Re: [PATCH] Fix gimplification of const var initialization from COND_EXPR (PR c++/80129)

[Richard Biener <rguenther at suse dot de>]

On Tue, 21 Mar 2017, Jakub Jelinek wrote:

> Hi!
> 
> For non-gimple reg types var = x ? y : z; is gimplified as
> x ? (var = y) : (var = z);.  The problem is that if var is TREE_READONLY,
> we can gimplify one of those assignments by promoting them to TREE_STATIC
> (if not addressable) and making the initializer into their DECL_INITIAL.
> That doesn't work well, because then we conditionally store into that and
> during optimizations, as it is a TREE_READONLY var with DECL_INITIAL, just
> use that initializer for optimizations.
> 
> The following patch fixes that by clearing TREE_READONLY flag if we are
> going to store it more than once.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2017-03-21  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/80129
> 	* gimplify.c (gimplify_modify_expr_rhs) <case COND_EXPR>: Clear
> 	TREE_READONLY on result if writing it more than once.
> 
> 	* g++.dg/torture/pr80129.C: New test.
> 
> --- gcc/gimplify.c.jj	2017-03-21 07:56:55.000000000 +0100
> +++ gcc/gimplify.c	2017-03-21 13:37:45.555612652 +0100
> @@ -5098,6 +5098,13 @@ gimplify_modify_expr_rhs (tree *expr_p,
>  	      if (ret != GS_ERROR)
>  		ret = GS_OK;
>  
> +	      /* If we are going to write RESULT more than once, clear
> +		 TREE_READONLY flag, otherwise we might gimplify it
> +		 incorrectly.  */
> +	      if (DECL_P (result)

Can it ever be that result is sth like decl.component?  At least I
don't see GENERIC restricted in a way that it cannot.  (and generally
assigning to sth TREE_READONLY is fishy at best...)

I see how this fixes the problem at hand (and in a safe way), so the
patch is probably ok, just looking how to avoid this situation in
a more general (and better) way.

> +		  && TREE_TYPE (TREE_OPERAND (cond, 1)) != void_type_node
> +		  && TREE_TYPE (TREE_OPERAND (cond, 2)) != void_type_node)
> +		TREE_READONLY (result) = 0;
>  	      if (TREE_TYPE (TREE_OPERAND (cond, 1)) != void_type_node)
>  		TREE_OPERAND (cond, 1)
>  		  = build2 (code, void_type_node, result,
> --- gcc/testsuite/g++.dg/torture/pr80129.C.jj	2017-03-21 13:40:04.179852313 +0100
> +++ gcc/testsuite/g++.dg/torture/pr80129.C	2017-03-21 13:41:32.121735570 +0100
> @@ -0,0 +1,14 @@
> +// PR c++/80129
> +// { dg-do run }
> +// { dg-options "-std=c++11" }
> +
> +struct A { bool a; int b; };
> +
> +int
> +main ()
> +{
> +  bool c = false;
> +  const A x = c ? A {true, 1} : A {false, 0};
> +  if (x.a)
> +    __builtin_abort ();
> +}
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)