[FFmpeg-devel] [PATCH 1/2] checkasm: aacpsdsp: Tolerate extra intermediate precision in stereo_interpolate

[Martin Storsjö]

On Tue, 10 Dec 2019, Martin Storsjö wrote:

> The stereo_interpolate functions add h_step to the values h
> BUF_SIZE times. Within the stereo_interpolate C functions, the
> values h (h0-h3, h00-h13) are declared as local float variables,
> but the compiler is free to keep them in a register with extra
> precision.
>
> If the accumulation is rounded to 32 bit float precision after
> each step, the less significant bits of h_step end up ignored
> and the sum can deviate, affecting the end result more than
> the currently set EPS.
>
> By clearing the log2(BUF_SIZE) lower bits of h_step, we make sure
> that the accumulation shouldn't differ significantly, regardless
> of any extra precision in the accmulating register/variable.
>
> This fixes the aacpsdsp checkasm test when built with clang for
> mingw/x86_32.
> ---
> tests/checkasm/aacpsdsp.c | 18 ++++++++++++++++++
> 1 file changed, 18 insertions(+)
>
> diff --git a/tests/checkasm/aacpsdsp.c b/tests/checkasm/aacpsdsp.c
> index ea68b39fa9..2ceef4341f 100644
> --- a/tests/checkasm/aacpsdsp.c
> +++ b/tests/checkasm/aacpsdsp.c
> @@ -17,6 +17,7 @@
>  */
> 
> #include "libavcodec/aacpsdsp.h"
> +#include "libavutil/intfloat.h"
> 
> #include "checkasm.h"
> 
> @@ -34,6 +35,16 @@
> 
> #define EPS 0.005
> 
> +static void clear_less_significant_bits(INTFLOAT *buf, int len, int bits)
> +{
> +    int i;
> +    for (i = 0; i < len; i++) {
> +        union av_intfloat32 u = { .f = buf[i] };
> +        u.i &= (0xffffffff << bits);
> +        buf[i] = u.f;
> +    }
> +}
> +
> static void test_add_squares(void)
> {
>     LOCAL_ALIGNED_16(INTFLOAT, dst0, [BUF_SIZE]);
> @@ -198,6 +209,13 @@ static void test_stereo_interpolate(PSDSPContext *psdsp)
>
>             randomize((INTFLOAT *)h, 2 * 4);
>             randomize((INTFLOAT *)h_step, 2 * 4);
> +            // Clear the least significant 14 bits of h_step, to avoid
> +            // divergence when accumulating h_step BUF_SIZE times into
> +            // a float variable which may or may not have extra intermediate
> +            // precision. Therefore clear roughly log2(BUF_SIZE) less
> +            // significant bits, to get the same result regardless of any
> +            // extra precision in the accumulator.
> +            clear_less_significant_bits((INTFLOAT *)h_step, 2 * 4, 14);
>
>             call_ref(l0, r0, h, h_step, BUF_SIZE);
>             call_new(l1, r1, h, h_step, BUF_SIZE);
> -- 
> 2.17.1

Ping

// Martin