[Ffmpeg-devel] Segfault when encoding MPEG with MMX support

Tobias Grimm listaccount
Mon Jul 11 20:15:33 CEST 2005

Michael Niedermayer schrieb:

>writing it or similar), the code makes no sense and cannot work

Ok - I will start with a fresh copy and try to make it PIC-compilable.

Let's take this codeblock:


asm volatile(
    "movd %%"REG_a", %%mm3        \n\t" // last_non_zero_p1
    "pxor %%mm7, %%mm7            \n\t" // 0
    "pxor %%mm4, %%mm4            \n\t" // 0
    "movq (%2), %%mm5            \n\t" // qmat[0]
    "pxor %%mm6, %%mm6            \n\t"
    "psubw (%3), %%mm6            \n\t" // -bias[0]
    "mov $-128, %%"REG_a"        \n\t"
    ".balign 16                \n\t"
    "1:                    \n\t"
    "pxor %%mm1, %%mm1            \n\t" // 0
    "movq (%1, %%"REG_a"), %%mm0    \n\t" // block[i]
    "pcmpgtw %%mm0, %%mm1        \n\t" // block[i] <= 0 ? 0xFF : 0x00
    "pxor %%mm1, %%mm0            \n\t"
    "psubw %%mm1, %%mm0            \n\t" // ABS(block[i])
    "psubusw %%mm6, %%mm0        \n\t" // ABS(block[i]) + bias[0]
    "pmulhw %%mm5, %%mm0        \n\t" // (ABS(block[i])*qmat[0] -
    "por %%mm0, %%mm4            \n\t"
    "pxor %%mm1, %%mm0            \n\t"
    "psubw %%mm1, %%mm0            \n\t" // out=((ABS(block[i])*qmat[0]
- bias[0]*qmat[0])>>16)*sign(block[i])
    "movq %%mm0, (%5, %%"REG_a")    \n\t"
    "pcmpeqw %%mm7, %%mm0        \n\t" // out==0 ? 0xFF : 0x00
    "movq (%4, %%"REG_a"), %%mm1    \n\t"
    "movq %%mm7, (%1, %%"REG_a")    \n\t" // 0
    "pandn %%mm1, %%mm0            \n\t"
    PMAXW(%%mm0, %%mm3)
    "add $8, %%"REG_a"            \n\t"
    " js 1b                \n\t"
    "movq %%mm3, %%mm0            \n\t"
    "psrlq $32, %%mm3            \n\t"
    PMAXW(%%mm0, %%mm3)
    "movq %%mm3, %%mm0            \n\t"
    "psrlq $16, %%mm3            \n\t"
    PMAXW(%%mm0, %%mm3)
    "movd %%mm3, %%"REG_a"        \n\t"
    "movzb %%al, %%"REG_a"        \n\t" // last_non_zero_p1
    : "+a" (last_non_zero_p1)
    : "r" (block+64), "r" (qmat), "r" (bias),
      "r" (inv_zigzag_direct16+64), "r" (temp_block+64)


The compiler complains this, when using -fPIC:

i386/mpegvideo_mmx_template.c: In Funktion ?dct_quantize_MMX?:
i386/mpegvideo_mmx_template.c:93: error: can't find a register in class
`GENERAL _REGS' while reloading `asm'

The problem seems to be with the operands - gcc seems to run out of
registers (one more register is available in non-PIC-mode). What can be
done? Split the asm block? How?


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