On Fri, 18 Jul 2025 at 14:47, Niklas Haas <ffm...@haasn.xyz> wrote:
>
> On Fri, 18 Jul 2025 14:38:04 +0200 Kacper Michajlow <kaspe...@gmail.com>
wrote:
> > > +static inline int ff_detect_range_c(const uint8_t *data, ptrdiff_t
stride,
> > > + ptrdiff_t width, ptrdiff_t
height,
> > > + int mpeg_min, int mpeg_max)
> > > +{
> > > + while (height--) {
> > > + for (int x = 0; x < width; x++) {
> > > + const uint8_t val = data[x];
> > > + if (val < mpeg_min || val > mpeg_max)
> > > + return 1;
> > > + }
> > > + data += stride;
> > > + }
> > > +
> > > + return 0;
> > > +}
> >
> > You could process width as a whole to allow better vectorization.
> > Assuming you don't process 10000x1 images, it will be faster on average.
>
> That's what I had in v1 of my patch, but it is significantly (50%) slower
> on GCC, which prefers the version I have written above.
>
> There is the not insignificant point that this C routine is also being
used
> to handle remaining elements that don't fit into a multiple of the SIMD
> kernel, for which the scalar code is actually preferred.
Interesting. It's my fault, I didn't check. GCC really doesn't like
bool/int there.
If that function is important you could try:
{
+ uint8_t min = mpeg_min, max = mpeg_max;
while (height--) {
+ uint8_t out_of_range = 0;
for (int x = 0; x < width; x++) {
const uint8_t val = data[x];
- if (val < mpeg_min || val > mpeg_max)
- return 1;
+ out_of_range |= val < min || val > max;
}
+ if (out_of_range)
+ return 1;
data += stride;
}
Side note, if you change function prototype to `uint8_t mpeg_min, uint8_t
mpeg_max` directly,
clang goes down to 267.6 ( 1.00x). Unless it's because of UB, lol.
So, gcc scalar version is a bit slower in this case, but I think there is
value in it,
because it scales very nicely with -ftree-vectorize, even with sse2 target.
Hopefully we will enable -ftree-vectorize by default soon, there is pending
patch for that.
before (gcc -fno-tree-vectorize)
detect_range_8_c: 5537.4 ( 1.00x)
detect_range_8_avx2: 149.7 (36.98x)
detect_range_8_avx512: 111.2 (49.80x)
after (gcc -fno-tree-vectorize)
detect_range_8_c: 7709.0 ( 1.00x)
detect_range_8_avx2: 137.6 (56.02x)
detect_range_8_avx512: 104.2 (73.97x)
after (gcc -ftree-vectorize --march=generic)
detect_range_8_c: 657.0 ( 1.00x)
detect_range_8_avx2: 161.7 ( 4.06x)
detect_range_8_avx512: 116.5 ( 5.64x)
after (gcc -ftree-vectorize --march=znver4)
detect_range_8_c: 285.6 ( 1.00x)
detect_range_8_avx2: 256.0 ( 1.12x)
detect_range_8_avx512: 107.6 ( 2.65x)
after (clang --march=generic)
detect_range_8_c: 1769.0 ( 1.00x)
detect_range_8_avx2: 231.8 ( 7.63x)
detect_range_8_avx512: 96.6 (18.32x)
after (clang --march=znver4)
detect_range_8_c: 952.9 ( 1.00x)
detect_range_8_avx2: 137.7 ( 6.92x)
detect_range_8_avx512: 95.9 ( 9.94x)
- Kacper
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