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Presently, CodeGenPrepare deletes all nearly empty (only phi and branch) basic blocks. This pass can delete loop preheaders which frequently creates critical edges. A preheader can be a convenient place to spill registers to the stack. If the entrance to a loop body is a critical edge, then spills may occur in the loop body rather than immediately before it. This patch protects loop preheaders from deletion in CodeGenPrepare even if they are nearly empty. Since the patch alters the CFG, it affects a large number of test cases. In most cases, the changes are merely cosmetic (basic blocks have different names or instruction orders change slightly). I am somewhat concerned about the test/CodeGen/Mips/brdelayslot.ll test case. If the loop preheader is not deleted, then the MIPS backend does not take advantage of a branch delay slot. Consequently, I would like some close review by a MIPS expert. The patch also partially subsumes D16893 from George Burgess IV. George correctly notes that CodeGenPrepare does not actually preserve the dominator tree. I think the dominator tree was usually not valid when CodeGenPrepare ran, but I am using LoopInfo to mark preheaders, so the dominator tree is now always valid before CodeGenPrepare. Author: Tom Jablin (tjablin) Reviewers: hfinkel george.burgess.iv vkalintiris dsanders kbarton cycheng http://reviews.llvm.org/D16984 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265397 91177308-0d34-0410-b5e6-96231b3b80d8 |
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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.