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 [EMAIL PROTECTED] for PGP public key. See complete headers for address and phone numbers.
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