swiftlang/swift

[SR-8960] Emit a symbol reference to ensure swiftrt.o is linked on Linux/Windows

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#51,465 opened on Oct 10, 2018

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Linuxbugcompilergood first issueruntime

Description
Previous ID SR-8960
Radar None
Original Reporter @jckarter
Type Bug
Votes 0
Component/s Compiler
Labels Bug, Linux, Runtime, StarterBug
Assignee None
Priority Medium

md5: 7f33519b3b36a320eb441265efc7bc17

Issue Description:

On Linux and Windows, Swift binaries rely on a small stub object file to inject a static constructor necessary to register with the Swift runtime on load. When Swift object files are incorporated with other language object files and build systems, it's easy to forget to explicitly link this object file, which can appear to work at first but leads to surprising behavior when the metadata for the binary cannot be found at runtime. We can have the linker help us out with this problem, by having swiftrt.o define a dummy absolute symbol with a conspicuous name, and having the Swift compiler emit a dummy reference to this symbol if the generated IR includes any data structures that require runtime registration, so that linking without including swiftrt.o fails.

It should be possible to do this in such a way that the resulting symbols get dead-stripped because they wouldn't in reality be used anywhere by the final linked image.

Contributor guide

Tech stack
swift
Domain
backend
Issue type
bug
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.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Swift compiler internalsLLVM IR basicslinker symbols
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate how the Swift compiler emits symbols for runtime registration. Focus on the IRGen component and the handling of swiftrt.o. Look at existing references to swiftrt.o in the codebase and how dummy symbols are defined. The goal is to emit a reference to a conspicuous symbol from swiftrt.o when IR includes runtime registration data. Consider dead stripping to avoid unused symbols. See linked discussion in the issue for more context.