On Tue, 29 Mar 2022, Martin Storsjö wrote:
On Fri, 25 Mar 2022, Ben Avison wrote:
Disable ff_add_pixels_clamped_arm, which was found to fail the test. As
this
is normally only used for Arms prior to Armv6 (ARM11) it seems quite
unlikely
that anyone is still using this, so I haven't put in the effort to debug
it.
I had a look at this function, and I see that the overflow checks are using
tst r6, #0x100
to see whether the addition overflowed (either above or below). However, if
block[] was e.g. 0x200, it's possible to overflow without setting this bit at
all.
If it would be the case that the valid range of block[] values would be e.g.
[-255,255], then this kind of overflow checking would work though. (As there
exists assembly for armv6, then this function probably hasn't been used much
in modern times, so this doesn't say much about what values actually are used
here.)
Secondly, the clamping seems to be done with
movne r6, r5, lsr #24
However that should use asr, not lsr, I think, to get proper clamping in both
ends?
On second thought, no, lsr #24 should be correct here. But "tst r6,
#0x100" probably is the main issue, given the range of input values set by
the current test. No idea what the actual value range is, for the decoders
that use this function though.
// Martin
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