Re: Silecne array bounds warnings in duplicated code

[Richard Biener <rguenther at suse dot de>]

On Thu, 11 Oct 2012, Jan Hubicka wrote:

> Hi,
> this patch address problem I run into with strenghtened cunroll pass.  I made
> cunroll to use loop_max_iterations bounds into an account that makes us to
> occasionally produce out of bounds loop accesses in loop like:
> 
> for (i=0;i<n;i++)
> {
>   <something>
>   if (test)
>     break
>   a[i]=1;
> }
> Here the constantly sized array appears in loop with multiple exits and we then
> in the last iteration produce out of bound array (since we need to duplicate 
> <something> and tree-ssa-vrp warns.
> 
> I think this is more generic problem (though appearing rarely): as soon as we
> specialize the code, we can no longer warn about out of bounds accesses when we
> prove array ref to be constant.  We have no idea if this code path will execute
> in practice.
> 
> Seems resonable?  I think similar problems can be constructed by inlining, too.
> Regtested/bootstrapped x86_64-linux.

No, I don't think this is reasonable.  There are other PRs for this
as well, btw.

The array-bounds warning in VRP needs to be conditional if the
path to it isn't always executed, too (thus a may-be warning).

Richard.

> Honza
> 
> 	* tree-cfg.c (gimple_duplicate_bb): Drop NO_WARNING on array refs.
> Index: tree-cfg.c
> ===================================================================
> *** tree-cfg.c	(revision 192360)
> --- tree-cfg.c	(working copy)
> *************** gimple_duplicate_bb (basic_block bb)
> *** 5504,5509 ****
> --- 5504,5519 ----
>         if (lhs && TREE_CODE (lhs) != SSA_NAME)
>   	{
>   	  tree base = get_base_address (lhs);
> + 	  tree op = lhs;
> +
> +	  /* Silence the array bounds warning.  After specializing the code we
> +	     no longer know if the particular code path can execute at runtime.  */
> + 	  while (handled_component_p (op))
> + 	    {
> + 	      if (TREE_CODE (op) == ARRAY_REF)
> + 		TREE_NO_WARNING (op) = 1;
> + 	      op =  TREE_OPERAND (op, 0);
> +   	    }
>   	  if (base
>   	      && (TREE_CODE (base) == VAR_DECL
>   		  || TREE_CODE (base) == RESULT_DECL)
> *************** gimple_duplicate_bb (basic_block bb)
> *** 5514,5519 ****
> --- 5521,5539 ----
>   		  || !DECL_HAS_VALUE_EXPR_P (base)))
>   	    DECL_NONSHAREABLE (base) = 1;
>   	}
> +
> +	/* Silence the array bounds warning.  After specializing the code we
> +	   no longer know if the particular code path can execute at runtime.  */
> +       if (gimple_code (stmt) == GIMPLE_ASSIGN)
> + 	{
> + 	  tree op = gimple_assign_rhs1 (stmt);
> + 	  while (handled_component_p (op))
> + 	    {
> + 	      if (TREE_CODE (op) == ARRAY_REF)
> + 		TREE_NO_WARNING (op) = 1;
> + 	      op =  TREE_OPERAND (op, 0);
> +   	    }
> + 	}
>   
>         /* Create new names for all the definitions created by COPY and
>   	 add replacement mappings for each new name.  */
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE / SUSE Labs
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