Opened 7 years ago
Closed 7 years ago
#6848 closed defect (fixed)
checkasm float_dsp.vector_dmul sometimes fails on i386
| Reported by: | James Cowgill | Owned by: | |
|---|---|---|---|
| Priority: | normal | Component: | undetermined |
| Version: | git-master | Keywords: | |
| Cc: | Blocked By: | ||
| Blocking: | Reproduced by developer: | no | |
| Analyzed by developer: | no |
Description
Sometimes (about 1 in 10 times), the float_dsp.vector_dmul checkasm test fails on i386.
Example:
$ tests/checkasm/checkasm --test=float_dsp 2642491962 checkasm: using random seed 2642491962 SSE: - float_dsp.vector_fmul [OK] - float_dsp.vector_fmac [OK] - float_dsp.butterflies_float [OK] - float_dsp.scalarproduct_float [OK] 93: -53.395181798898 - -53.395181798898 = -7.1054273576e-15 SSE2: vector_dmul_scalar_sse2 (float_dsp.c:171) - float_dsp.vector_dmul [FAILED] - float_dsp.vector_dmac [OK]
The failures are reprodicible given the same random seed. Note you probably need a toolchain which uses the x87 FPU to reproduce this (eg Fedora's gcc will not work).
I think this happens because the SSE routines compute the multiplication to double precision, but the reference C routines compute the multiplcation to 80-bit precision. Sometimes the results are different.
I noticed this patch which I think is an attempt to solve this. While it does reduce the probability of failure, I can still get the test to fail when run > 1000 times.
https://patchwork.ffmpeg.org/patch/5743/
Downstream bug: https://bugs.debian.org/882075
Fixed by Michael in 38f966b2222db4bfeeaca4642a63049253536c46