Re: [PATCH] v5: C++: more location wrapper nodes (PR c++/43064, PR c++/43486)

[David Malcolm <dmalcolm@redhat.com>]

On Mon, 2018-12-17 at 14:33 -0500, Jason Merrill wrote:
> On 12/14/18 7:17 PM, David Malcolm wrote:
> > +      /* Since default args are effectively part of the function
> > type,
> > +	 strip location wrappers here, since otherwise the
> > location of
> > +	 one function's default arguments is arbitrarily chosen
> > for
> > +	 all functions with similar signature (due to
> > canonicalization
> > +	 of function types).  */
> 
> Hmm, looking at this again, why would this happen?  I see that 
> type_list_equal uses == to compare default arguments, so two
> function 
> types with the same default argument but different location wrappers 
> shouldn't be combined.
> 
> Jason

Thanks.

I did some digging into this.  I added this strip to fix
  g++.dg/template/defarg6.C
but it looks like I was overzealous (the comment is correct, but it's
papering over a problem).

It turns out that type_list_equal is doing more than just pointer
equality; it's hitting the simple_cst_equal part of the && at line
7071:

7063	bool
7064	type_list_equal (const_tree l1, const_tree l2)
7065	{
7066	  const_tree t1, t2;
7067	
7068	  for (t1 = l1, t2 = l2; t1 && t2; t1 = TREE_CHAIN (t1), t2 = TREE_CHAIN (t2))
7069	    if (TREE_VALUE (t1) != TREE_VALUE (t2)
7070		|| (TREE_PURPOSE (t1) != TREE_PURPOSE (t2)
7071		    && ! (1 == simple_cst_equal (TREE_PURPOSE (t1), TREE_PURPOSE (t2))
7072			  && (TREE_TYPE (TREE_PURPOSE (t1))
7073			      == TREE_TYPE (TREE_PURPOSE (t2))))))
7074	      return false;
7075	
7076	  return t1 == t2;
7077	}

What's happening is that there are two different functions with
identical types apart from the locations of their (equal) default
arguments: both of the TREE_PURPOSEs are NON_LVALUE_EXPR wrappers
around a CONST_DECL enum value (at different source locations).

simple_cst_equal is stripping the location wrappers here:

7311	  if (CONVERT_EXPR_CODE_P (code1) || code1 == NON_LVALUE_EXPR)
7312	    {
7313	      if (CONVERT_EXPR_CODE_P (code2)
7314		  || code2 == NON_LVALUE_EXPR)
7315		return simple_cst_equal (TREE_OPERAND (t1, 0), TREE_OPERAND (t2, 0));
7316	      else
7317		return simple_cst_equal (TREE_OPERAND (t1, 0), t2);
7318	    }

and thus finds them to be equal; the iteration in type_list_equal
continues, and runs out of parameters with t1 == t2 == NULL, and thus
returns true, and thus the two function types hash to the same slot,
and the two function types get treated as being the same.

It's not clear to me yet what the best solution to this is:
- should simple_cst_equal regard different source locations as being
different?
- should function-type hashing use a custom version of type_list_equal
when comparing params, and make different source locations of default
args be different?
- something else?

Dave