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//===-- PPCBranchSelector.cpp - Emit long conditional branches-----*- C++ -*-=//
//
// The LLVM Compiler Infrastructure
//
// 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
// 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
// pass should be run last, just before the assembly printer.
//
//===----------------------------------------------------------------------===//
#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/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 PPCBSel : public MachineFunctionPass {
static char ID;
PPCBSel() : MachineFunctionPass(ID) {}
/// 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 Selector";
}
};
char PPCBSel::ID = 0;
}
/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
/// Pass
///
FunctionPass *llvm::createPPCBranchSelectionPass() {
return new PPCBSel();
}
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());
// 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();
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];
}
// 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);
// 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;
}
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