[PATCH] [InstCombine] Fix visitSwitchInst to use right operand types for sub cstexpr.

[Bruno Cardoso Lopes]

================
Comment at: lib/Transforms/InstCombine/InstructionCombining.cpp:2122
@@ +2121,3 @@
+              TruncCond ? ConstantInt::get(SI.getContext(),
+                                           CaseVal->getValue().sext(BitWidth))
+                        : CaseVal);
----------------
majnemer wrote:
> bruno wrote:
> > majnemer wrote:
> > > Is there any particular reason why you chose to sext instead of zext?
> > Being conservative to perform the right extension when coming back from the truncated negative condition cases. Would that be somehow redundant in this scenario?
> I don't think it will actually be faster.
> 
> `ConstantExpr::getSExt(CaseVal, Cond->getType())` will call `getFoldedCast` which will call `ConstantFoldCastInstruction` which will do the following:
>    case Instruction::SExt:
>      if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
>        uint32_t BitWidth = cast<IntegerType>(DestTy)->getBitWidth();
>        return ConstantInt::get(V->getContext(),
>                                CI->getValue().sext(BitWidth));
>      }
> 
> Seems like it will do the exact same thing.  No table lookups involved.
-----------
As I said before, it takes additional calls for every case statement. We don't need to bikeshed on that though, I guess that won't make that much difference either.

http://reviews.llvm.org/D6644

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