gnu /
gcc /
c98aabc1427a4d2a25a2176c89dc709148a04707 AArch64: Add implementation for pow2 bitmask division.
This adds an implementation for the new optab for unsigned pow2 bitmask for
AArch64.
The implementation rewrites:
x = y / (2 ^ (sizeof (y)/2)-1
into e.g. (for bytes)
(x + ((x + 257) >> 8)) >> 8
where it's required that the additions be done in double the precision of x
such that we don't lose any bits during an overflow.
Essentially the sequence decomposes the division into doing two smaller
divisions, one for the top and bottom parts of the number and adding the results
back together.
To account for the fact that shift by 8 would be division by 256 we add 1 to
both parts of x such that when 255 we still get 1 as the answer.
Because the amount we shift are half the original datatype we can use the
halfing instructions the ISA provides to do the operation instead of using
actual shifts.
For AArch64 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:
movi v3.16b, 0x1
umull2 v1.8h, v0.16b, v2.16b
umull v0.8h, v0.8b, v2.8b
addhn v5.8b, v1.8h, v3.8h
addhn v4.8b, v0.8h, v3.8h
uaddw v1.8h, v1.8h, v5.8b
uaddw v0.8h, v0.8h, v4.8b
uzp2 v0.16b, v0.16b, v1.16b
instead of:
umull v2.8h, v1.8b, v5.8b
umull2 v1.8h, v1.16b, v5.16b
umull v0.4s, v2.4h, v3.4h
umull2 v2.4s, v2.8h, v3.8h
umull v4.4s, v1.4h, v3.4h
umull2 v1.4s, v1.8h, v3.8h
uzp2 v0.8h, v0.8h, v2.8h
uzp2 v1.8h, v4.8h, v1.8h
shrn v0.8b, v0.8h, 7
shrn2 v0.16b, v1.8h, 7
Which results in significantly faster code.
Thanks for Wilco for the concept.
gcc/ChangeLog:
* config/aarch64/aarch64-simd.md (@aarch64_bitmask_udiv<mode>3): New.
* config/aarch64/aarch64.cc (aarch64_vectorize_can_special_div_by_constant): New.
gcc/testsuite/ChangeLog:
* gcc.target/aarch64/div-by-bitmask.c: New test.
3 files changed
