[FFmpeg-devel] fate-rv20-1239 failure on power8, aliasing bug

Carl Eugen Hoyos ceffmpeg at gmail.com
Tue Nov 27 02:25:14 EET 2018


2018-11-26 23:35 GMT+01:00, Michael Niedermayer <michael at niedermayer.cc>:
> On Mon, Nov 26, 2018 at 12:45:26AM +0100, Carl Eugen Hoyos wrote:
>> 2018-11-25 16:17 GMT+01:00, Lauri Kasanen <cand at gmx.com>:
>> > Hi,
>> >
>> > The lone power8 fate failing test seems like an aliasing issue.
>> > I've isolated it into the attached standalone test case. Compiling it
>> > with
>> > gcc -std=c11 -maltivec -mabi=altivec -mvsx -O3 -fno-tree-vectorize
>> > -o test test.c
>> >
>> > reproduces on gcc 8.2.0, dropping the optimization level fixes it. This
>> > was one of the "adding a printf made it work" things too.
>> >
>> > -Wstrict-aliasing=1 complains about the "register int *idataptr =
>> > (int*)dataptr;" cast. If I put "typedef int __attribute__((may_alias))
>> > int_alias;" at the top and change the cast and type to int_alias, the
>> > results become correct.
>>
>> Thank you for the analysis!
>>
>> Patch attached, Carl Eugen
>
>>  jrevdct.c |   17 +++++++++--------
>>  1 file changed, 9 insertions(+), 8 deletions(-)
>> fef3d295836588ff8e619d4aa42247b4d2f25e93
>> 0001-lavc-jrevdct-Avoid-an-aliasing-violation.patch
>> From e5403b832f2bcd360128d9986b602484e576c931 Mon Sep 17 00:00:00 2001
>> From: Carl Eugen Hoyos <ceffmpeg at gmail.com>
>> Date: Mon, 26 Nov 2018 00:43:46 +0100
>> Subject: [PATCH] lavc/jrevdct: Avoid an aliasing violation.
>>
>> Fixes fate on different PowerPC systems with some compilers.
>>
>> Analyzed-by: Lauri Kasanen
>> ---
>>  libavcodec/jrevdct.c |   17 +++++++++--------
>>  1 file changed, 9 insertions(+), 8 deletions(-)
>>
>> diff --git a/libavcodec/jrevdct.c b/libavcodec/jrevdct.c
>> index 3b15a52..c749468 100644
>> --- a/libavcodec/jrevdct.c
>> +++ b/libavcodec/jrevdct.c
>> @@ -63,6 +63,7 @@
>>   */
>>
>>  #include "libavutil/common.h"
>> +#include "libavutil/intreadwrite.h"
>>
>>  #include "dct.h"
>>  #include "idctdsp.h"
>> @@ -234,7 +235,7 @@ void ff_j_rev_dct(DCTBLOCK data)
>>       * row DCT calculations can be simplified this way.
>>       */
>>
>> -    register int *idataptr = (int*)dataptr;
>> +    register uint8_t *idataptr = (uint8_t*)dataptr;
>>
>>      /* WARNING: we do the same permutation as MMX idct to simplify the
>>         video core */
>> @@ -254,10 +255,10 @@ void ff_j_rev_dct(DCTBLOCK data)
>>            int16_t dcval = (int16_t) (d0 * (1 << PASS1_BITS));
>>            register int v = (dcval & 0xffff) | ((dcval * (1 << 16)) &
>> 0xffff0000);
>>
>> -          idataptr[0] = v;
>> -          idataptr[1] = v;
>> -          idataptr[2] = v;
>> -          idataptr[3] = v;
>> +          AV_WN32(&idataptr[ 0], v);
>> +          AV_WN32(&idataptr[ 4], v);
>> +          AV_WN32(&idataptr[ 8], v);
>> +          AV_WN32(&idataptr[12], v);
>
> this probably should be AV_WN32A()

Tested and applied with AV_WN32A().

Thank you, Carl Eugen


More information about the ffmpeg-devel mailing list