Re: one simple question
I compile a test C file. I notice there are a few lines at the
beginning of the assembly code. I want to know what it means, but
can't figure out one of them. Can anyone tell me what the
following line does please?
and$0xfff0,%esp
Hmmm, when I compile the simplest possible C file:
---
int
main (int argc, char *argv[])
{
return 0;
}
I get this:
.file test1.c
.version01.01
gcc2_compiled.:
.text
.p2align 2,0x90
.globl main
.typemain,@function
main:
pushl %ebp
movl %esp,%ebp
xorl %eax,%eax
jmp .L2
.p2align 2,0x90
.L2:
leave
ret
.Lfe1:
.sizemain,.Lfe1-main
.ident GCC: (GNU) c 2.95.4 20020320 [FreeBSD]
No such thing as:
and$0xfff0,%esp
Are you using gcc 3.3.x?
Anyway, this code looks like it would align the stack
the stack pointer...
best regards
Chungwei
--
Cordula's Web. http://www.cordula.ws/
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Re: one simple question
thank you very much for the reply
yes and I am using gcc 3.2.2
if you gdb the executable and disassemble main
you will see the line like that
but if you use gcc -S something.s something.c
it won't appear in the assembly code
and I google around, I think it does the alignment for optimization
purpose, in that case the memory access will be faster according to the
article.
best regards,
Chungwei
--- Cordula's Web [EMAIL PROTECTED] wrote:
I compile a test C file. I notice there are a few lines at the
beginning of the assembly code. I want to know what it means, but
can't figure out one of them. Can anyone tell me what the
following line does please?
and$0xfff0,%esp
Hmmm, when I compile the simplest possible C file:
---
int
main (int argc, char *argv[])
{
return 0;
}
I get this:
.file test1.c
.version01.01
gcc2_compiled.:
.text
.p2align 2,0x90
.globl main
.typemain,@function
main:
pushl %ebp
movl %esp,%ebp
xorl %eax,%eax
jmp .L2
.p2align 2,0x90
.L2:
leave
ret
.Lfe1:
.sizemain,.Lfe1-main
.ident GCC: (GNU) c 2.95.4 20020320 [FreeBSD]
No such thing as:
and$0xfff0,%esp
Are you using gcc 3.3.x?
Anyway, this code looks like it would align the stack
the stack pointer...
best regards
Chungwei
--
Cordula's Web. http://www.cordula.ws/
=
bear
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Re: one simple question
I compile a test C file. I notice there are a few lines at the beginning of the assembly code. I want to know what it means, but can't figure out one of them. Can anyone tell me what the following line does please? and$0xfff0,%esp gcc2_compiled.: .text .p2align 2,0x90 .globl main .typemain,@function main: pushl %ebp movl %esp,%ebp xorl %eax,%eax jmp .L2 .p2align 2,0x90 .L2: thank you very much for the reply yes and I am using gcc 3.2.2 if you gdb the executable and disassemble main you will see the line like that but if you use gcc -S something.s something.c it won't appear in the assembly code Ah, so it's being introduced by the assembler, not the compiler. That is perhaps the effect of alignement instructions like .text .p2align 2,0x90 this and similar defaults. and I google around, I think it does the alignment for optimization purpose, in that case the memory access will be faster according to the article. That may very well be the case. Considering that a cache line is also a few words worth, it may be sensible to start with an aligned stack frame too. best regards, Chungwei -- Cordula's Web. http://www.cordula.ws/ ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
one simple question
Hello.. I have a simple question, but I am not sure what the answer is. If anyone can possibly help me, it is really appreciated. I compile a test C file. I notice there are a few lines at the beginning of the assembly code. I want to know what it means, but can't figure out one of them. Can anyone tell me what the following line does please? and$0xfff0,%esp best regards Chungwei ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: one simple question
On Wed, 3 Mar 2004 18:13:43 + chungwei Hsiung [EMAIL PROTECTED] wrote: Hello.. I have a simple question, but I am not sure what the answer is. If anyone can possibly help me, it is really appreciated. I compile a test C file. I notice there are a few lines at the beginning of the assembly code. I want to know what it means, but can't figure out one of them. Can anyone tell me what the following line does please? and$0xfff0,%esp best regards Chungwei Hi Chungwei, I believe that instruction is used to align the stack pointer to a 16-byte boundary, for efficiency. However, this is just a guess, based on some discussions I've seen. I don't know for certain. You may have better luck asking on [EMAIL PROTECTED] -Chris ___ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
