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Conditional branch being fixed up is not the last MI in the BB, there is a

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unconditional branch following it. Simply invert the condition and swap
destinations if the conditional branch can reach the destination of the
unconditional branch:
beq L1
b   L2
=>
bne L2
b   L1

llvm-svn: 33548
This commit is contained in:
Evan Cheng 2007-01-26 20:38:26 +00:00
parent 1db79d3e23
commit 2b4f267d57
1 changed files with 51 additions and 13 deletions
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64
lib/Target/ARM/ARMConstantIslandPass.cpp
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@ -107,6 +107,7 @@ namespace {
void SplitBlockBeforeInstr(MachineInstr *MI);
void UpdateForInsertedWaterBlock(MachineBasicBlock *NewBB);
bool HandleConstantPoolUser(MachineFunction &Fn, CPUser &U);
bool BBIsInBranchRange(MachineInstr *MI, MachineBasicBlock *BB, unsigned D);
bool FixUpImmediateBranch(MachineFunction &Fn, ImmBranch &Br);
unsigned GetInstSize(MachineInstr *MI) const;
@ -561,6 +562,29 @@ bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, CPUser &U){
return true;
}
/// BBIsInBranchRange - Returns true is the distance between specific MI and
/// specific BB can fit in MI's displacement field.
bool ARMConstantIslands::BBIsInBranchRange(MachineInstr *MI,
MachineBasicBlock *DestBB,
unsigned MaxDisp) {
unsigned BrOffset = GetOffsetOf(MI);
unsigned DestOffset = GetOffsetOf(DestBB);
// Check to see if the destination BB is in range.
if (BrOffset < DestOffset) {
if (DestOffset - BrOffset < MaxDisp)
return true;
} else {
if (BrOffset - DestOffset <= MaxDisp)
return true;
}
return false;
}
static inline unsigned getUncondBranchDisp(int Opc) {
return (Opc == ARM::tB) ? (1<<10)*2 : (1<<23)*4;
}
/// FixUpImmediateBranch - Fix up immediate branches whose destination is too
/// far away to fit in its displacement field. If it is a conditional branch,
/// then it is converted to an inverse conditional branch + an unconditional
@ -571,17 +595,8 @@ ARMConstantIslands::FixUpImmediateBranch(MachineFunction &Fn, ImmBranch &Br) {
MachineInstr *MI = Br.MI;
MachineBasicBlock *DestBB = MI->getOperand(0).getMachineBasicBlock();
unsigned BrOffset = GetOffsetOf(MI);
unsigned DestOffset = GetOffsetOf(DestBB);
// Check to see if the destination BB is in range.
if (BrOffset < DestOffset) {
if (DestOffset - BrOffset < Br.MaxDisp)
return false;
} else {
if (BrOffset - DestOffset <= Br.MaxDisp)
return false;
}
if (BBIsInBranchRange(MI, DestBB, Br.MaxDisp))
return false;
if (!Br.isCond) {
// Unconditional branch. We have to insert a branch somewhere to perform
@ -604,7 +619,30 @@ ARMConstantIslands::FixUpImmediateBranch(MachineFunction &Fn, ImmBranch &Br) {
// direct the updated conditional branch to the fall-through block. Otherwise,
// split the MBB before the next instruction.
MachineBasicBlock *MBB = MI->getParent();
if (&MBB->back() != MI || !BBHasFallthrough(MBB)) {
MachineInstr *BackMI = &MBB->back();
bool NeedSplit = (BackMI != MI) || !BBHasFallthrough(MBB);
if (BackMI != MI) {
if (next(MachineBasicBlock::iterator(MI)) == MBB->back() &&
BackMI->getOpcode() == Br.UncondBr) {
// Last MI in the BB is a unconditional branch. Can we simply invert the
// condition and swap destinations:
// beq L1
// b L2
// =>
// bne L2
// b L1
MachineBasicBlock *NewDest = BackMI->getOperand(0).getMachineBasicBlock();
if (BBIsInBranchRange(MI, NewDest, Br.MaxDisp)) {
BackMI->getOperand(0).setMachineBasicBlock(DestBB);
MI->getOperand(0).setMachineBasicBlock(NewDest);
MI->getOperand(1).setImm(CC);
return true;
}
}
}
if (NeedSplit) {
SplitBlockBeforeInstr(MI);
// No need for the branch to the next block. We're adding a unconditional
// branch to the destination.
@ -617,7 +655,7 @@ ARMConstantIslands::FixUpImmediateBranch(MachineFunction &Fn, ImmBranch &Br) {
BuildMI(MBB, TII->get(MI->getOpcode())).addMBB(NextBB).addImm(CC);
Br.MI = &MBB->back();
BuildMI(MBB, TII->get(Br.UncondBr)).addMBB(DestBB);
unsigned MaxDisp = (Br.UncondBr == ARM::tB) ? (1<<10)*2 : (1<<23)*4;
unsigned MaxDisp = getUncondBranchDisp(Br.UncondBr);
ImmBranches.push_back(ImmBranch(&MBB->back(), MaxDisp, false, Br.UncondBr));
MI->eraseFromParent();