Re: i386 clang optimisation problem with stack alignment

[Tijl Coosemans]

On Tue, 10 Sep 2013 18:16:01 +0200 Tijl Coosemans wrote:
> I've attached a small test program extracted from multimedia/gstreamer-ffmpeg
> (libavcodec/h264_cabac.c:ff_h264_init_cabac_states(H264Context *h)).
> 
> When you compile and run it like this on FreeBSD/i386, it results in a
> SIGBUS:
> 
> % cc -o paddd paddd.c -O3 -msse2 -fPIE -fomit-frame-pointer 
> % ./paddd
> Bus error
> 
> The reason is this instruction where %esp isn't 16-byte aligned:
> paddd   (%esp), %xmm7
> 
> Is this an upstream bug or is this because of local changes (to make the
> stack 4 byte aligned by default or something)?

Sigh, let me just inline the attachment:

#define NUM 16

signed char state[NUM];
signed char tab[NUM][2];

void
init_states(int slice) {
    int i;

    /* calculate pre-state */
    for( i= 0; i < NUM; i++ ) {
        int pre = 2*(((tab[i][0] * slice) >>4 ) + tab[i][1]) - 127;

        pre^= pre>>31;
        if(pre > 124)
            pre= 124 + (pre&1);

        state[i] =  pre;
    }
}

int
main(void) {
        init_states(10);
}

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