[llvm-commits] [llvm] r51006 - /llvm/trunk/lib/CodeGen/BranchFolding.cpp

[Dale Johannesen]

Author: johannes
Date: Mon May 12 17:53:12 2008
New Revision: 51006

URL: http://llvm.org/viewvc/llvm-project?rev=51006&view=rev
Log:
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.


Modified:
    llvm/trunk/lib/CodeGen/BranchFolding.cpp

Modified: llvm/trunk/lib/CodeGen/BranchFolding.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/BranchFolding.cpp?rev=51006&r1=51005&r2=51006&view=diff

==============================================================================
--- llvm/trunk/lib/CodeGen/BranchFolding.cpp (original)
+++ llvm/trunk/lib/CodeGen/BranchFolding.cpp Mon May 12 17:53:12 2008
@@ -495,7 +495,18 @@
                                         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;