On Monday, 8 December 2014 at 17:05:09 UTC, John Colvin wrote:
On Monday, 8 December 2014 at 16:32:50 UTC, Martin Nowak wrote:
I want to do bounds checking of 2 (4 on avx) ulongs (64-bit)
at a time.
ulong2 vval = [v0, v1];
ulong2 vlow = [low, low];
ulong2 vhigh = [high, high];
int res = PMOVMSKB(vval >= vlow & vval < vhigh);
I figured out sort of a solution, but it seems way too
complicated, because there is only signed comparison.
Usually (scalar) I'd use this, which makes use of unsigned
wrap to safe one conditional
immutable size = cast(ulong)(vhigh - vlow);
if (cast(ulong)(v0 - vlow) < size) {}
if (cast(ulong)(v1 - vlow) < size) {}
over
if (v0 >= vlow && v0 < vhigh) {}
Maybe this can be used on SIMD too (saturated sub or so)?
-Martin
Well gcc gives me:
typedef unsigned long ulong4 __attribute__ ((vector_size (32)));
ulong4 foo(ulong4 a, ulong4 l, ulong4 h)
{
return (a >= l) & (a < h);
}
foo(unsigned long __vector, unsigned long __vector, unsigned
long __vector):
vmovdqa .LC0(%rip), %ymm3
vpsubq %ymm3, %ymm0, %ymm0
vpsubq %ymm3, %ymm2, %ymm2
vpsubq %ymm3, %ymm1, %ymm1
vpcmpgtq %ymm0, %ymm2, %ymm2
vpcmpgtq %ymm0, %ymm1, %ymm1
vpandn %ymm2, %ymm1, %ymm0
ret
.LC0:
.quad -9223372036854775808
.quad -9223372036854775808
.quad -9223372036854775808
.quad -9223372036854775808
To conceptually get what it's doing here, the trick is that it's
offsetting the values so as to simulate unsigned comparisons
using signed instructions.
