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audioconvert: avoid even more precision loss in F32 to S32 conversion

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This is somewhat similar to the S32->F32 conversion improvements,
but here things a bit more tricky...

The main consideration is that the limits to which we clamp
must be valid 32-bit signed integers, but not all such integers
are exactly losslessly representable in `float32_t`.

For example it we'd clamp to `2147483647`,
that is actually a `2147483648.0f`,
and `2147483648` is not a valid 32-bit signed integer,
so the post-clamp conversion would basically be UB.
We don't have this problem for negative bound, though.

But as we know, any 25-bit signed integer is losslessly
round-trippable through float32_t, and since multiplying by 2
only changes the float's exponent, we can clamp to `2147483520`!
The algorithm of selection of the pre-clamping scale is unaffected.

This additionally avoids right-shift, and thus is even faster.

As `test_lossless_s32_lossless_subset` shows,
if the integer is in the form of s25+shift,
the maximal absolute error is finally zero.

Without going through `float`->`double`->`int`,
i'm not sure if the `float`->`int` conversion
can be improved further.
This commit is contained in:
Roman Lebedev 2024-06-25 19:20:42 +03:00
parent f4c89b1b40
commit 7c40cafa7c
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4 changed files with 50 additions and 70 deletions
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10
spa/plugins/audioconvert/fmt-ops.h
View file

@ -121,10 +121,14 @@
#define S32_MIN -2147483648
#define S32_MAX 2147483647
#define S32_SCALE 2147483648.0f
#define S32_TO_F32(v) ITOF(int32_t, v, S32_SCALE, 0.0f)
#define S32_SCALE_I2F 2147483648.0f
#define S32_TO_F32(v) ITOF(int32_t, v, S32_SCALE_I2F, 0.0f)
#define S32S_TO_F32(v) S32_TO_F32(bswap_32(v))
#define F32_TO_S32_D(v,d) S25_32_TO_S32(F32_TO_S25_32_D(v,d))
#define S32_MIN_F2I ((int32_t)(((uint32_t)(S25_MIN)) << 7))
#define S32_MAX_F2I ((S25_MAX) << 7)
#define S32_SCALE_F2I (-((float)(S32_MIN_F2I)))
#define F32_TO_S32_D(v,d) FTOI(int32_t, v, S32_SCALE_F2I, 0.0f, d, S32_MIN_F2I, S32_MAX_F2I)
#define F32_TO_S32(v) F32_TO_S32_D(v, 0.0f)
#define F32_TO_S32S(v) bswap_32(F32_TO_S32(v))
#define F32_TO_S32S_D(v,d) bswap_32(F32_TO_S32_D(v,d))