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Teach PHIElimination to handle <undef> operands.

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When a PHI use is <undef>, don't emit a copy in the predecessor block,
but insert an IMPLICIT_DEF instruction instead. This ensures that
virtual register uses are always jointly dominated by defs, even if some
of them are IMPLICIT_DEF.

llvm-svn: 159121
This commit is contained in:
Jakob Stoklund Olesen 2012-06-25 03:36:12 +00:00
parent 6b556f824d
commit 70ed924e18
1 changed files with 34 additions and 19 deletions
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53
llvm/lib/CodeGen/PHIElimination.cpp
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@ -171,21 +171,28 @@ bool PHIElimination::EliminatePHINodes(MachineFunction &MF,
return true; return true;
} }
/// isImplicitlyDefined - Return true if all defs of VirtReg are implicit-defs.
/// This includes registers with no defs.
static bool isImplicitlyDefined(unsigned VirtReg,
const MachineRegisterInfo *MRI) {
for (MachineRegisterInfo::def_iterator DI = MRI->def_begin(VirtReg),
DE = MRI->def_end(); DI != DE; ++DI)
if (!DI->isImplicitDef())
return false;
return true;
}
/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node /// isSourceDefinedByImplicitDef - Return true if all sources of the phi node
/// are implicit_def's. /// are implicit_def's.
static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi, static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi,
const MachineRegisterInfo *MRI) { const MachineRegisterInfo *MRI) {
for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) { for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
unsigned SrcReg = MPhi->getOperand(i).getReg(); if (!isImplicitlyDefined(MPhi->getOperand(i).getReg(), MRI))
const MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
if (!DefMI || !DefMI->isImplicitDef())
return false; return false;
}
return true; return true;
} }
/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block, /// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
/// under the assuption that it needs to be lowered in a way that supports /// under the assuption that it needs to be lowered in a way that supports
/// atomic execution of PHIs. This lowering method is always correct all of the /// atomic execution of PHIs. This lowering method is always correct all of the
@ -287,7 +294,8 @@ void PHIElimination::LowerAtomicPHINode(
for (int i = NumSrcs - 1; i >= 0; --i) { for (int i = NumSrcs - 1; i >= 0; --i) {
unsigned SrcReg = MPhi->getOperand(i*2+1).getReg(); unsigned SrcReg = MPhi->getOperand(i*2+1).getReg();
unsigned SrcSubReg = MPhi->getOperand(i*2+1).getSubReg(); unsigned SrcSubReg = MPhi->getOperand(i*2+1).getSubReg();
bool SrcUndef = MPhi->getOperand(i*2+1).isUndef() ||
isImplicitlyDefined(SrcReg, MRI);
assert(TargetRegisterInfo::isVirtualRegister(SrcReg) && assert(TargetRegisterInfo::isVirtualRegister(SrcReg) &&
"Machine PHI Operands must all be virtual registers!"); "Machine PHI Operands must all be virtual registers!");
@ -295,14 +303,6 @@ void PHIElimination::LowerAtomicPHINode(
// path the PHI. // path the PHI.
MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB(); MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB();
// If source is defined by an implicit def, there is no need to insert a
// copy.
MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
if (DefMI->isImplicitDef()) {
ImpDefs.insert(DefMI);
continue;
}
// Check to make sure we haven't already emitted the copy for this block. // Check to make sure we haven't already emitted the copy for this block.
// This can happen because PHI nodes may have multiple entries for the same // This can happen because PHI nodes may have multiple entries for the same
// basic block. // basic block.
@ -315,12 +315,27 @@ void PHIElimination::LowerAtomicPHINode(
findPHICopyInsertPoint(&opBlock, &MBB, SrcReg); findPHICopyInsertPoint(&opBlock, &MBB, SrcReg);
// Insert the copy. // Insert the copy.
if (!reusedIncoming && IncomingReg) if (!reusedIncoming && IncomingReg) {
BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(), if (SrcUndef) {
TII->get(TargetOpcode::COPY), IncomingReg).addReg(SrcReg, 0, SrcSubReg); // The source register is undefined, so there is no need for a real
// COPY, but we still need to ensure joint dominance by defs.
// Insert an IMPLICIT_DEF instruction.
BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(),
TII->get(TargetOpcode::IMPLICIT_DEF), IncomingReg);
// Clean up the old implicit-def, if there even was one.
if (MachineInstr *DefMI = MRI->getVRegDef(SrcReg))
if (DefMI->isImplicitDef())
ImpDefs.insert(DefMI);
} else {
BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(),
TII->get(TargetOpcode::COPY), IncomingReg)
.addReg(SrcReg, 0, SrcSubReg);
}
}
// Now update live variable information if we have it. Otherwise we're done // Now update live variable information if we have it. Otherwise we're done
if (!LV) continue; if (SrcUndef || !LV) continue;
// We want to be able to insert a kill of the register if this PHI (aka, the // We want to be able to insert a kill of the register if this PHI (aka, the
// copy we just inserted) is the last use of the source value. Live // copy we just inserted) is the last use of the source value. Live