commit 97b9b97853d7e4fbb5c8460ef28126013c76e9a9
author Evan Cheng <evan.cheng@apple.com> Tue Aug 17 01:20:36 2010 +0000
committer Evan Cheng <evan.cheng@apple.com> Tue Aug 17 01:20:36 2010 +0000
tree 00177661907fa80d2ee615063908e042e57911b9
parent 922157c83440e0197e7fd962cf7030491a8069b1

PHI elimination should not break back edge. It can cause some significant code placement issues. rdar://8263994

good:
LBB0_2:
  mov     r2, r0
  . . .
  mov     r1, r2
  bne     LBB0_2

bad:
LBB0_2:
  mov     r2, r0
  . . .
@ BB#3:
  mov     r1, r2
  b       LBB0_2


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111221 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineBasicBlock.cpp

diff --git a/lib/CodeGen/MachineBasicBlock.cpp b/lib/CodeGen/MachineBasicBlock.cpp
index a27ee47..895653d 100644
--- a/lib/CodeGen/MachineBasicBlock.cpp
+++ b/lib/CodeGen/MachineBasicBlock.cpp
@@ -439,6 +439,14 @@
   if (TII->AnalyzeBranch(*this, TBB, FBB, Cond))
     return NULL;
 
+  // Avoid splitting backedges of loops. It would introduce small out-of-line
+  // blocks into the loop which is very bad for code placement.
+  if (this == Succ)
+    return NULL;
+  MachineLoopInfo *MLI = P->getAnalysisIfAvailable<MachineLoopInfo>();
+  if (MLI->isLoopHeader(Succ))
+    return NULL;
+
   MachineBasicBlock *NMBB = MF->CreateMachineBasicBlock();
   MF->insert(llvm::next(MachineFunction::iterator(this)), NMBB);
   DEBUG(dbgs() << "PHIElimination splitting critical edge:"
@@ -471,8 +479,7 @@
         P->getAnalysisIfAvailable<MachineDominatorTree>())
     MDT->addNewBlock(NMBB, this);
 
-  if (MachineLoopInfo *MLI =
-        P->getAnalysisIfAvailable<MachineLoopInfo>())
+  if (MLI)
     if (MachineLoop *TIL = MLI->getLoopFor(this)) {
       // If one or the other blocks were not in a loop, the new block is not
       // either, and thus LI doesn't need to be updated.

lib/CodeGen/PHIElimination.cpp

diff --git a/lib/CodeGen/PHIElimination.cpp b/lib/CodeGen/PHIElimination.cpp
index 7bf0aa2..105f20b 100644
--- a/lib/CodeGen/PHIElimination.cpp
+++ b/lib/CodeGen/PHIElimination.cpp
@@ -20,6 +20,7 @@
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Function.h"
@@ -44,9 +45,9 @@
 
 void llvm::PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addPreserved<LiveVariables>();
+  AU.addRequired<MachineLoopInfo>();
   AU.addPreserved<MachineDominatorTree>();
-  // rdar://7401784 This would be nice:
-  // AU.addPreservedID(MachineLoopInfoID);
+  AU.addPreservedID(MachineLoopInfoID);
   MachineFunctionPass::getAnalysisUsage(AU);
 }
 
@@ -382,6 +383,7 @@
   if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad())
     return false;   // Quick exit for basic blocks without PHIs.
 
+  bool Changed = false;
   for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end();
        BBI != BBE && BBI->isPHI(); ++BBI) {
     for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) {
@@ -391,7 +393,7 @@
       // (not considering PHI nodes). If the register is live in to this block
       // anyway, we would gain nothing from splitting.
       if (!LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB))
-        PreMBB->SplitCriticalEdge(&MBB, this);
+        Changed |= PreMBB->SplitCriticalEdge(&MBB, this) != 0;
     }
   }
   return true;