yahoo.com> In-Reply-To: <20040306001150.GQ67801@wantadilla.lemis.com> References: <4048CA38.6040203@yahoo.com> <20040305233209.GO67801@wantadilla.lemis.com> <4049108F.5080703@yahoo.com> <20040306001150.GQ67801@wantadilla.lemis.com>
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Greg 'groggy' Lehey wrote: >On Friday, 5 March 2004 at 18:43:11 -0500, Chungwei Hsiung wrote: > > >>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.. >>>> >>>>0x8048201 <main+9>: mov $0x0,%eax >>>>0x8048206 <main+14>: sub %eax,%esp >>>>... >>>> >>>>I don't know if at line 5, we move zero to %eax. why do we need to sub >>>> >>>> >>>>>eax, %esp? why do we need to substract 0 from the stack pointer?? >>>>> >>>>> >>>>Any help is really appreciated. >>>> >>>> >>>This is probably because you didn't optimize the output. You'd be >>>surprised how many redundant instructions the compiler puts in under >>>these circumstances. Try optimizing and see what the code looks like. >>> >>>If this *was* done with optimization, let's see the source code. >>> >>> >>Hello.. thank you very much for the reply >>I actually don't know how to use the optimization. >> >> > >Use the gcc command line options. See below. > > > >>I just compile it with gcc 3.2.2, and use gdb to disassemble main to >>get this assembly. Is it possible I can get the non-redundent output? >>here is the code I compile.. >> >>... >> >> > >The best way to look at the assembly output is to generate it directly >from the compiler. I get: > >$ cc -O -pipe -mcpu=pentiumpro -S exec.c >$ cat exec.s >.LC0: > .string "/bin/sh" >... >main: > pushl %ebp > movl %esp, %ebp > subl $24, %esp > andl $-16, %esp > movl $.LC0, -8(%ebp) > leal -8(%ebp), %edx > movl $0, 4(%edx) > movl -8(%ebp), %eax > movl %eax, (%esp) > movl %edx, 4(%esp) > movl $0, 8(%esp) > call execve > movl $0, %eax > movl %ebp, %esp > popl %ebp > ret > >This doesn't look that much like your code. Without the -O (optimize) >flag I get: > >$ cc -pipe -mcpu=pentiumpro -S exec.c >$ cat exec.s >... >main: > pushl %ebp > movl %esp, %ebp > subl $24, %esp > andl $-16, %esp > movl $0, %eax > subl %eax, %esp > movl $.LC0, -8(%ebp) > >So yes, it looks as if you're not optimizing. > >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 greg.. yes.. it does the difference.. thanks a lot for your help.. Chungwei
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