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Make isSafeToClobberEFLAGS more aggressive. Teach it to scan backwards

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(for uses marked kill and defs marked dead) a few instructions in
addition to forwards. Also, increase the maximum number of instructions
to scan, as it appears to help in a fair number of cases.

llvm-svn: 84061
This commit is contained in:
Dan Gohman 2009-10-14 00:08:59 +00:00
parent 49946908b4
commit 76221cc874
1 changed files with 32 additions and 7 deletions
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39
lib/Target/X86/X86InstrInfo.cpp
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@ -853,7 +853,7 @@ X86InstrInfo::isReallyTriviallyReMaterializable(const MachineInstr *MI,
/// isSafeToClobberEFLAGS - Return true if it's safe insert an instruction that
/// would clobber the EFLAGS condition register. Note the result may be
/// conservative. If it cannot definitely determine the safety after visiting
/// two instructions it assumes it's not safe.
/// a few instructions in each direction it assumes it's not safe.
static bool isSafeToClobberEFLAGS(MachineBasicBlock &MBB,
MachineBasicBlock::iterator I) {
// It's always safe to clobber EFLAGS at the end of a block.
@ -861,11 +861,13 @@ static bool isSafeToClobberEFLAGS(MachineBasicBlock &MBB,
return true;
// For compile time consideration, if we are not able to determine the
// safety after visiting 2 instructions, we will assume it's not safe.
for (unsigned i = 0; i < 2; ++i) {
// safety after visiting 4 instructions in each direction, we will assume
// it's not safe.
MachineBasicBlock::iterator Iter = I;
for (unsigned i = 0; i < 4; ++i) {
bool SeenDef = false;
for (unsigned j = 0, e = I->getNumOperands(); j != e; ++j) {
MachineOperand &MO = I->getOperand(j);
for (unsigned j = 0, e = Iter->getNumOperands(); j != e; ++j) {
MachineOperand &MO = Iter->getOperand(j);
if (!MO.isReg())
continue;
if (MO.getReg() == X86::EFLAGS) {
@ -878,10 +880,33 @@ static bool isSafeToClobberEFLAGS(MachineBasicBlock &MBB,
if (SeenDef)
// This instruction defines EFLAGS, no need to look any further.
return true;
++I;
++Iter;
// If we make it to the end of the block, it's safe to clobber EFLAGS.
if (I == MBB.end())
if (Iter == MBB.end())
return true;
}
Iter = I;
for (unsigned i = 0; i < 4; ++i) {
// If we make it to the beginning of the block, it's safe to clobber
// EFLAGS iff EFLAGS is not live-in.
if (Iter == MBB.begin())
return !MBB.isLiveIn(X86::EFLAGS);
--Iter;
bool SawKill = false;
for (unsigned j = 0, e = Iter->getNumOperands(); j != e; ++j) {
MachineOperand &MO = Iter->getOperand(j);
if (MO.isReg() && MO.getReg() == X86::EFLAGS) {
if (MO.isDef()) return MO.isDead();
if (MO.isKill()) SawKill = true;
}
}
if (SawKill)
// This instruction kills EFLAGS and doesn't redefine it, so
// there's no need to look further.
return true;
}