The input complex factors are constant for each iterations. This
substitudes 4 loads, 2 additions and 2 subtractions per iteration of
the inner-loop with another 4 loads. Thus effectively 4 arithmetic
operations per iteration of the inner loop are avoided, i.e. 24
operations per iteration of the outer loop, or 24 * (n - 1) operations
in total.
If the inner loop is not unrolled by the compiler, this also might
also save some pointer arithmetic as most instruction sets do not
have addressing modes with negated register offsets (12 - j). Unless
the compiler is optimising for code size, this is unlikely though.
This is more spec-compliant because it does not rely
on dead-code elimination by the compiler. Especially
MSVC has problems with this, as can be seen in
https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/296373.html
or
https://ffmpeg.org/pipermail/ffmpeg-devel/2022-May/297022.html
This commit does not eliminate every instance where we rely
on dead code elimination: It only tackles branching to
the initialization of arch-specific dsp code, not e.g. all
uses of CONFIG_ and HAVE_ checks. But maybe it is already
enough to compile FFmpeg with MSVC with whole-programm-optimizations
enabled (if one does not disable too many components).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Fixes: signed integer overflow: -1539565182 + -798086761 cannot be represented in type 'int'
Fixes: 14807/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-564925382682214
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Fixes: signed integer overflow: -1813244069 + -1407981383 cannot be represented in type 'int'
Fixes: 8823/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-5643295618236416
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Fixes: signed integer overflow: 1073741842 + 1784008138 cannot be represented in type 'int'
Fixes: 6792/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AAC_FIXED_fuzzer-5677589835284480
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Fixes: runtime error: signed integer overflow: 1939661764 - -454942263 cannot be represented in type 'int'
Fixes: 3191/clusterfuzz-testcase-minimized-5688798451073024
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
Fixes runtime error: signed integer overflow: 1997494407 + 613252359 cannot be represented in type 'int'
Fixes: 2014/clusterfuzz-testcase-minimized-5186337030275072
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
