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OTorg
New Contributor II
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Optimization bug which can lead to access violation

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Hello!

I compiled the x264 project with Intel Compiler 15.0, windows, x86, and found the buggy-generated code at O2/O3 optimization level.

static float predict_row_size( x264_t *h, int y, float qscale )
{
    x264_ratecontrol_t *rc = h->rc;
    float pred_s = predict_size( &rc->row_pred[0], qscale, h->fdec->i_row_satd );
    if( h->sh.i_type == SLICE_TYPE_I || qscale >= h->fref[0][0]->f_row_qscale )
    {
        if( h->sh.i_type == SLICE_TYPE_P
            && h->fref[0][0]->i_type == h->fdec->i_type
            && h->fref[0][0]->f_row_qscale > 0
            && h->fref[0][0]->i_row_satd > 0
            && (abs(h->fref[0][0]->i_row_satd - h->fdec->i_row_satd) < h->fdec->i_row_satd/2))
        {
            float pred_t = h->fref[0][0]->i_row_bits * h->fdec->i_row_satd / h->fref[0][0]->i_row_satd
                         * h->fref[0][0]->f_row_qscale / qscale;
            return (pred_s + pred_t) * 0.5f;
        }
        return pred_s;
    }
    else
    {
        float pred_intra = predict_size( &rc->row_pred[1], qscale, h->fdec->i_row_satds[0][0] );
        return pred_intra + pred_s;
    }
}

Inspecting the generated asm, you can see that "h->fref[0][0]->i_type" value is read from memory (for the purpose to store it on stack) BEFORE executing "h->sh.i_type == SLICE_TYPE_I" condition. But "h->fref[0][0]" is NULL when "h->sh.i_type == SLICE_TYPE_I", which leads to access violation.

I can publish more details about build parameters and command line for resulting executable run with crash, if need.

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Accepted Solutions
44 Views

FYI. This issue is fixed in v15.0 update 2 (w_ccompxe_2015.2.179.exe). I've verified the fix and it works now.

Thanks,

Shenghong

View solution in original post

6 Replies
44 Views

Hi dj_alek,

yes, do publish more details (download link/version of x264, how to build, how to run the executable which will crash, version of compiler etc.) I will have a try to see whether I can reproduce the issue and submit to developer to fix. Thank you very much.

Thanks,

Shenghong

OTorg
New Contributor II
44 Views

steps to reproduce the problem:

1. we need machine with microsoft visual studio 2010 or 2013 (c/c++) + intel compiler 14 or 15

2. files to download:
2.1. x264 source code: https://drive.google.com/file/d/0B8SCkOT4os4HMmF5eXRwTGlFbVE/view?usp=sharing
2.2. test requisites: https://drive.google.com/file/d/0B8SCkOT4os4Hc04zWmNyd1p2VWM/view?usp=sharing
2.3. build tools (mingw's msys + yasm): https://drive.google.com/file/d/0B8SCkOT4os4HNm9vNDM2cGhHX0E/view?usp=sharing

3. unpack downloaded archives into e.g C:\x264src, C:\test, C:\mgw_msys

4. run icl command line environment for ia32, e.g: "*\Start Menu\*\Intel Parallel Studio XE 2013\Command prompt\Parallel Studio XE with Intel Compiler XE v14.0 Update 3\IA-32 Visual Studio 2010 mode.lnk"

5. inside that cmdline:
5.1. run "C:\mgw_msys\msys.bat"
5.2. go to the source folder: "cd C:/x264src"
5.3. run "CC=icl configure --enable-debug --extra-cflags=-Zp8 --disable-gpl --enable-win32thread --disable-opencl --disable-avs --disable-swscale --disable-lavf --disable-ffms --disable-gpac --disable-lsmash"
5.4. run "make"
5.5. exit

6. copy builded x264.exe from C:\x264src to C:\test

7. check x264.exe works: run "C:\test\_ok.bat", which should make result.mkv, which can be played with e.g media player classic

8. and now start x264.exe with parameters that utilize buggy-compiled code: "C:\test\_av.bat", which leads to access violation

44 Views

Hi dj_alek,

Thank you for providing these details, I have been able to reproduce the issue based on the steps (Intel Compiler v15). The issue also exists with O1.

I will report it to dev team to fix.

Thanks,

Shenghong

44 Views

I have submitted the issue to developer to fix, the issue happens when compiler is trying to vectorize the loop.

By the way, you may workaround it by adding "#pragma optimization_level 0" before function predict_row_size to disable the optimization for this function only as a workaround.

Thanks,

Shenghong

OTorg
New Contributor II
44 Views

Thank you.

I have already found and use workaround: add "__declspec(noinline)" before "static float predict_row_size( x264_t *h, int y, float qscale )"

45 Views

FYI. This issue is fixed in v15.0 update 2 (w_ccompxe_2015.2.179.exe). I've verified the fix and it works now.

Thanks,

Shenghong

View solution in original post