Discussion:
What's purpose of "gather" instructions?
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Branimir Maksimovic
2021-05-27 10:37:25 UTC
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I tried with them recenlty and they are slow, slow,
slower then manualy loading ;)
I mean like "loop" instruction, uselless ;)
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Terje Mathisen
2021-05-27 14:23:59 UTC
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Post by Branimir Maksimovic
I tried with them recenlty and they are slow, slow,
slower then manualy loading ;)
I mean like "loop" instruction, uselless ;)
Gather is supposed to run at minimum one word per cycle, but preferably
all loads that come from the same cache line should happen in a single
cycle, so that looking up stuff in a compact structure should be
reasonably fast, and much faster than scalar loads.

The first Larrabee CPU had gather implemented in an external chip, so it
was effectively a coprocessor. The idea was that you would setup a bunch
of these as part of a big processing loop, then stream the results through.

I.e. typical GPU optimizing for bandwidth, not latency.

Terje
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Anton Ertl
2021-05-27 14:56:51 UTC
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Post by Branimir Maksimovic
I tried with them recenlty and they are slow, slow,
slower then manualy loading ;)
I mean like "loop" instruction, uselless ;)
Possible explanations:

1) An instruction set designer thought that this could be implemented
better than by using scalar loads, but

a) the hardware designers did not get around to it.
b) the hardware designers tried, but the result was buggy, and was
disabled in delivered hardware.

Still, there is a slight benefit to having these instructions: If
there ever is a useful hardware implementation, software people can
use it in the knowledge that their code will at least run on a
variety of hardware (some may have a switch between using gather
instructions and scalar code, but not everyone can afford
development time for all CPU variations).

2) The instruction already worked better than the scalar code in the
Xeon Phi (I dimly remember reading something like that, although
looking at the cycle numbers I found the claim questionable), and
was added to other CPUs to support software that uses the
instruction. The problem with this theory is that Xeon Phi
supports (a variant of) AVX-512, but the Haswell and Skylake
(client) support only AVX2.

- anton
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