Re: [FFmpeg-devel] [PATCH v2] avcodec/ppc/h264dsp: Fix unaligned stores

[Sean McGovern <gseanmcg@gmail.com>]

Andreas:

On Wed, Mar 13, 2024 at 7:31 AM Andreas Rheinhardt
<andreas.rheinhardt@outlook.com> wrote:
>
> Also fix an effective-type violation.
> Exposed by https://fate.ffmpeg.org/report.cgi?time=20240312011016&slot=ppc-linux-gcc-13.2-ubsan-altivec-qemu
>
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>  libavcodec/ppc/h264dsp.c | 35 +++++++++++++++++------------------
>  1 file changed, 17 insertions(+), 18 deletions(-)
>
> diff --git a/libavcodec/ppc/h264dsp.c b/libavcodec/ppc/h264dsp.c
> index c02733dda2..f50f2553a2 100644
> --- a/libavcodec/ppc/h264dsp.c
> +++ b/libavcodec/ppc/h264dsp.c
> @@ -401,30 +401,29 @@ static inline void write16x4(uint8_t *dst, int dst_stride,
>                               register vec_u8 r0, register vec_u8 r1,
>                               register vec_u8 r2, register vec_u8 r3) {
>      DECLARE_ALIGNED(16, unsigned char, result)[64];
> -    uint32_t *src_int = (uint32_t *)result, *dst_int = (uint32_t *)dst;
> -    int int_dst_stride = dst_stride/4;
> +    uint32_t *src_int = (uint32_t *)result;
>
>      vec_st(r0, 0, result);
>      vec_st(r1, 16, result);
>      vec_st(r2, 32, result);
>      vec_st(r3, 48, result);
>      /* FIXME: there has to be a better way!!!! */
> -    *dst_int = *src_int;
> -    *(dst_int+   int_dst_stride) = *(src_int + 1);
> -    *(dst_int+ 2*int_dst_stride) = *(src_int + 2);
> -    *(dst_int+ 3*int_dst_stride) = *(src_int + 3);
> -    *(dst_int+ 4*int_dst_stride) = *(src_int + 4);
> -    *(dst_int+ 5*int_dst_stride) = *(src_int + 5);
> -    *(dst_int+ 6*int_dst_stride) = *(src_int + 6);
> -    *(dst_int+ 7*int_dst_stride) = *(src_int + 7);
> -    *(dst_int+ 8*int_dst_stride) = *(src_int + 8);
> -    *(dst_int+ 9*int_dst_stride) = *(src_int + 9);
> -    *(dst_int+10*int_dst_stride) = *(src_int + 10);
> -    *(dst_int+11*int_dst_stride) = *(src_int + 11);
> -    *(dst_int+12*int_dst_stride) = *(src_int + 12);
> -    *(dst_int+13*int_dst_stride) = *(src_int + 13);
> -    *(dst_int+14*int_dst_stride) = *(src_int + 14);
> -    *(dst_int+15*int_dst_stride) = *(src_int + 15);
> +    AV_WN32(dst,                   AV_RN32A(src_int + 0));
> +    AV_WN32(dst +      dst_stride, AV_RN32A(src_int + 1));
> +    AV_WN32(dst +  2 * dst_stride, AV_RN32A(src_int + 2));
> +    AV_WN32(dst +  3 * dst_stride, AV_RN32A(src_int + 3));
> +    AV_WN32(dst +  4 * dst_stride, AV_RN32A(src_int + 4));
> +    AV_WN32(dst +  5 * dst_stride, AV_RN32A(src_int + 5));
> +    AV_WN32(dst +  6 * dst_stride, AV_RN32A(src_int + 6));
> +    AV_WN32(dst +  7 * dst_stride, AV_RN32A(src_int + 7));
> +    AV_WN32(dst +  8 * dst_stride, AV_RN32A(src_int + 8));
> +    AV_WN32(dst +  9 * dst_stride, AV_RN32A(src_int + 9));
> +    AV_WN32(dst + 10 * dst_stride, AV_RN32A(src_int + 10));
> +    AV_WN32(dst + 11 * dst_stride, AV_RN32A(src_int + 11));
> +    AV_WN32(dst + 12 * dst_stride, AV_RN32A(src_int + 12));
> +    AV_WN32(dst + 13 * dst_stride, AV_RN32A(src_int + 13));
> +    AV_WN32(dst + 14 * dst_stride, AV_RN32A(src_int + 14));
> +    AV_WN32(dst + 15 * dst_stride, AV_RN32A(src_int + 15));
>  }
>
>  /** @brief performs a 6x16 transpose of data in src, and stores it to dst
> --
> 2.40.1
>
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First of all, thank you for looking into this.

Second, do we feel that this change covers the FIXME immediately above
it that exclaims "there has to be a better way!!!!"?
If so, we can remove the comment.

I did not perform a full FATE run as it is expensive on my QEMU setup,
but I can confirm that this fixes the checkasm-h264dsp test under GCC
UBsan there as well as on a POWER7 (ppc64) and a POWER9 (ppc64le).

Thanks,
Sean McGovern
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