[PATCH] look harder for MEM_REF operand equality to avoid -Wstringop-truncation (PR 84561)

[Richard Biener]

On Thu, Aug 30, 2018 at 7:39 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 08/30/2018 11:22 AM, Richard Biener wrote:
> > On August 30, 2018 6:54:21 PM GMT+02:00, Martin Sebor <msebor@gmail.com> wrote:
> >> On 08/30/2018 02:35 AM, Richard Biener wrote:
> >>> On Thu, Aug 30, 2018 at 2:12 AM Martin Sebor <msebor@gmail.com>
> >> wrote:
> >>>>
> >>>> The attached patch adds code to work harder to determine whether
> >>>> the destination of an assignment involving MEM_REF is the same
> >>>> as the destination of a prior strncpy call.  The included test
> >>>> case demonstrates when this situation comes up.  During ccp,
> >>>> dstbase and lhsbase returned by get_addr_base_and_unit_offset()
> >>>> end up looking like this:
> >>>
> >>> "During CCP" means exactly when?  The CCP lattice tracks copies
> >>> so CCP should already know that _1 == _8.  I suppose during
> >>> substitute_and_fold then?  But that replaces uses before folding
> >>> the stmt.
> >>
> >> Yes, when ccp_finalize() performs the final substitution during
> >> substitute_and_fold().
> >
> > But then you shouldn't need the loop but at most look at the pointer SSA Def to get at the non-invariant ADDR_EXPR.
>
> I don't follow.   Are you suggesting to compare
> SSA_NAME_DEF_STMT (dstbase) to SSA_NAME_DEF_STMT (lhsbase) for
> equality?  They're not equal.

No.

> The first loop iterates once and retrieves
>
>    1.  _8 = &pb_3(D)->a;
>
> The second loop iterates three times and retrieves:
>
>    1.  _1 = _9
>    2.  _9 = _8
>    3.  _8 = &pb_3(D)->a;
>
> How do I get from _1 to &pb_3(D)->a without iterating?  Or are
> you saying to still iterate but compare the SSA_NAME_DEF_STMT?

I say you should retrieve _8 = &pb_3(D)->a immediately since the
copies should be
propagated out at this stage.

> Martin
>
> >
> > Richard.
> >
> >> Martin
> >>
> >>>
> >>> So I'm confused.
> >>>
> >>>>
> >>>>    _8 = &pb_3(D)->a;
> >>>>    _9 = _8;
> >>>>    _1 = _9;
> >>>>    strncpy (MEM_REF (&pb_3(D)->a), ...);
> >>>>    MEM[(struct S *)_1].a[n_7] = 0;
> >>>>
> >>>> so the loops follow the simple assignments until we get at
> >>>> the ADDR_EXPR assigned to _8 which is the same as the strncpy
> >>>> destination.
> >>>>
> >>>> Tested on x86_64-linux.
> >>>>
> >>>> Martin
> >
>