[PATCH] D25913: [InstCombine] Fold nuw left-shifts in `ugt`/`ule` comparisons.

[Eli Friedman via llvm-commits]

efriedma added inline comments.


================
Comment at: lib/Transforms/InstCombine/InstCombineCompares.cpp:1960
+    if (X->getType()->isVectorTy())
+      CTy = VectorType::get(CTy, X->getType()->getVectorNumElements());
+    // Derivation for the ult case:
----------------
bryant wrote:
> efriedma wrote:
> > Isn't CTy always equal to "X->getType()"?
> Since we're morphing `(x << s) ugt c` into `x ugt (c >> s)`, CTy needs to be enough to hold `c >> s`.
> 
> `c`'s type would be sufficient thanks to the right shift.
> 
> But unlike other binops, shift nodes place no constraint between the types of shiftee and shift amount (I'm really just inferring from my memory of TargetSelectionDAG.td, so I could be wrong).
> 
> So X-getType() might not be wide enough to hold `c >> s`.
In IR the two operands of an shl always have the same type.  (The rules change once you get to SelectionDAG.)


Repository:
  rL LLVM

https://reviews.llvm.org/D25913