Author: Robert Hyatt
Date: 08:07:04 05/21/04
Go up one level in this thread
On May 21, 2004 at 09:23:01, Anthony Cozzie wrote:
>On May 21, 2004 at 09:15:36, Christophe Drieu wrote:
>
>>Hi, i'm learning Inline assembly with gcc, but have some trouble with global C
>>variables:
>>
>>int a=4, b=0;
>>
>>int main()
>>{
>> asm("incl _a"); // a++
>> asm("movl _a,%eax"); // eax=a
>> asm("movl %eax,_b"); // b=eax
>>
>> printf("a=%d b=%d\n", a, b); // print a=5 b=5
>>
>> a=4;
>>
>> asm("incl _a"); // a++
>> asm("movl _a,%eax"); // eax=a
>> asm("movl %eax,_b"); // b=eax
>>
>> printf("a=%d b=%d\n", a, b); // print a=4 b=5
>>}
>>
>>My question is, why a==4 in second case and not 5 ? Thank you.
>
>
>Dunno if this is the issue, but registers are not guaranteed to be preserved
>between asm calls.
>
>gcc -S will show you the actual assembly that is generated - I'd take a look at
>that.
>
>This is my personal inline assembly reference:
>
>http://www.delorie.com/djgpp/doc/brennan/brennan_att_inline_djgpp.html
>
>anthony
He also needs to study the gcc "clobber list". His asm changes memory. The
compiler might not realize that and use a previous value of _a that was kept in
a register across the asm. He also needs to indicate that he is clobbering the
registers he modifies so that the compiler will know they are changed after the
asm...
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