From d4d40670e8b9ff752362d157806e41028d523007 Mon Sep 17 00:00:00 2001 From: Bob Wilson Date: Tue, 24 Nov 2009 23:35:49 +0000 Subject: Refactor target hook for tail duplication as requested by Chris. Make tail duplication of indirect branches much more aggressive (for targets that indicate that it is profitable), based on further experience with this transformation. I compiled 3 large applications with and without this more aggressive tail duplication and measured minimal changes in code size. ("size" on Darwin seems to round the text size up to the nearest page boundary, so I can only say that any code size increase was less than one 4k page.) Radar 7421267. llvm-svn: 89814 --- llvm/lib/CodeGen/BranchFolding.cpp | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) (limited to 'llvm/lib/CodeGen/BranchFolding.cpp') diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp index f807e8fa261..0fd3c23085d 100644 --- a/llvm/lib/CodeGen/BranchFolding.cpp +++ b/llvm/lib/CodeGen/BranchFolding.cpp @@ -1043,9 +1043,18 @@ bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB, // of one less than the tail-merge threshold. When optimizing for size, // duplicate only one, because one branch instruction can be eliminated to // compensate for the duplication. - unsigned MaxDuplicateCount = - MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ? - 1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1); + unsigned MaxDuplicateCount; + if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize)) + MaxDuplicateCount = 1; + else if (TII->isProfitableToDuplicateIndirectBranch() && + !TailBB->empty() && TailBB->back().getDesc().isIndirectBranch()) + // If the target has hardware branch prediction that can handle indirect + // branches, duplicating them can often make them predictable when there + // are common paths through the code. The limit needs to be high enough + // to allow undoing the effects of tail merging. + MaxDuplicateCount = 20; + else + MaxDuplicateCount = TailMergeSize - 1; // Check the instructions in the block to determine whether tail-duplication // is invalid or unlikely to be profitable. -- cgit v1.2.3