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Author	SHA1	Message	Date
Ben Avison	2e26847780	avcodec/vc1: Introduce fast path for unescaping bitstream buffer Includes a checkasm test. Signed-off-by: Ben Avison <bavison@riscosopen.org> Signed-off-by: Martin Storsjö <martin@martin.st>	3 years ago
Ben Avison	2698bfdc93	checkasm: Add vc1dsp inverse transform tests This test deliberately doesn't exercise the full range of inputs described in the committee draft VC-1 standard. It says: input coefficients in frequency domain, D, satisfy -2048 <= D < 2047 intermediate coefficients, E, satisfy -4096 <= E < 4095 fully inverse-transformed coefficients, R, satisfy -512 <= R < 511 For one thing, the inequalities look odd. Did they mean them to go the other way round? That would make more sense because the equations generally both add and subtract coefficients multiplied by constants, including powers of 2. Requiring the most-negative values to be valid extends the number of bits to represent the intermediate values just for the sake of that one case! For another thing, the extreme values don't look to occur in real streams - both in my experience and supported by the following comment in the AArch32 decoder: tNhalf is half of the value of tN (as described in vc1_inv_trans_8x8_c). This is done because sometimes files have input that causes tN + tM to overflow. To avoid this overflow, we compute tNhalf, then compute tNhalf + tM (which doesn't overflow), and then we use vhadd to compute (tNhalf + (tNhalf + tM)) >> 1 which does not overflow because it is one instruction. My AArch64 decoder goes further than this. It calculates tNhalf and tM then does an SRA (essentially a fused halve and add) to compute (tN + tM) >> 1 without ever having to hold (tNhalf + tM) in a 16-bit element without overflowing. It only encounters difficulties if either tNhalf or tM overflow in isolation. I haven't had sight of the final standard, so it's possible that these issues were dealt with during finalisation, which could explain the lack of usage of extreme inputs in real streams. Or a preponderance of decoders that only support 16-bit intermediate values in their inverse transforms might have caused encoders to steer clear of such cases. I have effectively followed this approach in the test, and limited the scale of the coefficients sufficient that both the existing AArch32 decoder and my new AArch64 decoder both pass. Signed-off-by: Ben Avison <bavison@riscosopen.org> Signed-off-by: Martin Storsjö <martin@martin.st>	3 years ago
Ben Avison	20cb43ea8b	checkasm: Add vc1dsp in-loop deblocking filter tests Note that the benchmarking results for these functions are highly dependent upon the input data. Therefore, each function is benchmarked twice, corresponding to the best and worst case complexity of the reference C implementation. The performance of a real stream decode will fall somewhere between these two extremes. Signed-off-by: Ben Avison <bavison@riscosopen.org> Signed-off-by: Martin Storsjö <martin@martin.st>	3 years ago

Author

SHA1

Message

Date

Ben Avison

2e26847780

avcodec/vc1: Introduce fast path for unescaping bitstream buffer

Includes a checkasm test.

Signed-off-by: Ben Avison <bavison@riscosopen.org>
Signed-off-by: Martin Storsjö <martin@martin.st>

3 years ago

Ben Avison

2698bfdc93

checkasm: Add vc1dsp inverse transform tests

This test deliberately doesn't exercise the full range of inputs described in
the committee draft VC-1 standard. It says:

input coefficients in frequency domain, D, satisfy   -2048 <= D < 2047
intermediate coefficients, E, satisfy                -4096 <= E < 4095
fully inverse-transformed coefficients, R, satisfy    -512 <= R <  511

For one thing, the inequalities look odd. Did they mean them to go the
other way round? That would make more sense because the equations generally
both add and subtract coefficients multiplied by constants, including powers
of 2. Requiring the most-negative values to be valid extends the number of
bits to represent the intermediate values just for the sake of that one case!

For another thing, the extreme values don't look to occur in real streams -
both in my experience and supported by the following comment in the AArch32
decoder:

    tNhalf is half of the value of tN (as described in vc1_inv_trans_8x8_c).
    This is done because sometimes files have input that causes tN + tM to
    overflow. To avoid this overflow, we compute tNhalf, then compute
    tNhalf + tM (which doesn't overflow), and then we use vhadd to compute
    (tNhalf + (tNhalf + tM)) >> 1 which does not overflow because it is
    one instruction.

My AArch64 decoder goes further than this. It calculates tNhalf and tM
then does an SRA (essentially a fused halve and add) to compute
(tN + tM) >> 1 without ever having to hold (tNhalf + tM) in a 16-bit element
without overflowing. It only encounters difficulties if either tNhalf or
tM overflow in isolation.

I haven't had sight of the final standard, so it's possible that these
issues were dealt with during finalisation, which could explain the lack
of usage of extreme inputs in real streams. Or a preponderance of decoders
that only support 16-bit intermediate values in their inverse transforms
might have caused encoders to steer clear of such cases.

I have effectively followed this approach in the test, and limited the
scale of the coefficients sufficient that both the existing AArch32 decoder
and my new AArch64 decoder both pass.

Signed-off-by: Ben Avison <bavison@riscosopen.org>
Signed-off-by: Martin Storsjö <martin@martin.st>

3 years ago

Ben Avison

20cb43ea8b

checkasm: Add vc1dsp in-loop deblocking filter tests

Note that the benchmarking results for these functions are highly dependent
upon the input data. Therefore, each function is benchmarked twice,
corresponding to the best and worst case complexity of the reference C
implementation. The performance of a real stream decode will fall somewhere
between these two extremes.

Signed-off-by: Ben Avison <bavison@riscosopen.org>
Signed-off-by: Martin Storsjö <martin@martin.st>

3 years ago

3 Commits (6ca16906439f689e4627b7cb4e6466bd45cc696f)