From: Dale Johannesen <dalej@apple.com>
Date: Mon, 12 May 2008 22:53:12 +0000 (+0000)
Subject: Be more aggressive about tail-merging small blocks
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=30562b7d4403414936da0661d7c6bda392f26c59;p=oota-llvm.git

Be more aggressive about tail-merging small blocks
if those blocks consist entirely of common instructions;
merging will not add an extra branch in this case.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51006 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index a8fee1c9ac3..1807f98cb13 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -495,7 +495,18 @@ unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
                                         CurMPIter->second,
                                         I->second,
                                         TrialBBI1, TrialBBI2);
-      if (CommonTailLen >= minCommonTailLength) {
+      // If we will have to split a block, there should be at least
+      // minCommonTailLength instructions in common; if not, at worst
+      // we will be replacing a fallthrough into the common tail with a
+      // branch, which at worst breaks even with falling through into
+      // the duplicated common tail, so 1 instruction in common is enough.
+      // We will always pick a block we do not have to split as the common
+      // tail if there is one.
+      // (Empty blocks will get forwarded and need not be considered.)
+      if (CommonTailLen >= minCommonTailLength ||
+          (CommonTailLen > 0 &&
+           (TrialBBI1==CurMPIter->second->begin() ||
+            TrialBBI2==I->second->begin()))) {
         if (CommonTailLen > maxCommonTailLength) {
           SameTails.clear();
           maxCommonTailLength = CommonTailLen;