-//===-- PowerPCBranchSelector.cpp - Emit long conditional branches-*- C++ -*-=//
-//
+//===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=//
+//
// The LLVM Compiler Infrastructure
//
-// This file was developed by Nate Baegeman and is distributed under the
-// University of Illinois Open Source License. See LICENSE.TXT for details.
-//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
//===----------------------------------------------------------------------===//
//
-// This file contains a pass that scans a machine function to determine which
+// This file contains a pass that scans a machine function to determine which
// conditional branches need more than 16 bits of displacement to reach their
// target basic block. It does this in two passes; a calculation of basic block
// positions pass, and a branch psuedo op to machine branch opcode pass. This
//
//===----------------------------------------------------------------------===//
-#define DEBUG_TYPE "bsel"
-#include "PowerPC.h"
-#include "PowerPCInstrBuilder.h"
-#include "PowerPCInstrInfo.h"
+#define DEBUG_TYPE "ppc-branch-select"
+#include "PPC.h"
+#include "PPCInstrBuilder.h"
+#include "PPCInstrInfo.h"
+#include "PPCPredicates.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
-#include "llvm/CodeGen/MachineFunction.h"
-#include "Support/Debug.h"
-#include <map>
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/MathExtras.h"
using namespace llvm;
+STATISTIC(NumExpanded, "Number of branches expanded to long format");
+
namespace {
- struct BSel : public MachineFunctionPass {
- // OffsetMap - Mapping between BB and byte offset from start of function
- std::map<MachineBasicBlock*, unsigned> OffsetMap;
+ struct PPCBSel : public MachineFunctionPass {
+ static char ID;
+ PPCBSel() : MachineFunctionPass(&ID) {}
- /// bytesForOpcode - A convenience function for totalling up the number of
- /// bytes in a basic block.
- ///
- static unsigned bytesForOpcode(unsigned opcode) {
- switch (opcode) {
- case PPC32::COND_BRANCH:
- // while this will be 4 most of the time, if we emit 12 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 12;
- case PPC32::MovePCtoLR:
- // MovePCtoLR is actually a combination of a branch-and-link (bl)
- // followed by a move from link register to dest reg (mflr)
- return 8;
- break;
- case PPC32::IMPLICIT_DEF: // no asm emitted
- return 0;
- break;
- default:
- return 4; // PowerPC instructions are all 4 bytes
- break;
- }
- }
-
- virtual bool runOnMachineFunction(MachineFunction &Fn) {
- // Running total of instructions encountered since beginning of function
- unsigned ByteCount = 0;
+ /// BlockSizes - The sizes of the basic blocks in the function.
+ std::vector<unsigned> BlockSizes;
- // For each MBB, add its offset to the offset map, and count up its
- // instructions
- for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
- ++MFI) {
- MachineBasicBlock *MBB = MFI;
- OffsetMap[MBB] = ByteCount;
+ virtual bool runOnMachineFunction(MachineFunction &Fn);
- for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
- MBBI != EE; ++MBBI)
- ByteCount += bytesForOpcode(MBBI->getOpcode());
- }
+ virtual const char *getPassName() const {
+ return "PowerPC Branch Selector";
+ }
+ };
+ char PPCBSel::ID = 0;
+}
- // We're about to run over the MBB's again, so reset the ByteCount
- ByteCount = 0;
+/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
+/// Pass
+///
+FunctionPass *llvm::createPPCBranchSelectionPass() {
+ return new PPCBSel();
+}
- // 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
- //
- // long branch:
- // bInverseCC $PC+8
- // b .L_TARGET_MBB
- // b .L_FALLTHROUGH_MBB
+bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
+ const PPCInstrInfo *TII =
+ static_cast<const PPCInstrInfo*>(Fn.getTarget().getInstrInfo());
+ // Give the blocks of the function a dense, in-order, numbering.
+ Fn.RenumberBlocks();
+ BlockSizes.resize(Fn.getNumBlockIDs());
- for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
- ++MFI) {
- MachineBasicBlock *MBB = MFI;
+ // 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;
+
+ unsigned BlockSize = 0;
+ for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
+ MBBI != EE; ++MBBI)
+ BlockSize += TII->GetInstSizeInBytes(MBBI);
+
+ BlockSizes[MBB->getNumber()] = BlockSize;
+ FuncSize += BlockSize;
+ }
+
+ // 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 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
+ //
+ 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 += TII->GetInstSizeInBytes(I);
+ continue;
+ }
+
+ // Determine the offset from the current branch to the destination
+ // block.
+ MachineBasicBlock *Dest = I->getOperand(2).getMBB();
- for (MachineBasicBlock::iterator MBBI = MBB->begin(), EE = MBB->end();
- MBBI != EE; ++MBBI) {
- if (MBBI->getOpcode() == PPC32::COND_BRANCH) {
- // condbranch operands:
- // 0. CR0 register
- // 1. bc opcode
- // 2. target MBB
- // 3. fallthrough MBB
- MachineBasicBlock *trueMBB =
- MBBI->getOperand(2).getMachineBasicBlock();
- MachineBasicBlock *falseMBB =
- MBBI->getOperand(3).getMachineBasicBlock();
-
- int Displacement = OffsetMap[trueMBB] - ByteCount;
- unsigned Opcode = MBBI->getOperand(1).getImmedValue();
- unsigned Inverted = PowerPCInstrInfo::invertPPCBranchOpcode(Opcode);
+ 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;
- MachineInstr *MI = MBBI;
- if (Displacement >= -32768 && Displacement <= 32767) {
- BuildMI(*MBB, MBBI, Opcode, 2).addReg(PPC32::CR0).addMBB(trueMBB);
- } else {
- BuildMI(*MBB, MBBI, Inverted, 2).addReg(PPC32::CR0).addSImm(8);
- BuildMI(*MBB, MBBI, PPC32::B, 1).addMBB(trueMBB);
- BuildMI(*MBB, MBBI, PPC32::B, 1).addMBB(falseMBB);
- }
- MBB->erase(MI);
- }
- ByteCount += bytesForOpcode(MBBI->getOpcode());
+ for (unsigned i = MBB.getNumber(), e = Dest->getNumber(); i != e; ++i)
+ BranchSize += BlockSizes[i];
}
- }
- OffsetMap.clear();
- return true;
- }
+ // If this branch is in range, ignore it.
+ if (isInt<16>(BranchSize)) {
+ MBBStartOffset += 4;
+ continue;
+ }
+
+ // 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;
+ DebugLoc dl = OldBranch->getDebugLoc();
+
+ // Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
+ BuildMI(MBB, I, dl, TII->get(PPC::BCC))
+ .addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
+
+ // Uncond branch to the real destination.
+ I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
- virtual const char *getPassName() const {
- return "PowerPC Branch Selection";
+ // 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;
+ MadeChange = true;
+ }
}
- };
+ EverMadeChange |= MadeChange;
+ }
+
+ BlockSizes.clear();
+ return true;
}
-/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
-/// Pass
-///
-FunctionPass *llvm::createPPCBranchSelectionPass() {
- return new BSel();
-}