[JumpThreading] Change a return of ComputeValueKnownInPredecessors() to prevent triggering an assert

[haicheng]

Summary

Change a return statement of ComputeValueKnownInPredecessors() to be the same as the rest return statements of the function. Otherwise, it might return true with an empty Result when the current basic block has no predecessors and trigger the first assert of JumpThreading::ProcessThreadableEdges.

[mssimpso]

LGTM.

[mcrosier]

Please add a test case. In the test please comment that you're just making sure the code doesn't assert.

Chad

[haicheng]

In D16952#345938, @mcrosier wrote:

Please add a test case. In the test please comment that you're just making sure the code doesn't assert.

It is quite difficult to create a test case for this without D16809. I will add the test case in D16809.

[mcrosier]

In D16952#346361, @haicheng wrote:

In D16952#345938, @mcrosier wrote:

Please add a test case. In the test please comment that you're just making sure the code doesn't assert.

It is quite difficult to create a test case for this without D16809. I will add the test case in D16809.

SGTM. Feel free to commit and include the test in D16809.

[haicheng]

Committed in r260110.

lib/Transforms/Scalar/JumpThreading.cpp

ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
   // stack pops back out again.
   RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
 
   // If V is a constant, then it is known in all predecessors.
   if (Constant *KC = getKnownConstant(V, Preference)) {
     for (BasicBlock *Pred : predecessors(BB))
       Result.push_back(std::make_pair(KC, Pred));
 
-    return true;
+    return !Result.empty();
   }
 
   // If V is a non-instruction value, or an instruction in a different block,
   // then it can't be derived from a PHI.
   Instruction *I = dyn_cast<Instruction>(V);
   if (!I || I->getParent() != BB) {
 
     // Okay, if this is a live-in value, see if it has a known value at the end