[PATCH] D36247: [LVI] Constant-propagate a zero extension of the switch condition value through case edges

[Sanjoy Das via Phabricator via llvm-commits]

sanjoy added a comment.

Code wise this looks correct to me, but I'd prefer to phrase the solution a bit differently in the commit message and in the comments: IIUC the bug we had was that we were assuming `Condition != T` (in the default block) implied `f(Condition) != f(T)`.  However this isn't true for a non injective `f` (like `f(i32 x) = and i32 x, 2`).

For instance we //could// still do this transformation for

    x = cond + 2
    switch cond, label %left, [ i32 20, label %right]
  
  left:
    ...
  right:
   ...

In `left` we know that `cond` is not `20` and thus `x` is not `22`, since `f(x) = x + 2` is injective.

(I'm not suggesting we actually do the above btw, I think the case this patch is handling (`f(x) = x`) is enough for now.)



================
Comment at: lib/Analysis/LazyValueInfo.cpp:158
+    }
+    return Optional<APInt>();
+  }
----------------
Can you `return None;` here?


================
Comment at: test/Transforms/CorrelatedValuePropagation/range.ll:587
+    i64 4, label %exit2 
+  ]
+
----------------
Can we demonstrate the issue with just one `switch`?


https://reviews.llvm.org/D36247