Re: [PATCH] c++: Fix up build_m_component_ref [PR113599]

[Patrick Palka <ppalka@redhat.com>]

On Thu, 25 Jan 2024, Jakub Jelinek wrote:

> Hi!
> 
> The following testcase reduced from GDB is miscompiled starting with
> r14-5503 PR112427 change.
> The problem is in the build_m_component_ref hunk, which changed
> -      datum = fold_build_pointer_plus (fold_convert (ptype, datum), component);
> +      datum = cp_convert (ptype, datum, complain);
> +      if (!processing_template_decl)
> +       datum = build2 (POINTER_PLUS_EXPR, ptype,
> +                       datum, convert_to_ptrofftype (component));
> +      datum = cp_fully_fold (datum);
> Component is e, (sizetype) e is 16, offset of c inside of C.
> ptype is A *, pointer to type of C::c and datum is &d.
> Now, previously the above created ((A *) &d) p+ (sizetype) e which is correct,
> but in the new code cp_convert sees that C has A as base class and
> instead of returning (A *) &d, it returns &d.D.2800 where D.2800 is
> the FIELD_DECL for the A base at offset 8 into C.
> So, instead of computing ((A *) &d) p+ (sizetype) e it computes
> &d.D.2800 p+ (sizetype) e, which is ((A *) &d) p+ 24.
> 
> The following patch fixes it by using convert instead of cp_convert which
> eventually calls build_nop (ptype, datum).

LGTM.  IIUC using 'convert' would work here too thanks to its special
case for indirect types, but I naively went with cp_convert since it has
a complain parameter :/

> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2024-01-25  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/113599
> 	* typeck2.cc (build_m_component_ref): Use convert instead of
> 	cp_convert for pointer conversion.
> 
> 	* g++.dg/expr/ptrmem11.C: New test.
> 
> --- gcc/cp/typeck2.cc.jj	2024-01-03 12:01:23.672476417 +0100
> +++ gcc/cp/typeck2.cc	2024-01-25 14:11:40.656361310 +0100
> @@ -2378,7 +2378,7 @@ build_m_component_ref (tree datum, tree
>        /* Build an expression for "object + offset" where offset is the
>  	 value stored in the pointer-to-data-member.  */
>        ptype = build_pointer_type (type);
> -      datum = cp_convert (ptype, datum, complain);
> +      datum = convert (ptype, datum);
>        if (!processing_template_decl)
>  	datum = build2 (POINTER_PLUS_EXPR, ptype,
>  			datum, convert_to_ptrofftype (component));
> --- gcc/testsuite/g++.dg/expr/ptrmem11.C.jj	2024-01-25 14:13:11.736089567 +0100
> +++ gcc/testsuite/g++.dg/expr/ptrmem11.C	2024-01-25 14:18:47.720398222 +0100
> @@ -0,0 +1,17 @@
> +// PR c++/113599
> +// { dg-do run }
> +
> +struct A { void *a; };
> +struct B { void *b; };
> +struct C : public B, public A { A c; };
> +static C d;
> +
> +int
> +main ()
> +{
> +  A C::*e = &C::c;
> +  A *f = &(d.*e);
> +  A *g = &d.c;
> +  if (f != g)
> +    __builtin_abort ();
> +}
> 
> 	Jakub
> 
>