[JumpThread] Try to fold SwitchInst before attempting to jumpthread it

[trentxintong]

Summary

Try to fold SwitchInst before attempting to jumpthread it.

[trentxintong]

Do not call getPredicateAt when its not a foldable terminator.

[efriedma]

test/Transforms/JumpThreading/basic.ll

21 ↗

(On Diff #90901)

IIRC, SimplifyCFG converts a switch on i1 to a conditional branch. Given that, I can't see how you could ever trigger this in practice.

[trentxintong]

efriedma is right about simplifycfg converting switch(i1) to condbr. so this patch is now
a comment change =).

[efriedma]

lib/Transforms/Scalar/JumpThreading.cpp

810

I think the original comment was suggesting that jump threading could use getConstantRange or something like that to prune impossible switch destinations.

lib/Transforms/Scalar/JumpThreading.cpp

if (ProcessThreadableEdges(Condition, BB, Preference, Terminator))
       return true;
     return false;
   }
 
   if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
     // If we're branching on a conditional, LVI might be able to determine
     // it's value at the branch instruction.  We only handle comparisons
     // against a constant at this time.
-    // TODO: This should be extended to handle switches as well.
+    //
+    // Switch(i1) will be converted to CondBr by SimplifyCFG and therefore
+    // handled by the CondBr folding here..
     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
     if (CondBr && CondConst) {
       // We should have returned as soon as we turn a conditional branch to
       // unconditional. Because its no longer interesting as far as jump
       // threading is concerned.
       assert(CondBr->isConditional() && "Threading on unconditional terminator");