[ObjC] Support emission of selector and class stubs calls instead of objc_msgSend.#12570
Open
ahatanaka wants to merge 2 commits intostable/21.xfrom
Open
[ObjC] Support emission of selector and class stubs calls instead of objc_msgSend.#12570ahatanaka wants to merge 2 commits intostable/21.xfrom
ahatanaka wants to merge 2 commits intostable/21.xfrom
Conversation
…nd. (llvm#183922) This optimizes objc_msgSend calls by emitting "selector stubs" instead. Usually, the linker redirects calls to external symbols to a symbol stub it generates, which loads the target function's address from the GOT and branches to it: <symbol stub for _func:> adrp x16, _func@GOTPAGE ldr x16, [x16, _func@GOTPAGEOFF] br x16 with msgSend selector stubs, we extend that to compute the selector as well: <selector stub for "foo":> adrp x1, <selector ref for "foo">@page ldr x1, [x1, <selector ref for "foo">@PAGEOFF] adrp x16, _objc_msgSend@GOTPAGE ldr x16, [x16, _objc_msgSend@GOTPAGEOFF] br x16 This lets us avoid loading the selector in the compiler, hopefully leading to codesize reductions. We're considering two kinds of stubs, the combined one above with the objc_msgSend dispatch included, or a shorter one that forwards to the existing stub for objc_msgSend. That's a decision to be made in the linker. Concretely, instead of something like: %sel = load i8*, i8** @<selector ref for "foo"> call ... @objc_msgSend(i8* self, i8* sel) we emit a call to a newly-declared external "function": call ... @"objc_msgSend$foo"(i8* self, i8* undef) where "objc_msgSend$foo" is treated as any other external symbol reference throughout the compiler, and, at link-time, is recognized by the linker as a selector stub. There are other ways to implement this. An obvious one is to combine away the objc_msgSend(self, load(sel)) pattern at the IR level. Both seem feasible, but the IRGen approach might save us a tiny bit of compile-time, with the assumption that loads need more mid-level analysis than boring external functions. This optimization requires linker version 811.2 or newer for arm64, arm64e, and arm64_32. rdar://84437635
Instead of translating class messages to `objc_msgSend` calls, clang now emits calls to stub functions that are synthesized by the linker. Each stub loads the class reference and the selector name and forwards them to `objc_msgSend`. The stub function is named using the following format: `objc_msgSendClass$selName$_OBJC_CLASS_$_className` Note that the optimization is disabled in the following cases: - When the class name is unknown at compile time (e.g, `[id classMethod]`). - The selector name contains a `$`, which serves as the delimiter in stub function names. - The class is annotated with either `objc_class_stub` or `objc_runtime_visible`. In such cases, clang reverts to the existing message send optimization, where each stub loads only the selector name. Also add class objects to llvm.used when class msgSend optimization is enabled. This is needed as the linker currently cannot generate the stub, which references the class object, if LTO internalizes the global variable for the class object. rdar://147604613 --------- Co-authored-by: Ahmed Bougacha <ahmed@bougacha.org>
Author
|
@swift-ci test |
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
No description provided.