golang/go

strings/bytes: LastIndexByte is significantly slower than IndexByte

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#36,891 opened on Jan 30, 2020

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Description

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

yes.

What operating system and processor architecture are you using (go env)?

Both Linux/MacOS

What did you do?

I was using multipart.NewReader() to process multi-part responses from Cloud REST API. It turned out that ~1/3 of profile is spent in mime/multipart/multipart.go :: scanUntilBoundary() -> bytes.LastIndexByte().

After looking into it, it is no wonder as bytes.LastIndexByte() is not using any optimisations and compiled into simple loop iterating over bytes, no REP SCASB instruction is used on Intel (nor SSE).

What did you expect to see?

bytes.LastIndexByte() to use SSE or at least REP SCASB optimised code.

What did you see instead?

simple byte to byte loop in asm code.

Contributor guide

Research direction
Investigate the current implementation of bytes.LastIndexByte and compare with bytes.IndexByte. Trace the assembly code to understand why LastIndexByte is slower. Look into using SSE or REP SCASB instructions to optimize it.
Tech stack
go
Domain
backend
Issue type
Performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GoAssembly
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30