/// 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.
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) {
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;
}