PR1909: Tail merging pass ran wild. It makes no sense to merge blocks in order to save a single instruction since a branch will be inserted for each BB.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47301 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index dc67593..2d7836f 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -451,7 +451,10 @@
bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
MachineBasicBlock* PredBB) {
- unsigned minCommonTailLength = (SuccBB ? 1 : 2);
+ // It doesn't make sense to save a single instruction since tail merging
+ // will add a jump.
+ // FIXME: Ask the target to provide the threshold?
+ unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1;
MadeChange = false;
// Sort by hash value so that blocks with identical end sequences sort
@@ -476,9 +479,9 @@
// Look through all the pairs of blocks that have the same hash as this
// one, and find the pair that has the largest number of instructions in
// common.
- // Since instructions may get combined later (e.g. single stores into
+ // Since instructions may get combined later (e.g. single stores into
// store multiple) this measure is not particularly accurate.
- MachineBasicBlock::iterator BBI1, BBI2;
+ MachineBasicBlock::iterator BBI1, BBI2;
unsigned FoundI = ~0U, FoundJ = ~0U;
unsigned maxCommonTailLength = 0U;
@@ -541,6 +544,12 @@
continue;
}
+ MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
+ unsigned CommonTailLen = ComputeCommonTailLength(CurMBB, MBB2,
+ TrialBBI1, TrialBBI2);
+ if (CommonTailLen < minCommonTailLength)
+ continue;
+
// Decide whether we want to split CurMBB or MBB2.
if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, PredBB)) {
CurMBB = SplitMBBAt(*CurMBB, BBI1);