Re: [PATCH] Add LVAL argument to c_fully_fold* and propagate it through (PR c/66618, PR c/69960)

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Nov 08, 2017 at 05:42:07PM +0100, Marek Polacek wrote:
> > Not sure about the COND_EXPR/VEC_COND_EXPR cases, right now I'm passing
> > false as LVAL for the first operand (condition) and lval as LVAL for the
> > other two (i.e. if called with lval == true on the whole *_COND_EXPR
> > decl_constant_value_for_optimization etc. isn't performed on op1/op2, while
> > without it it is).  Can one take address of the whole COND_EXPR, or
> > have it on LHS of anything in C?  
> 
> I don't think so, the ?: operator does not yield an lvalue.

Ok, I'll pass false then.

> > @@ -218,12 +235,51 @@ c_fully_fold_internal (tree expr, bool i
> >        op2 = TREE_OPERAND (expr, 2);
> >        op3 = TREE_OPERAND (expr, 3);
> >        op0 = c_fully_fold_internal (op0, in_init, maybe_const_operands,
> > -				   maybe_const_itself, for_int_const);
> > +				   maybe_const_itself, for_int_const, lval);
> >        STRIP_TYPE_NOPS (op0);
> >        op1 = c_fully_fold_internal (op1, in_init, maybe_const_operands,
> > -				   maybe_const_itself, for_int_const);
> > +				   maybe_const_itself, for_int_const, false);
> >        STRIP_TYPE_NOPS (op1);
> > -      op1 = decl_constant_value_for_optimization (op1);
> > +      /* Fold "foo"[2] in initializers.  */
> > +      if (!lval
> > +	  && in_init
> > +	  && TREE_CODE (op0) == STRING_CST
> > +	  && TREE_CODE (op1) == INTEGER_CST
> > +	  && TREE_CODE (TREE_TYPE (op0)) == ARRAY_TYPE
> 
> Does this also handle 2["foobar"]?

Yes, because build_array_ref has:
2607	  if (TREE_CODE (TREE_TYPE (array)) != ARRAY_TYPE
2608	      && TREE_CODE (TREE_TYPE (array)) != POINTER_TYPE
2609	      /* Allow vector[index] but not index[vector].  */
2610	      && !VECTOR_TYPE_P (TREE_TYPE (array)))
2611	    {
2612	      if (TREE_CODE (TREE_TYPE (index)) != ARRAY_TYPE
2613		  && TREE_CODE (TREE_TYPE (index)) != POINTER_TYPE)
2614		{
2615	          error_at (loc,
2616	            "subscripted value is neither array nor pointer nor vector");
2617	
2618		  return error_mark_node;
2619		}
2620	      std::swap (array, index);
2621	      swapped = true;
2622	    }


> > +	  && tree_fits_uhwi_p (op1))
> > +	{
> > +	  tree ary = op0;
> > +	  tree index = op1;
> > +	  unsigned len = 0;
> > +	  tree elem_type = TREE_TYPE (TREE_TYPE (ary));
> > +	  unsigned elem_nchars = (TYPE_PRECISION (elem_type)
> > +				  / TYPE_PRECISION (char_type_node));
> > +	  len = (unsigned) TREE_STRING_LENGTH (ary) / elem_nchars;
> > +
> > +	  tree nelts = array_type_nelts (TREE_TYPE (ary));
> > +	  bool dummy1 = true, dummy2 = true;
> > +	  nelts = c_fully_fold_internal (nelts, in_init, &dummy1, &dummy2,
> > +					 for_int_const, false);
> > +	  unsigned HOST_WIDE_INT i = tree_to_uhwi (index);
> > +	  if (tree_int_cst_le (index, nelts)
> > +	      && i < len
> > +	      && i + elem_nchars <= len)
> > +	    {
> > +	      tree type = TREE_TYPE (expr);
> > +	      if (elem_nchars == 1)
> > +		ret = build_int_cst (type,
> > +				     TREE_STRING_POINTER (ary)[i]);
> > +	      else
> > +		{
> > +		  const unsigned char *ptr
> > +		    = ((const unsigned char *)TREE_STRING_POINTER (ary)
> > +		       + i * elem_nchars);
> > +		  ret = native_interpret_expr (type, ptr, elem_nchars);
> > +		}
> > +	      if (ret)
> > +		goto out;
> > +	    }
> > +	}
> 
> So this code comes from gimple-fold.c?  I would probably move it to a new
> function.

No, this comes from very simplified cp/constexpr.c (extract_string_elt,
cxx_eval_array_reference).  Can outline it into a separate function, sure.

	Jakub