swiftlang/swift

[SR-1111] Non-optimized builds should avoid redundant thick_to_objc_metatype conversions

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#43,724 opened on Mar 30, 2016

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IRGencompilergood first issueimprovementnon-optimized only

Description
Previous ID SR-1111
Radar None
Original Reporter @belkadan
Type Improvement
Votes 0
Component/s Compiler
Labels Improvement, IRGen, NotOptimizedOnly, StarterBug
Assignee None
Priority Medium

md5: ee5fa37243b2596a51a6125675863696

Issue Description:

In this code:

import Foundation
func test() -> Bool {
  let a = RunLoop.current
  let b = RunLoop.main
  return a === b
}

We're smart enough to avoid emitting two references to the metadata for NSRunLoop, but not smart enough (in -Onone builds) to avoid emitting the conversion to an ObjC Class twice, which involves a branch. We should be able to reuse the value from the first thick_to_objc_metatype conversion.

Contributor guide

Tech stack
swift
Domain
performance
Issue type
feature
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.
half day
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.
clear
Prerequisites
Swift compiler internalsIRGenthick to objc metatype concept
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
The issue involves reusing the result of thick to objc metatype conversion in non optimized builds. The relevant code is likely in the IRGen component of the Swift compiler. A contributor should search for where thick to objc metatype is emitted and check if redundant conversions are avoided. Given the issue is old, verify if this improvement is already implemented or still relevant.
[SR-1111] Non-optimized builds should avoid redundant thick_to_objc_metatype conversions · swiftlang/swift#43724 | Good First Issue