* Re: precise debugging of inline asm
2008-06-01 17:27 ` Claudio Fontana
@ 2008-06-08 12:36 ` lynx.abraxas
0 siblings, 0 replies; 3+ messages in thread
From: lynx.abraxas @ 2008-06-08 12:36 UTC (permalink / raw)
To: linux-assembly
On 01/06/08 19:27:26, Claudio Fontana wrote:
> On x86, try for example
>
> (gdb) disassemble $eip $eip+20
>
> Also see
> (gdb) info registers
Thanks Claudio Fontana. That did the trick. So the first line of output from
disassemble $eip $eip+20 is then the line where the segfault occured? In this
example "mov %cx,(%esi,%edx,2)"?
(gdb) disassemble $eip $eip+20
Dump of assembler code from 0x87b29f8 to 0x87b2a0c:
0x087b29f8 <.lable1+3>: mov %cx,(%esi,%edx,2)
0x087b29fc <.lable1+7>: inc %edx
0x087b29fd <.lable1+8>: jne 0x87b29ca <.lable0>
0x087b29ff <.lable1+10>: mov %ebp,0xffffffd0(%ebp)
0x087b2a02 <.lable1+13>: pop %ebx
0x087b2a03 <.lable1+14>: mov %eax,0xffffffa8(%ebp)
0x087b2a06 <.lable1+17>: addl $0x1,0xffffffb0(%ebp)
0x087b2a0a <.lable1+21>: cmpl $0x2f,0xffffffb0(%ebp)
End of assembler dump.
Well investigating that I think I should be sure of what each line does so
here I have some more questions:
As in the code below, will gcc not use registers in the clobber list as
registers for %0 - %5 ?
Or asked differently: How would one code an asm swap?
1.
__asm__("movl %0,%4 \n\t"
"movl %1,%0 \n\t"
"movl %4,%1 \n\t"
: "=r" (a), "=r" (b) : "0" (a), "1" (b), "r" (c));
or
2.
__asm__("movl %0,%%ebx \n\t"
"movl %1,%0 \n\t"
"movl %%ebx,%1 \n\t"
: "=r" (a), "=r" (b) : "0" (a), "1" (b) : %ebx);
Well if some one likes the challange: Does anybody see the mistakes I mad
translating the intel inline-asm MSC code to at&t inline-asm gcc code? Did I
do something wrong with the register lowding or the constrains?
Thanks again
Lynx
_asm {
mov edx, endX
mov edi, pDestPixel
mov esi, dataPtr
mov ecx, startX
lea edi, [edi + 2*edx]
sub ecx, edx
mov ebx, transDataPtr
L0:
mov dx, [esi]
add esi, 2
xor eax, eax
mov ax, dx
cmp eax, 4
jge L1
mov edx, tileData[4*eax]
test edx, edx
jz L2
add edx, 2
mov tileData[4*eax], edx
mov dx, [edx-2]
jmp L1
L2:
mov dx, [ebx]
L1:
add ebx, 2
mov [edi + 2*ecx], dx
inc ecx
jnz L0
mov transDataPtr, ebx
mov dataPtr, esi
}
#else
printf("%s L%d: Using __asm__ tile blending!\n", __FILE__, __LINE__);
__asm__ (
// "movl $endX, %edx \n\t" //done by gcc! %3
// "movl $pDestPixel, %edi \n\t" //done by gcc! %2
// "movl $dataPtr, %esi \n\t" //done by gcc! %1
// "movl $startX, %ecx \n\t" //done by gcc! %4
"leal (%2,%3,2),%2 \n\t"//load value %2 + 2 * %3 is pointing to
"subl %3,%4 \n\t" //%3 is now reuseable
// "movl $transDataPtr, %ebx \n\t" //done by gcc!
".lable0: \n\t"
"movw (%1),%w3 \n\t" //reusing edx (%3)
"addl $2,%1 \n\t"
// "movl %%eax, %4 \n\t"
"pushl %%ebx \n\t" // save reg for PIC!
// "pushl %%ebp \n\t" // save ebp, NEVER! use -fomit-frame-pointer
//even if ebp is in clobberlist gcc uses it here to reference memory!
"movl %0, %%ebp \n\t"
"xorl %%ebx,%%ebx \n\t" //make %ebx = 0, see below, we use ebx
"movw %w3,%%bx \n\t" //because of this ebx has to be static
"cmpl $4,%%ebx \n\t" //because of above line eax may be != 0
"jge .lable1 \n\t"
"movl (%5,%%ebx,4), %3 \n\t" // ** tileData can't be passed as "m"!
"testl %3,%3 \n\t" //check if %3 is zero, set Z-bit
"jz .lable2 \n\t"
"addl $2,%3 \n\t"
"movl %3, (%5,%%ebx,4) \n\t" // (%eax + %ebx * 4) = %3
"movw -2(%3),%w3 \n\t" // %w3 first (or last?) 2 bytes of %3
"jmp .lable1 \n\t"
".lable2: \n\t"
"movw (%%ebp),%w3 \n\t"//movw (%3),%%dx
".lable1: \n\t"
"addl $2,%%ebp \n\t"
"movw %w3,(%2,%4,2) \n\t"
"incl %4 \n\t"
"jnz .lable0 \n\t"
"movl %%ebp, %0 \n\t"// this wouldn't work without -fomit-frame-pointer
// "popl %%ebp \n\t"// restore ebp, NEVER use this!
"popl %%ebx \n\t"// restore reg for PIC!
// "movl %esi, $dataPtr \n\t" //does gcc
: "=g" (transDataPtr), "=r" (dataPtr) // "=D" (pDestPixel)
: "r" (pDestPixel), "r" (endX), "r" (startX), "r" (tileData), "0" (transDataPtr), "1" (dataPtr)
: "%ebp", "cc"
#if !defined(PIC) && !defined(__PIC__) //don't tell gcc about what was done to ebx!!!
, "%ebx" //but do, if PIC is not enabled! It might use it else wise!!!
#endif
);
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