Rewrite the branch selector to be correct in the face of large functions.

The algorithm it used before wasn't 100% correct, we now use an iterative
expansion model.  This fixes assembler errors when compiling 403.gcc with
tail merging enabled.

Change the way the branch selector works overall: Now, the isel generates
PPC::BCC instructions (as it used to) directly, and these BCC instructions
are emitted to the output or jitted directly if branches don't need
expansion.  Only if branches need expansion are instructions rewritten
and created.  This should make branch select faster, and eliminates the
Bxx instructions from the .td file.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31837 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 101 insertions, 84 deletions

diff --git a/lib/Target/PowerPC/PPCBranchSelector.cpp b/lib/Target/PowerPC/PPCBranchSelector.cpp
index db85738ccc..f3d3124358 100644
--- a/lib/Target/PowerPC/PPCBranchSelector.cpp
+++ b/lib/Target/PowerPC/PPCBranchSelector.cpp
@@ -24,6 +24,7 @@
 #include "llvm/Target/TargetAsmInfo.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/MathExtras.h"
 using namespace llvm;
 
 static Statistic<> NumExpanded("ppc-branch-select",
@@ -31,13 +32,13 @@ static Statistic<> NumExpanded("ppc-branch-select",
 
 namespace {
   struct VISIBILITY_HIDDEN PPCBSel : public MachineFunctionPass {
-    /// OffsetMap - Mapping between BB # and byte offset from start of function.
-    std::vector<unsigned> OffsetMap;
+    /// BlockSizes - The sizes of the basic blocks in the function.
+    std::vector<unsigned> BlockSizes;
 
     virtual bool runOnMachineFunction(MachineFunction &Fn);
 
     virtual const char *getPassName() const {
-      return "PowerPC Branch Selection";
+      return "PowerPC Branch Selector";
     }
   };
 }
@@ -54,11 +55,6 @@ FunctionPass *llvm::createPPCBranchSelectionPass() {
 ///
 static unsigned getNumBytesForInstruction(MachineInstr *MI) {
   switch (MI->getOpcode()) {
-  case PPC::BCC:
-    // while this will be 4 most of the time, if we emit 8 it is just a
-    // minor pessimization that saves us from having to worry about
-    // keeping the offsets up to date later when we emit long branch glue.
-    return 8;
   case PPC::IMPLICIT_DEF_GPRC: // no asm emitted
   case PPC::IMPLICIT_DEF_G8RC: // no asm emitted
   case PPC::IMPLICIT_DEF_F4:   // no asm emitted
@@ -77,98 +73,119 @@ static unsigned getNumBytesForInstruction(MachineInstr *MI) {
 
 
 bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
-  // Running total of instructions encountered since beginning of function
-  unsigned ByteCount = 0;
-  
-  OffsetMap.resize(Fn.getNumBlockIDs());
-  
-  // For each MBB, add its offset to the offset map, and count up its
-  // instructions
+  // Give the blocks of the function a dense, in-order, numbering.
+  Fn.RenumberBlocks();
+  BlockSizes.resize(Fn.getNumBlockIDs());
+
+  // Measure each MBB and compute a size for the entire function.
+  unsigned FuncSize = 0;
   for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
        ++MFI) {
     MachineBasicBlock *MBB = MFI;
-    OffsetMap[MBB->getNumber()] = ByteCount;
-    
+
+    unsigned BlockSize = 0;
     for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
          MBBI != EE; ++MBBI)
-      ByteCount += getNumBytesForInstruction(MBBI);
+      BlockSize += getNumBytesForInstruction(MBBI);
+    
+    BlockSizes[MBB->getNumber()] = BlockSize;
+    FuncSize += BlockSize;
   }
   
-  // We're about to run over the MBB's again, so reset the ByteCount
-  ByteCount = 0;
+  // If the entire function is smaller than the displacement of a branch field,
+  // we know we don't need to shrink any branches in this function.  This is a
+  // common case.
+  if (FuncSize < (1 << 15)) {
+    BlockSizes.clear();
+    return false;
+  }
   
-  // For each MBB, find the conditional branch pseudo instructions, and
-  // calculate the difference between the target MBB and the current ICount
-  // to decide whether or not to emit a short or long branch.
-  //
-  // short branch:
-  // bCC .L_TARGET_MBB
+  // For each conditional branch, if the offset to its destination is larger
+  // than the offset field allows, transform it into a long branch sequence
+  // like this:
+  //   short branch:
+  //     bCC MBB
+  //   long branch:
+  //     b!CC $PC+8
+  //     b MBB
   //
-  // long branch:
-  // bInverseCC $PC+8
-  // b .L_TARGET_MBB
-  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
-       ++MFI) {
-    MachineBasicBlock *MBB = MFI;
-    
-    for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
-         MBBI != EE; ++MBBI) {
-      // We may end up deleting the MachineInstr that MBBI points to, so
-      // remember its opcode now so we can refer to it after calling erase()
-      unsigned ByteSize = getNumBytesForInstruction(MBBI);
-      if (MBBI->getOpcode() != PPC::BCC) {
-        ByteCount += ByteSize;
-        continue;
-      }
-      
-      // condbranch operands:
-      // 0. CR register
-      // 1. PPC branch opcode
-      // 2. Target MBB
-      MachineBasicBlock *DestMBB = MBBI->getOperand(2).getMachineBasicBlock();
-      PPC::Predicate Pred = (PPC::Predicate)MBBI->getOperand(0).getImm();
-      unsigned CRReg = MBBI->getOperand(1).getReg();
-      int Displacement = OffsetMap[DestMBB->getNumber()] - ByteCount;
+  bool MadeChange = true;
+  bool EverMadeChange = false;
+  while (MadeChange) {
+    // Iteratively expand branches until we reach a fixed point.
+    MadeChange = false;
+  
+    for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
+         ++MFI) {
+      MachineBasicBlock &MBB = *MFI;
+      unsigned MBBStartOffset = 0;
+      for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
+           I != E; ++I) {
+        if (I->getOpcode() != PPC::BCC || I->getOperand(2).isImm()) {
+          MBBStartOffset += getNumBytesForInstruction(I);
+          continue;
+        }
+        
+        // Determine the offset from the current branch to the destination
+        // block.
+        MachineBasicBlock *Dest = I->getOperand(2).getMachineBasicBlock();
+        
+        int BranchSize;
+        if (Dest->getNumber() <= MBB.getNumber()) {
+          // If this is a backwards branch, the delta is the offset from the
+          // start of this block to this branch, plus the sizes of all blocks
+          // from this block to the dest.
+          BranchSize = MBBStartOffset;
+          
+          for (unsigned i = Dest->getNumber(), e = MBB.getNumber(); i != e; ++i)
+            BranchSize += BlockSizes[i];
+        } else {
+          // Otherwise, add the size of the blocks between this block and the
+          // dest to the number of bytes left in this block.
+          BranchSize = -MBBStartOffset;
+
+          for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
+            BranchSize += BlockSizes[i];
+        }
 
-      bool ShortBranchOk = Displacement >= -32768 && Displacement <= 32767;
-      
-      // Branch on opposite condition if a short branch isn't ok.
-      if (!ShortBranchOk)
-        Pred = PPC::InvertPredicate(Pred);
+        // If this branch is in range, ignore it.
+        if (isInt16(BranchSize)) {
+          MBBStartOffset += 4;
+          continue;
+        }
         
-      unsigned Opcode;
-      switch (Pred) {
-      default: assert(0 && "Unknown cond branch predicate!");
-      case PPC::PRED_LT: Opcode = PPC::BLT; break;
-      case PPC::PRED_LE: Opcode = PPC::BLE; break;
-      case PPC::PRED_EQ: Opcode = PPC::BEQ; break;
-      case PPC::PRED_GE: Opcode = PPC::BGE; break;
-      case PPC::PRED_GT: Opcode = PPC::BGT; break;
-      case PPC::PRED_NE: Opcode = PPC::BNE; break;
-      case PPC::PRED_UN: Opcode = PPC::BUN; break;
-      case PPC::PRED_NU: Opcode = PPC::BNU; break;
-      }
-      
-      MachineBasicBlock::iterator MBBJ;
-      if (ShortBranchOk) {
-        MBBJ = BuildMI(*MBB, MBBI, Opcode, 2).addReg(CRReg).addMBB(DestMBB);
-      } else {
-        // Long branch, skip next branch instruction (i.e. $PC+8).
+        // Otherwise, we have to expand it to a long branch.
+        // The BCC operands are:
+        // 0. PPC branch predicate
+        // 1. CR register
+        // 2. Target MBB
+        PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
+        unsigned CRReg = I->getOperand(1).getReg();
+        
+        MachineInstr *OldBranch = I;
+        
+        // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
+        BuildMI(MBB, I, PPC::BCC, 3)
+          .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
+        
+        // Uncond branch to the real destination.
+        I = BuildMI(MBB, I, PPC::B, 1).addMBB(Dest);
+
+        // Remove the old branch from the function.
+        OldBranch->eraseFromParent();
+        
+        // Remember that this instruction is 8-bytes, increase the size of the
+        // block by 4, remember to iterate.
+        BlockSizes[MBB.getNumber()] += 4;
+        MBBStartOffset += 8;
         ++NumExpanded;
-        BuildMI(*MBB, MBBI, Opcode, 2).addReg(CRReg).addImm(2);
-        MBBJ = BuildMI(*MBB, MBBI, PPC::B, 1).addMBB(DestMBB);
+        MadeChange = true;
       }
-      
-      // Erase the psuedo BCC instruction, and then back up the
-      // iterator so that when the for loop increments it, we end up in
-      // the correct place rather than iterating off the end.
-      MBB->erase(MBBI);
-      MBBI = MBBJ;
-      ByteCount += ByteSize;
     }
+    EverMadeChange |= MadeChange;
   }
   
-  OffsetMap.clear();
+  BlockSizes.clear();
   return true;
 }