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


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.

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?

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



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