[llvm] r191090 - InstCombine: Canonicalize (gep i8* X, -(ptrtoint Y)) to (sub (ptrtoint X), (ptrtoint Y))

[Benjamin Kramer]

On 21.09.2013, at 01:28, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Fri, Sep 20, 2013 at 12:12 PM, Benjamin Kramer <benny.kra at gmail.com> wrote:
> 
> On 20.09.2013, at 20:59, Eli Friedman <eli.friedman at gmail.com> wrote:
> 
> > On Fri, Sep 20, 2013 at 7:38 AM, Benjamin Kramer <benny.kra at googlemail.com> wrote:
> > Author: d0k
> > Date: Fri Sep 20 09:38:44 2013
> > New Revision: 191090
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=191090&view=rev
> > Log:
> > InstCombine: Canonicalize (gep i8* X, -(ptrtoint Y)) to (sub (ptrtoint X), (ptrtoint Y))
> >
> > The GEP pattern is what SCEV expander emits for "ugly geps". The latter is what
> > you get for pointer subtraction in C code. The rest of instcombine already
> > knows how to deal with that so just canonicalize on that.
> >
> >
> >
> > Is this really a good idea?  We lose aliasing information from turning the GEP into a sub, and it seems like it could easily trigger on regular C++ code.  (If the only user of the GEP is a ptrtoint, it's a different story, but you're not checking that.)
> 
> I was thinking the same first, but does subtracting two pointers and using the result as a pointer really make sense? I can't see how that would be useful.
> 
> 
> No, but just because we see a ptrtoint doesn't mean Y is actually represents a pointer.

And how do non-pointers disguised as pointers affect aliasing? ;)

- Ben