[IPSCCP] Add assertion to surface cases where we zap returns with overdefined users.

fhahn · fhahn · commit 5354c83ece00 · 2019-07-25T09:37:09.000Z
We should only zap returns in functions, where all live users have a replace-able value (are not overdefined). Unused return values should be undefined. This should make it easier to detect bugs like in PR42738. Alternatively we could bail out of zapping the function returns, but I think it would be better to address those divergences between function and call-site values where they are actually caused. Reviewers: davide, efriedma Reviewed By: davide, efriedma Differential Revision: https://reviews.llvm.org/D65222 llvm-svn: 366998
diff --git a/llvm/lib/Transforms/Scalar/SCCP.cpp b/llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -1924,6 +1924,21 @@ static void findReturnsToZap(Function &F,
     return;
   }
 
+  assert(
+      all_of(F.users(),
+             [&Solver](User *U) {
+               if (isa<Instruction>(U) &&
+                   !Solver.isBlockExecutable(cast<Instruction>(U)->getParent()))
+                 return false;
+               if (U->getType()->isStructTy()) {
+                 return all_of(
+                     Solver.getStructLatticeValueFor(U),
+                     [](const LatticeVal &LV) { return !LV.isOverdefined(); });
+               }
+               return !Solver.getLatticeValueFor(U).isOverdefined();
+             }) &&
+      "We can only zap functions where all live users have a concrete value");
+
   for (BasicBlock &BB : F) {
     if (CallInst *CI = BB.getTerminatingMustTailCall()) {
       LLVM_DEBUG(dbgs() << "Can't zap return of the block due to present "
