From: Chris Lattner <sabre@nondot.org>
Date: Wed, 8 Nov 2006 01:03:21 +0000 (+0000)
Subject: optimize single MBB loops better.  In particular, produce:
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=5d056952ba2f97729c5db69a3f14250595fa1ee8;p=oota-llvm.git

optimize single MBB loops better.  In particular, produce:

LBB1_57:        #bb207.i
        movl 72(%esp), %ecx
        movb (%ecx,%eax), %cl
        movl 80(%esp), %edx
        movb %cl, 1(%edx,%eax)
        incl %eax
        cmpl $143, %eax
        jne LBB1_57     #bb207.i
        jmp LBB1_64     #cond_next255.i

intead of:

LBB1_57:        #bb207.i
        movl 72(%esp), %ecx
        movb (%ecx,%eax), %cl
        movl 80(%esp), %edx
        movb %cl, 1(%edx,%eax)
        incl %eax
        cmpl $143, %eax
        je LBB1_64      #cond_next255.i
        jmp LBB1_57     #bb207.i

This eliminates a branch per iteration of the loop.  This hurted PPC
particularly, because the extra branch meant another dispatch group for each
iteration of the loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31530 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index fea03a5f407..ddd3bca03b9 100644
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -687,6 +687,23 @@ void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
                                        !CurCond.empty(),
                                        ++MachineFunction::iterator(MBB));
 
+    // If this is a two-way branch, and the FBB branches to this block, reverse 
+    // the condition so the single-basic-block loop is faster.  Instead of:
+    //    Loop: xxx; jcc Out; jmp Loop
+    // we want:
+    //    Loop: xxx; jncc Loop; jmp Out
+    if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
+      std::vector<MachineOperand> NewCond(CurCond);
+      if (!TII->ReverseBranchCondition(NewCond)) {
+        TII->RemoveBranch(*MBB);
+        TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
+        MadeChange = true;
+        ++NumBranchOpts;
+        return OptimizeBlock(MBB);
+      }
+    }
+    
+    
     // If this branch is the only thing in its block, see if we can forward
     // other blocks across it.
     if (CurTBB && CurCond.empty() && CurFBB == 0 &&