BranchRelaxation: Recompute live-ins when splitting a block

Factors out and reuses live-in computation code from BranchFolding.

Differential Revision: https://reviews.llvm.org/D27558

llvm-svn: 290013

llvm/include/llvm/CodeGen/LivePhysRegs.h

@@ -155,6 +155,13 @@ inline raw_ostream &operator<<(raw_ostream &OS, const LivePhysRegs& LR) {
   return OS;
 }
 
+/// Compute the live-in list for \p MBB assuming all of its successors live-in
+/// lists are up-to-date. Uses the given LivePhysReg instance \p LiveRegs; This
+/// is just here to avoid repeated heap allocations when calling this multiple
+/// times in a pass.
+void computeLiveIns(LivePhysRegs &LiveRegs, const TargetRegisterInfo &TRI,
+                    MachineBasicBlock &MBB);
+
 } // end namespace llvm
 
 #endif // LLVM_CODEGEN_LIVEPHYSREGS_H

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -289,6 +289,9 @@ public:
   /// LiveIn insertion.
   void sortUniqueLiveIns();
 
+  /// Clear live in list.
+  void clearLiveIns();
+
   /// Add PhysReg as live in to this block, and ensure that there is a copy of
   /// PhysReg to a virtual register of class RC. Return the virtual register
   /// that is a copy of the live in PhysReg.

llvm/lib/CodeGen/BranchFolding.cpp

@@ -349,37 +349,16 @@ static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
   return TailLen;
 }
 
-void BranchFolder::computeLiveIns(MachineBasicBlock &MBB) {
-  if (!UpdateLiveIns)
-    return;
-
-  LiveRegs.init(*TRI);
-  LiveRegs.addLiveOutsNoPristines(MBB);
-  for (MachineInstr &MI : make_range(MBB.rbegin(), MBB.rend()))
-    LiveRegs.stepBackward(MI);
-
-  for (unsigned Reg : LiveRegs) {
-    // Skip the register if we are about to add one of its super registers.
-    bool ContainsSuperReg = false;
-    for (MCSuperRegIterator SReg(Reg, TRI); SReg.isValid(); ++SReg) {
-      if (LiveRegs.contains(*SReg)) {
-        ContainsSuperReg = true;
-        break;
-      }
-    }
-    if (ContainsSuperReg)
-      continue;
-    MBB.addLiveIn(Reg);
-  }
-}
-
 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
 /// after it, replacing it with an unconditional branch to NewDest.
 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
                                            MachineBasicBlock *NewDest) {
   TII->ReplaceTailWithBranchTo(OldInst, NewDest);
 
-  computeLiveIns(*NewDest);
+  if (UpdateLiveIns) {
+    NewDest->clearLiveIns();
+    computeLiveIns(LiveRegs, *TRI, *NewDest);
+  }
 
   ++NumTailMerge;
 }
@@ -417,7 +396,8 @@ MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
   // NewMBB inherits CurMBB's block frequency.
   MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
 
-  computeLiveIns(*NewMBB);
+  if (UpdateLiveIns)
+    computeLiveIns(LiveRegs, *TRI, *NewMBB);
 
   // Add the new block to the funclet.
   const auto &FuncletI = FuncletMembership.find(&CurMBB);

llvm/lib/CodeGen/BranchFolding.h

@@ -137,7 +137,6 @@ namespace llvm {
                        MachineBasicBlock* PredBB,
                        unsigned MinCommonTailLength);
     void setCommonTailEdgeWeights(MachineBasicBlock &TailMBB);
-    void computeLiveIns(MachineBasicBlock &MBB);
     void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
                                  MachineBasicBlock *NewDest);
     MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,

llvm/lib/CodeGen/BranchRelaxation.cpp

@@ -10,6 +10,7 @@
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/LivePhysRegs.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/RegisterScavenging.h"
 #include "llvm/Target/TargetInstrInfo.h"
@@ -69,8 +70,10 @@ class BranchRelaxation : public MachineFunctionPass {
 
   SmallVector<BasicBlockInfo, 16> BlockInfo;
   std::unique_ptr<RegScavenger> RS;
+  LivePhysRegs LiveRegs;
 
   MachineFunction *MF;
+  const TargetRegisterInfo *TRI;
   const TargetInstrInfo *TII;
 
   bool relaxBranchInstructions();
@@ -252,6 +255,10 @@ MachineBasicBlock *BranchRelaxation::splitBlockBeforeInstr(MachineInstr &MI,
   // All BBOffsets following these blocks must be modified.
   adjustBlockOffsets(*OrigBB);
 
+  // Need to fix live-in lists if we track liveness.
+  if (TRI->trackLivenessAfterRegAlloc(*MF))
+    computeLiveIns(LiveRegs, *TRI, *NewBB);
+
   ++NumSplit;
 
   return NewBB;
@@ -411,8 +418,9 @@ bool BranchRelaxation::relaxBranchInstructions() {
   for (MachineFunction::iterator I = MF->begin(); I != MF->end(); ++I) {
     MachineBasicBlock &MBB = *I;
 
-    auto Last = MBB.rbegin();
+    // Empty block?
-    if (Last == MBB.rend()) // Empty block.
+    MachineBasicBlock::iterator Last = MBB.getLastNonDebugInstr();
+    if (Last == MBB.end())
       continue;
 
     // Expand the unconditional branch first if necessary. If there is a
@@ -473,7 +481,7 @@ bool BranchRelaxation::runOnMachineFunction(MachineFunction &mf) {
   const TargetSubtargetInfo &ST = MF->getSubtarget();
   TII = ST.getInstrInfo();
 
-  const TargetRegisterInfo *TRI = ST.getRegisterInfo();
+  TRI = ST.getRegisterInfo();
   if (TRI->trackLivenessAfterRegAlloc(*MF))
     RS.reset(new RegScavenger());
 

llvm/lib/CodeGen/LivePhysRegs.cpp

@@ -197,3 +197,26 @@ void LivePhysRegs::addLiveIns(const MachineBasicBlock &MBB) {
     addPristines(*this, MF, MFI, *TRI);
   addBlockLiveIns(MBB);
 }
+
+void llvm::computeLiveIns(LivePhysRegs &LiveRegs, const TargetRegisterInfo &TRI,
+                          MachineBasicBlock &MBB) {
+  assert(MBB.livein_empty());
+  LiveRegs.init(TRI);
+  LiveRegs.addLiveOutsNoPristines(MBB);
+  for (MachineInstr &MI : make_range(MBB.rbegin(), MBB.rend()))
+    LiveRegs.stepBackward(MI);
+
+  for (unsigned Reg : LiveRegs) {
+    // Skip the register if we are about to add one of its super registers.
+    bool ContainsSuperReg = false;
+    for (MCSuperRegIterator SReg(Reg, &TRI); SReg.isValid(); ++SReg) {
+      if (LiveRegs.contains(*SReg)) {
+        ContainsSuperReg = true;
+        break;
+      }
+    }
+    if (ContainsSuperReg)
+      continue;
+    MBB.addLiveIn(Reg);
+  }
+}

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -1288,3 +1288,7 @@ MachineBasicBlock::getEndClobberMask(const TargetRegisterInfo *TRI) const {
   // care what kind of return it is, putting a mask after it is a no-op.
   return isReturnBlock() && !succ_empty() ? TRI->getNoPreservedMask() : nullptr;
 }
+
+void MachineBasicBlock::clearLiveIns() {
+  LiveIns.clear();
+}