Allow machine-cse to look across MBB boundary when cse'ing instructions that

define physical registers. It's currently very restrictive, only catching
cases where the CE is in an immediate (and only) predecessor. But it catches
a surprising large number of cases.

rdar://10660865


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147827 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 54 insertions, 15 deletions

diff --git a/lib/CodeGen/MachineCSE.cpp b/lib/CodeGen/MachineCSE.cpp
index 8c02cd7ddb..5dd51a7944 100644
--- a/lib/CodeGen/MachineCSE.cpp
+++ b/lib/CodeGen/MachineCSE.cpp
@@ -26,13 +26,14 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/RecyclingAllocator.h"
-
 using namespace llvm;
 
 STATISTIC(NumCoalesces, "Number of copies coalesced");
 STATISTIC(NumCSEs,      "Number of common subexpression eliminated");
 STATISTIC(NumPhysCSEs,
           "Number of physreg referencing common subexpr eliminated");
+STATISTIC(NumCrossBBCSEs,
+          "Number of cross-MBB physreg referencing CS eliminated");
 STATISTIC(NumCommutes,  "Number of copies coalesced after commuting");
 
 namespace {
@@ -82,9 +83,11 @@ namespace {
                                 MachineBasicBlock::const_iterator E) const ;
     bool hasLivePhysRegDefUses(const MachineInstr *MI,
                                const MachineBasicBlock *MBB,
-                               SmallSet<unsigned,8> &PhysRefs) const;
+                               SmallSet<unsigned,8> &PhysRefs,
+                               SmallVector<unsigned,2> &PhysDefs) const;
     bool PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
-                          SmallSet<unsigned,8> &PhysRefs) const;
+                          SmallSet<unsigned,8> &PhysRefs,
+                          bool &NonLocal) const;
     bool isCSECandidate(MachineInstr *MI);
     bool isProfitableToCSE(unsigned CSReg, unsigned Reg,
                            MachineInstr *CSMI, MachineInstr *MI);
@@ -189,7 +192,8 @@ MachineCSE::isPhysDefTriviallyDead(unsigned Reg,
 /// instruction does not uses a physical register.
 bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
                                        const MachineBasicBlock *MBB,
-                                       SmallSet<unsigned,8> &PhysRefs) const {
+                                       SmallSet<unsigned,8> &PhysRefs,
+                                       SmallVector<unsigned,2> &PhysDefs) const{
   MachineBasicBlock::const_iterator I = MI; I = llvm::next(I);
   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
     const MachineOperand &MO = MI->getOperand(i);
@@ -207,6 +211,8 @@ bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
         (MO.isDead() || isPhysDefTriviallyDead(Reg, I, MBB->end())))
       continue;
     PhysRefs.insert(Reg);
+    if (MO.isDef())
+      PhysDefs.push_back(Reg);
     for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias)
       PhysRefs.insert(*Alias);
   }
@@ -215,20 +221,39 @@ bool MachineCSE::hasLivePhysRegDefUses(const MachineInstr *MI,
 }
 
 bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
-                                  SmallSet<unsigned,8> &PhysRefs) const {
+                                  SmallSet<unsigned,8> &PhysRefs,
+                                  bool &NonLocal) const {
   // For now conservatively returns false if the common subexpression is
-  // not in the same basic block as the given instruction.
-  MachineBasicBlock *MBB = MI->getParent();
-  if (CSMI->getParent() != MBB)
-    return false;
+  // not in the same basic block as the given instruction. The only exception
+  // is if the common subexpression is in the sole predecessor block.
+  const MachineBasicBlock *MBB = MI->getParent();
+  const MachineBasicBlock *CSMBB = CSMI->getParent();
+
+  bool CrossMBB = false;
+  if (CSMBB != MBB) {
+    if (MBB->pred_size() == 1 && *MBB->pred_begin() == CSMBB)
+      CrossMBB = true;
+    else
+      return false;
+  }
   MachineBasicBlock::const_iterator I = CSMI; I = llvm::next(I);
   MachineBasicBlock::const_iterator E = MI;
+  MachineBasicBlock::const_iterator EE = CSMBB->end();
   unsigned LookAheadLeft = LookAheadLimit;
   while (LookAheadLeft) {
     // Skip over dbg_value's.
-    while (I != E && I->isDebugValue())
+    while (I != E && I != EE && I->isDebugValue())
       ++I;
 
+    if (I == EE) {
+      assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
+      CrossMBB = false;
+      NonLocal = true;
+      I = MBB->begin();
+      EE = MBB->end();
+      continue;
+    }
+
     if (I == E)
       return true;
 
@@ -393,16 +418,18 @@ bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
     // If the instruction defines physical registers and the values *may* be
     // used, then it's not safe to replace it with a common subexpression.
     // It's also not safe if the instruction uses physical registers.
+    bool CrossMBBPhysDef = false;
     SmallSet<unsigned,8> PhysRefs;
-    if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs)) {
+    SmallVector<unsigned, 2> PhysDefs;
+    if (FoundCSE && hasLivePhysRegDefUses(MI, MBB, PhysRefs, PhysDefs)) {
       FoundCSE = false;
 
-      // ... Unless the CS is local and it also defines the physical register
-      // which is not clobbered in between and the physical register uses 
-      // were not clobbered.
+      // ... Unless the CS is local or is in the sole predecessor block
+      // and it also defines the physical register which is not clobbered
+      // in between and the physical register uses were not clobbered.
       unsigned CSVN = VNT.lookup(MI);
       MachineInstr *CSMI = Exps[CSVN];
-      if (PhysRegDefsReach(CSMI, MI, PhysRefs))
+      if (PhysRegDefsReach(CSMI, MI, PhysRefs, CrossMBBPhysDef))
         FoundCSE = true;
     }
 
@@ -457,6 +484,18 @@ bool MachineCSE::ProcessBlock(MachineBasicBlock *MBB) {
         MRI->replaceRegWith(CSEPairs[i].first, CSEPairs[i].second);
         MRI->clearKillFlags(CSEPairs[i].second);
       }
+
+      if (CrossMBBPhysDef) {
+        // Add physical register defs now coming in from a predecessor to MBB
+        // livein list.
+        while (!PhysDefs.empty()) {
+          unsigned LiveIn = PhysDefs.pop_back_val();
+          if (!MBB->isLiveIn(LiveIn))
+            MBB->addLiveIn(LiveIn);
+        }
+        ++NumCrossBBCSEs;
+      }
+
       MI->eraseFromParent();
       ++NumCSEs;
       if (!PhysRefs.empty())