3 ms·

Sure, that would make sense, but in AVX-512, there is also a comparison variant for 2x256-bit inputs that outputs a 32-bit mask and another one for 2x128 inputs

Sure, that would make sense, but in AVX-512, there is also a comparison variant for 2x256-bit inputs that outputs a 32-bit mask and another one for 2x128 inputs and a 16-bit output mask. I would use those in different ways, occasionally mixing with the old AVX2 variant, depending on what I’m doing. I have tried to create a generalizable SIMD framework several times in the past, mostly in 2015-17, and still don’t have a good abstraction even between AVX2 and AVX-512.

dzaima 2y ago
You'd just produce the ≤AVX2-style result; clang will switch to using the mask-returning comparison instrs if the usage ends up being a mask (gcc doesn't do such fancy things, so I guess getting the last bits of speedup from this strategy depends on how much control over compiler selection you choose to have). And if you really care about doing different things for ≤AVX2 vs AVX-512 masks, you're definitely in the "squeeze out the last 1.4x" camp and not the "simple ~8x speedup" one. And, realistically, you won't care about this on every single comparison, so where you really want to explicitly use mask-returning comparisons you could just switch to using intrinsics directly (or a different abstraction over them) temporarily. As a side-note, mask-returning comparisons have 2x less throughput than homogeneous-bit-returning ones for xmm/ymm as far as uops.info data goes[1] (and use port 5!), so this strategy is kinda just what you want really. (another strategy can be to just return the arch-specific result type, still preferring the ≤AVX2 style (with, if desired, a separate set of comparison ops for the AVX-512-mask output), and making mask-consuming ops polymorphic over the two) [1]: https://uops.info/table.html?search=cmpeqb%20ymm%29&cb_lat=on&cb_tp=on&cb_ports=on&cb_SKX=on&cb_TGL=on&cb_ADLP=on&cb_ZEN4=on&cb_measurements=on&cb_avx2=on&cb_avx512=on https://uops.info/table.html?search=cmpeqb%20ymm%29&cb_lat=o...

dzaima 2y ago

You'd just produce the ≤AVX2-style result; clang will switch to using the mask-returning comparison instrs if the usage ends up being a mask (gcc doesn't do such fancy things, so I guess getting the last bits of speedup from this strategy depends on how much control over compiler selection you choose to have). And if you really care about doing different things for ≤AVX2 vs AVX-512 masks, you're definitely in the "squeeze out the last 1.4x" camp and not the "simple ~8x speedup" one. And, realistically, you won't care about this on every single comparison, so where you really want to explicitly use mask-returning comparisons you could just switch to using intrinsics directly (or a different abstraction over them) temporarily. As a side-note, mask-returning comparisons have 2x less throughput than homogeneous-bit-returning ones for xmm/ymm as far as uops.info data goes[1] (and use port 5!), so this strategy is kinda just what you want really. (another strategy can be to just return the arch-specific result type, still preferring the ≤AVX2 style (with, if desired, a separate set of comparison ops for the AVX-512-mask output), and making mask-consuming ops polymorphic over the two) [1]: https://uops.info/table.html?search=cmpeqb%20ymm%29&cb_lat=on&cb_tp=on&cb_ports=on&cb_SKX=on&cb_TGL=on&cb_ADLP=on&cb_ZEN4=on&cb_measurements=on&cb_avx2=on&cb_avx512=on https://uops.info/table.html?search=cmpeqb%20ymm%29&cb_lat=o...