[llvm-dev] Normalize a SCEV Expression

[Stefanos Baziotis via llvm-dev]

Indeed you're right, thanks! I need a nuw (which I have to invent, with a
run-time check, since it doesn't exist in the original but anyway).

Best,
Stefanos

Στις Παρ, 31 Ιουλ 2020 στις 12:36 μ.μ., ο/η Florian Hahn <
florian_hahn at apple.com> έγραψε:

>
>
> > On Jul 30, 2020, at 21:03, Stefanos Baziotis via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > I have a SCEV like this: {16,+,8}. Say that this came from a loop like:
> > int64_t *p;
> > for (int64_t i = 0; i < ...; ++i)
> >   p[i+2]...
> >
> > And assuming that I'm on a 64-bit machine. What I would like to do is
> normalize it
> > like that, basically this: {2,+,1} i.e. map it to the index.
> >
> > Now, I tried to get the underlying element size of the pointer, then
> getUDivExpr(OriginalSCEV, ElementSize); But I don't get the desired SCEV
> back. Instead I'm getting:
> > {16,+,8} \u 8, i.e. just adding udiv in the original expression.
> >
>
>
> I guess the problem here is that the AddRec is missing overflow flags,
> which means the expression may overflow and if that happens, the folded
> expression may not be equivalent to the original one in all cases.
>
> If the AddRec has the overflow flags set, it looks something like
> `{16,+,8}<nuw><nsw><%loop>`. I’d check if the original IR had nuw/nsw flags
> on add instruction for the AddRec. The way those flags are managed in SCEV
> are sometimes a bit surprising, because the expressions are not tied to a
> specific location and the flags need to be valid for the whole scope the
> expression can be evaluated in.
>
> Cheers,
> Florian
>
>
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