int BranchProbabilities::CheckIntegerHeuristic()
{
// Heuristic fails if the last instruction is not a conditional branch
BranchInst *BI = dyn_cast<BranchInst>(_TI);
if ((!BI) || (BI->isUnconditional()))
return -1;
// All integer comparisons are done with the icmp instruction
ICmpInst *icmp = dyn_cast<ICmpInst>(BI->getCondition());
if (!icmp)
return -1;
Value *v[2];
v[0] = icmp->getOperand(0);
v[1] = icmp->getOperand(1);
// If neither is a constant, nothing to do
if (!isa<ConstantInt>(v[0]) && !isa<ConstantInt>(v[1]))
return -1;
// If we're dealing with something other than ints, nothing to do
if (!isa<IntegerType>(v[0]->getType()))
return -1;
// Get comparison
ICmpInst::Predicate pred = icmp->getPredicate();
// Eq and Not Eq are easy cases
if (pred == ICmpInst::ICMP_EQ)
return 1;
else if (pred == ICmpInst::ICMP_NE)
return 0;
ConstantInt *CI = dyn_cast<ConstantInt>(v[1]);
// If the right side isn't a constant, swap the predicate so we can pretend
if (!CI)
{
pred = icmp->getSwappedPredicate();
CI = cast<ConstantInt>(v[0]);
}
// Choose the appropriate branch depending on the const val and predicate
if (CI->isZero())
{
switch (pred)
{
case ICmpInst::ICMP_UGE:
assert("UGE zero always returns true");
return -1;
case ICmpInst::ICMP_ULT:
assert("ULT zero always returns false");
return -1;
case ICmpInst::ICMP_UGT:
case ICmpInst::ICMP_SGT:
case ICmpInst::ICMP_SGE:
return 0;
case ICmpInst::ICMP_ULE:
case ICmpInst::ICMP_SLT:
case ICmpInst::ICMP_SLE:
return 1;
default:
return -1;
}
}
else if (CI->isOne())
{
switch (pred)
{
case ICmpInst::ICMP_UGE:
case ICmpInst::ICMP_SGE:
return 0;
case ICmpInst::ICMP_ULT:
case ICmpInst::ICMP_SLT:
return 1;
default:
return -1;
}
}
else if (CI->isAllOnesValue())
{
switch (pred)
{
case ICmpInst::ICMP_SGT:
return 0;
case ICmpInst::ICMP_SLE:
return 1;
default:
return -1;
}
}
return -1;
}
