> Ah - tree_ssa_useless_type_conversion and callers, during gimplification.
> I'd like to get rid of it but maybe simply delete the VIEW_CONVERT_EXPR
> case from it for now (and return true unconditionally for NON_LVALUE_EXPR).
> 
> Index: gcc/tree-ssa.c
> ===================================================================
> --- gcc/tree-ssa.c      (revision 229517)
> +++ gcc/tree-ssa.c      (working copy)
> @@ -1142,13 +1161,16 @@ delete_tree_ssa (struct function *fn)
>  bool
>  tree_ssa_useless_type_conversion (tree expr)
>  {
> +  /* Not strictly a conversion but this function is used to strip
> +     useless stuff from trees returned from GENERIC folding.  */
> +  if (TREE_CODE (expr) == NON_LVALUE_EXPR)
> +    return true;
> +
>    /* If we have an assignment that merely uses a NOP_EXPR to change
>       the top of the RHS to the type of the LHS and the type conversion
>       is "safe", then strip away the type conversion so that we can
>       enter LHS = RHS into the const_and_copies table.  */
> -  if (CONVERT_EXPR_P (expr)
> -      || TREE_CODE (expr) == VIEW_CONVERT_EXPR
> -      || TREE_CODE (expr) == NON_LVALUE_EXPR)
> +  if (CONVERT_EXPR_P (expr))
>      return useless_type_conversion_p
>        (TREE_TYPE (expr),
>         TREE_TYPE (TREE_OPERAND (expr, 0)));

The patch introduces GIMPLE checking failures:

FAIL:   c52103m
FAIL:   c52103r
FAIL:   c52104m
FAIL:   c52104r

of the form:

slice9.adb: In function 'Slice9':
slice9.adb:1:1: error: conversion of an SSA_NAME on the left hand side
VIEW_CONVERT_EXPR<character[D.4195:iftmp.10]>("ABCDE");

D.4379 = &VIEW_CONVERT_EXPR<character[D.4195:iftmp.10]>("ABCDE")[D.4378 ...]
{lb: D.4195 sz: 1};

      if (TREE_CODE (expr) == VIEW_CONVERT_EXPR)
	{
	  /* For VIEW_CONVERT_EXPRs which are allowed here too, we only check
	     that their operand is not an SSA name or an invariant when
	     requiring an lvalue (this usually means there is a SRA or IPA-SRA
	     bug).  Otherwise there is nothing to verify, gross mismatches at
	     most invoke undefined behavior.  */
	  if (require_lvalue
	      && (TREE_CODE (op) == SSA_NAME
		  || is_gimple_min_invariant (op)))
	    {
	      error ("conversion of an SSA_NAME on the left hand side");
	      debug_generic_stmt (expr);
	      return true;
	    }
	  else if (TREE_CODE (op) == SSA_NAME
		&& TYPE_SIZE (TREE_TYPE (expr)) != TYPE_SIZE (TREE_TYPE (op)))
	    {
	      error ("conversion of register to a different size");
	      debug_generic_stmt (expr);
	      return true;
	    }
	  else if (!handled_component_p (op))
	    return false;
	}

It's related to dynamic slicing (reduced testcase attached).


	* gnat.dg/slice9.adb: New test.

-- 
Eric Botcazou