[RewriteStatepointsForGC] Fix debug assertion during derivable pointer rematerialization

[igor-laevsky]

Summary

Correct assertion would be that there is no other uses from chain we are currently cloning. It is ok to have other uses of values not from this chain.

[sanjoy]

LGTM with comments.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

1947 ↗

(On Diff #26155)

IIUC, the invariant that is interesting to check here is: before the replaceUsesOfWith call, ClonedValue uses exactly one of the values in ChainToBase and that value is LastValue. Is there a way you can rewrite the assert to check that instead? Basically, before ClonedValue->replaceUsesOfWith, have something like:

for (auto OpValue : ...) {
  if(find(ChainToBase.begin(), .end(), OpValue))
    assert(OpValue == LastValue);

[sanjoy]

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

1947 ↗

(On Diff #26155)

Actually a slightly stronger invariant can be enforced by:

bool FoundLastValue = false;
for (auto OpValue : ...) {
  if(find(ChainToBase.begin(), .end(), OpValue)) {
    assert(OpValue == LastValue);
    FoundLastValue = true;
  }
assert(FoundLastValue);

llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

auto rematerializeChain = [&ChainToBase](Instruction *InsertBefore) {
         ClonedValue->setName(Instr->getName() + ".remat");
 
         // If it is not first instruction in the chain then it uses previously
         // cloned value. We should update it to use cloned value.
         if (LastClonedValue) {
           assert(LastValue);
           ClonedValue->replaceUsesOfWith(LastValue, LastClonedValue);
 #ifndef NDEBUG
-          // Assert that cloned instruction does not use any instructions
-          // other than LastClonedValue
-          for (auto OpValue: ClonedValue->operand_values()) {
-            if (isa<Instruction>(OpValue))
-              assert(OpValue == LastClonedValue &&
-                     "unexpected use found in rematerialized value");
+          // Assert that cloned instruction does not use any instructions from
+          // this chain other than LastClonedValue
+          for (auto OpValue : ClonedValue->operand_values()) {
+            assert(std::find(ChainToBase.begin(), ChainToBase.end(), OpValue) ==
+                       ChainToBase.end() &&
+                   "incorrect use in rematerialization chain");
           }
 #endif
         }
 
         LastClonedValue = ClonedValue;
         LastValue = Instr;
       }
       assert(LastClonedValue);