Teach jump threading to "look through" a select when the branch direction of a terminator depends on it.
When it sees a promising select it now tries to figure out whether the condition of the select is known in any of the predecessors and if so it maps the operands appropriately.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121859 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp
index b44bc5e..e59ae31 100644
--- a/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/lib/Transforms/Scalar/JumpThreading.cpp
@@ -538,6 +538,40 @@
}
}
+ if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
+ // Handle select instructions where at least one operand is a known constant
+ // and we can figure out the condition value for any predecessor block.
+ Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
+ Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
+ PredValueInfoTy Conds;
+ if ((TrueVal || FalseVal) &&
+ ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
+ WantInteger)) {
+ for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
+ Constant *Cond = Conds[i].first;
+
+ // Figure out what value to use for the condition.
+ bool KnownCond;
+ if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
+ // A known boolean.
+ KnownCond = CI->isOne();
+ } else {
+ assert(isa<UndefValue>(Cond) && "Unexpected condition value");
+ // Either operand will do, so be sure to pick the one that's a known
+ // constant.
+ // FIXME: Do this more cleverly if both values are known constants?
+ KnownCond = (TrueVal != 0);
+ }
+
+ // See if the select has a known constant value for this predecessor.
+ if (Constant *Val = KnownCond ? TrueVal : FalseVal)
+ Result.push_back(std::make_pair(Val, Conds[i].second));
+ }
+
+ return !Result.empty();
+ }
+ }
+
// If all else fails, see if LVI can figure out a constant value for us.
Constant *CI = LVI->getConstant(V, BB);
if (Constant *KC = getKnownConstant(CI, Preference)) {