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.

llvm-svn: 121859

1 files changed, 34 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index b44bc5efef5..e59ae31e134 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -538,6 +538,40 @@ ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
     }
   }
 
+  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)) {