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math/big: optimize amd64 asm shlVU and shrVU for shift==0 case #31097

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josharian opened this issue Mar 28, 2019 · 3 comments
Open

math/big: optimize amd64 asm shlVU and shrVU for shift==0 case #31097

josharian opened this issue Mar 28, 2019 · 3 comments
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help wanted NeedsFix The path to resolution is known, but the work has not been done. Performance
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@josharian
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When shift == 0, shlVU and shrVU reduce to a memcopy. When z.ptr == x.ptr, it further reduces to a no-op. The pure Go implementation has these optimizations, as of https://go-review.googlesource.com/c/go/+/164967. The arm64 implementation has one of them (see #31084 (comment)). We should add both to the amd64 implementation.

cc @griesemer
@josharian josharian added Performance help wanted NeedsFix The path to resolution is known, but the work has not been done. labels Mar 28, 2019
@josharian josharian added this to the Go1.13 milestone Mar 28, 2019
@josharian josharian self-assigned this Mar 29, 2019
@nsajko nsajko mentioned this issue Mar 31, 2019
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Change https://golang.org/cl/170257 mentions this issue: math/big: optimize amd64 asm shlVU and shrVU for shift==0 case

josharian added a commit to josharian/go that referenced this issue May 7, 2019
@josharian
math/big: optimize amd64 shlVU for shift 0 case
15a4a83 
DO NOT MAIL

TODO: shrVU too
TODO: benchmarks
TODO: fuzz for confidence
TODO: better commit message

When shift == 0, shlVU and shrVU reduce to a memcopy. When z.ptr == x.ptr, it further reduces to a no-op. The pure Go implementation has these optimizations, as of https://go-review.googlesource.com/c/go/+/164967. The arm64 implementation has one of them (see golang#31084 (comment)). We should add both to the amd64 implementation.

cc @griesemer

Fixes golang#31097

Change-Id: I3979d7c82a63e1840c8191636a8947e8f440af3b
@andybons andybons modified the milestones: Go1.13, Go1.14 Jul 8, 2019
@rsc rsc modified the milestones: Go1.14, Backlog Oct 9, 2019
@nightlyone
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Can this be done in the wrappers/callers instead so the per-arch assembler as well as the generic can just assume that this optimization has been applied?

This also allows SSA to see where these conditions might be constant either now or in the future.

@josharian
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Good question. As of this moment there aren’t any pure go wrappers for these functions—they all go straight to the assembly implementations. Now that we have mid-stack inlining, it might make sense to change that, and do optimizations like this in the wrappers, so they can skip the call entirely. Want to experiment and send a CL for 1.16 if appropriate?

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math/big: optimize amd64 asm shlVU and shrVU for shift==0 case nsajko/go-1
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