swiftlang/swift

[SR-1103] Redundant metadata accesses when invoking ObjC protocol methods

Open

#43,716 opened on Mar 29, 2016

View on GitHub
 (1 comment) (0 reactions) (0 assignees)Swift (69,989 stars) (10,719 forks)batch import
IRGenbugcompilergood first issue

Description
Previous ID SR-1103
Radar None
Original Reporter @jckarter
Type Bug
Votes 0
Component/s Compiler
Labels Bug, IRGen, StarterBug
Assignee None
Priority Medium

md5: 970c1d7db6290aee45b5ecc9fd1e9432

Issue Description:

Protocol methods are modeled as functions generic on Self, so IRGen tries to get the type metadata to satisfy the formal type parameter, even though it's not used for an ObjC or C protocol method:

define void @invoke_protocol_methods(%objc_object*) #​0 {
entry:
  %.Type = call %swift.type* @swift_getObjectType(%objc_object* %0) #​2
  %1 = bitcast %objc_object* %0 to i8*
  call void @mutateSomeState(i8* %1)
  ret void
}

The call is marked readnone so gets optimized away, but we shouldn't emit it at all for the sake of unoptimized code.

Contributor guide

Tech stack
swift
Domain
toolingperformance
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.
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 internalsIRGen
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the IRGen code for protocol method dispatch. The redundant call to swift getObjectType is emitted but unused. Look at the code generation in lib/IRGen/GenProtocol.cpp or similar to conditionally skip metadata retrieval for ObjC protocols. Review the existing logic for Self type metadata in protocol method invocation.
[SR-1103] Redundant metadata accesses when invoking ObjC protocol methods · swiftlang/swift#43716 | Good First Issue