[PATCH] [RewriteStatepointsForGC] Fix debug assertion during derivable pointer rematerialization

[Sanjoy Das]

LGTM with comments.


REPOSITORY
  rL LLVM

================
Comment at: lib/Transforms/Scalar/RewriteStatepointsForGC.cpp:1947
@@ -1946,7 +1946,3 @@
 #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
----------------
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);
```

http://reviews.llvm.org/D9882

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