golang/go

runtime/race: simplify meta shadow mapping

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#24,133 opened on Feb 26, 2018

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NeedsFixRaceDetectorcompiler/runtimehelp wanted

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Description

Currently, the race detector runtime for Go assumes that the heap is contiguous and grows up. In particular, MapShadow always maps the meta shadow contiguously. According to @dvyukov's comment, this was done to deal with the fact that the Go heap grew irregularly and the meta shadow mapping has large alignment constraints.

As of 2b415549, the Go heap is no longer contiguous and doesn't necessarily grow up. However, it also now grows in large, aligned blocks (as of 78846472, 4MB on Windows, 64MB elsewhere). This easily satisfies the alignment constraints on the meta shadow. Hence, the meta shadow mapping code can be simplified to take advantage of this while at the same time adding support for Go's new sparse heap layout.

Contributor guide

Research direction
Review the recent changes to Go heap layout (commits 2b415549 and 78846472) that made the heap sparse and aligned. Then examine the MetaShadow mapping code in compiler rt/tsan/rtl/tsan rtl.cc. Understand the alignment constraints and simplify the mapping to use the new layout, removing assumptions of contiguous heap growing up.
Tech stack
go
Domain
compilers
Issue type
Refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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
GoGitruntime internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45