[llvm-commits] InstCombining, small bug alloca + bitcast

[Stepan Dyatkovskiy]

Hi Duncan. Well... now I see that this "if" + old test (that by some 
strange reasons uses nsw instead of nuw) looks really strange. Let me 
some time, I either commented well this function or try to change this 
"if" with some way.

-Stepan.

Duncan Sands wrote:
> Hi Stepan,
>
>> I also added some usefull checks to test and fixed InstCombineCasts.cpp.
>
> what was wrong with InstCombineCasts?  In fact the existing check
>
>       // Cannot look past anything that might overflow.
>       OverflowingBinaryOperator *OBI = dyn_cast<OverflowingBinaryOperator>(Val);
>       if (OBI&&  !OBI->hasNoUnsignedWrap()) {
>         Scale = 1;
>         Offset = 0;
>         return Val;
>       }
>
> looks bogus to me: without it, you correctly get Scale and Offset such that
> Val = X*Scale + Offset, which is what the routine is supposed to do according
> to the comment describing it.
>
> So if this check is really needed, it must be because of some undocumented
> semantics in how the routine is supposed to behave.  What are these semantics?
> They should be documented.  Once it is clear what this routine is supposed to
> be doing, it should become clear whether your change is correct or not.
>
> Ciao, Duncan.
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