e1414d1760
Summary: Instruments is a useful tool for finding performance issues in LLVM but it can be difficult to identify regions of interest on the timeline that we can use to filter the profiler or allocations instrument. Xcode 10 and the latest macOS/iOS/etc. added support for the os_signpost() API which allows us to annotate the timeline with information that's meaningful to LLVM. This patch causes timer start and end events to emit signposts. When used with -time-passes, this causes the passes to be annotated on the Instruments timeline. In addition to visually showing the duration of passes on the timeline, it also allows us to filter the profile and allocations instrument down to an individual pass allowing us to find the issues within that pass without being drowned out by the noise from other parts of the compiler. Using this in conjunction with the Time Profiler (in high frequency mode) and the Allocations instrument is how I found the SparseBitVector that should have been a BitVector and the DenseMap that could be replaced by a sorted vector a couple months ago. I added NamedRegionTimers to TableGen and used the resulting annotations to identify the slow portions of the Register Info Emitter. Some of these were placed according to educated guesses while others were placed according to hot functions from a previous profile. From there I filtered the profile to a slow portion and the aforementioned issues stood out in the profile. To use this feature enable LLVM_SUPPORT_XCODE_SIGNPOSTS in CMake and run the compiler under Instruments with -time-passes like so: instruments -t 'Time Profiler' bin/llc -time-passes -o - input.ll' Then open the resulting trace in Instruments. There was a talk at WWDC 2018 that explained the feature which can be found at https://developer.apple.com/videos/play/wwdc2018/405/ if you'd like to know more about it. Reviewers: bogner Reviewed By: bogner Subscribers: jdoerfert, mgorny, kristina, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D52954 llvm-svn: 354365 |
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README.txt
The LLVM Compiler Infrastructure ================================ This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you are writing a package for LLVM, see docs/Packaging.rst for our suggestions.