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The retpoline mitigation for variant 2 of CVE-2017-5715 inhibits the branch predictor, and as a result it can lead to a measurable loss of performance. We can reduce the performance impact of retpolined virtual calls by replacing them with a special construct known as a branch funnel, which is an instruction sequence that implements virtual calls to a set of known targets using a binary tree of direct branches. This allows the processor to speculately execute valid implementations of the virtual function without allowing for speculative execution of of calls to arbitrary addresses. This patch extends the whole-program devirtualization pass to replace certain virtual calls with calls to branch funnels, which are represented using a new llvm.icall.jumptable intrinsic. It also extends the LowerTypeTests pass to recognize the new intrinsic, generate code for the branch funnels (x86_64 only for now) and lay out virtual tables as required for each branch funnel. The implementation supports full LTO as well as ThinLTO, and extends the ThinLTO summary format used for whole-program devirtualization to support branch funnels. For more details see RFC: http://lists.llvm.org/pipermail/llvm-dev/2018-January/120672.html Differential Revision: https://reviews.llvm.org/D42453 llvm-svn: 327163 |
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Low Level Virtual Machine (LLVM) ================================ 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.