tree 9584f1731112b2a6a175c0a1799329a2a6474ed0
parent c98aabc1427a4d2a25a2176c89dc709148a04707
author Tamar Christina <tamar.christina@arm.com> 1668441222 +0000
committer Tamar Christina <tamar.christina@arm.com> 1668447693 +0000

AArch64: Add SVE2 implementation for pow2 bitmask division

In plenty of image and video processing code it's common to modify pixel values
by a widening operation and then scale them back into range by dividing by 255.

This patch adds an named function to allow us to emit an optimized sequence
when doing an unsigned division that is equivalent to:

   x = y / (2 ^ (bitsize (y)/2)-1)

For SVE2 this means we generate for:

void draw_bitmap1(uint8_t* restrict pixel, uint8_t level, int n)
{
  for (int i = 0; i < (n & -16); i+=1)
    pixel[i] = (pixel[i] * level) / 0xff;
}

the following:

        mov     z3.b, #1
.L3:
        ld1b    z0.h, p0/z, [x0, x3]
        mul     z0.h, p1/m, z0.h, z2.h
        addhnb  z1.b, z0.h, z3.h
        addhnb  z0.b, z0.h, z1.h
        st1b    z0.h, p0, [x0, x3]
        inch    x3
        whilelo p0.h, w3, w2
        b.any   .L3

instead of:

.L3:
        ld1b    z0.h, p1/z, [x0, x3]
        mul     z0.h, p0/m, z0.h, z1.h
        umulh   z0.h, p0/m, z0.h, z2.h
        lsr     z0.h, z0.h, #7
        st1b    z0.h, p1, [x0, x3]
        inch    x3
        whilelo p1.h, w3, w2
        b.any   .L3

Which results in significantly faster code.

gcc/ChangeLog:

	* config/aarch64/aarch64-sve2.md (@aarch64_bitmask_udiv<mode>3): New.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/sve2/div-by-bitmask_1.c: New test.