Re: determining aggregate member from MEM_REF

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Feb 15, 2018 at 6:28 PM, Martin Sebor <msebor@gmail.com> wrote:
> There are APIs to determine the base object and an offset
> into it from all sorts of expressions, including ARRAY_REF,
> COMPONENT_REF, and MEM_REF, but none of those I know about
> makes it also possible to discover the member being referred
> to.
>
> Is there an API that I'm missing or a combination of calls
> to some that would let me determine the (approximate) member
> and/or element of an aggregate from a MEM_REF expression,
> plus the offset from its beginning?
>
> Say, given
>
>   struct A
>   {
>      void *p;
>      char b[3][9];
>   } a[2];
>
> and an expression like
>
>   a[1].b[2] + 3
>
> represented as the expr
>
>   MEM_REF (char[9], a, 69)

 &MEM_REF (&a, 69)

you probably mean.

> where offsetof (struct A, a[1].b[2]) == 66
>
> I'd like to be able to determine that expr refers to the field
> b of struct A, and more specifically, b[2], plus 3.  It's not
> important what the index into the array a is, or any other
> arrays on the way to b.

There is code in initializer folding that searches for a field in
a CONSTRUCTOR by base and offset.  There's no existing
helper that gives you exactly what you want -- I guess you'd
ideally want to have a path to the refered object.  But it may
be possible to follow what fold_ctor_reference does and build
such a helper.

> I realize the reference can be ambiguous in some cases (arrays
> of structs with multiple array members) and so the result wouldn't
> be guaranteed to be 100% reliable.  It would only be used in
> diagnostics.  (I think with some effort the type of the MEM_REF
> could be used to disambiguate the majority (though not all) of
> these references in practice.)

Given you have the address of the MEM_REF in your example above
the type of the MEM_REF doesn't mean anything.

I think ambiguity only happens with unions given MEM_REF offsets
are constant.

Note that even the type of 'a' might not be correct as it may have had
a different dynamic type.

So not sure what context you are trying to use this in diagnostics.

Richard.


>
> Thanks
> Martin