forked from OSchip/llvm-project
d66607bd8c
This is part of solving PR27344: https://llvm.org/bugs/show_bug.cgi?id=27344 CGP should undo the SimplifyCFG transform for the same reason that earlier patches have used this same mechanism: it's possible that passes between SimplifyCFG and CGP may be able to optimize the IR further with a select in place. For the TLI hook default, >99% taken or not taken is chosen as the default threshold for a highly predictable branch. Even the most limited HW branch predictors will be correct on this branch almost all the time, so even a massive mispredict penalty perf loss would be overcome by the win from all the times the branch was predicted correctly. As a follow-up, we could make the default target hook less conservative by using the SchedMachineModel's MispredictPenalty. Or we could just let targets override the default by implementing the hook with that and other target-specific options. Note that trying to statically determine mispredict rates for close-to-balanced profile weight data is generally impossible if the HW is sufficiently advanced. Ie, 50/50 taken/not-taken might still be 100% predictable. Finally, note that this patch as-is will not solve PR27344 because the current __builtin_unpredictable() branch weight default values are 4 and 64. A proposal to change that is in D19435. Differential Revision: http://reviews.llvm.org/D19488 llvm-svn: 267572 |
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README.txt
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.