[ast-optimizer-branch]: Simplify STMT_EXPR's

[Diego Novillo]

On Thu, 02 May 2002, Daniel Berlin wrote:

> 
> Transforms stmt expressions like
> 
> a
> ({<....>})
> b
> 
> into
> 
> a
> {<....>}
> b
> 
> 
> Trying to transform those that return values doesn't always work.
> 
> 
> We generate what looks like correct code, i.e.:
> 
> 
> a
> b = ({<.....> <value>})
> 
> becomes
> 
> a
> {<....> <new temporary> = <value>}
> b = <new temporary>
> 
> 
> but during rtl expansion, it aborts failing the following test in 
> varasm.c:
> 
>   /* Check that we are not being given an automatic variable.  */
>   /* A weak alias has TREE_PUBLIC set but not the other bits.  */
>   if (TREE_CODE (decl) == PARM_DECL
>       || TREE_CODE (decl) == RESULT_DECL
>       || (TREE_CODE (decl) == VAR_DECL
>           && !TREE_STATIC (decl)
>           && !TREE_PUBLIC (decl)
>           && !DECL_EXTERNAL (decl)
>           && !DECL_REGISTER (decl)))
>     abort ();
> 
> 
> 
> So, until I figure out what to do, it only transforms those that do not 
> return values.
> 
> Index: c-simplify.c
> ===================================================================
> RCS file: /cvs/gcc/gcc/gcc/Attic/c-simplify.c,v
> retrieving revision 1.1.2.14
> diff -c -3 -p -w -B -b -r1.1.2.14 c-simplify.c
> *** c-simplify.c	1 May 2002 00:30:40 -0000	1.1.2.14
> --- c-simplify.c	2 May 2002 14:30:54 -0000
> *************** simplify_decl_stmt (t, post_p)
> *** 879,885 ****
>   	            post_p);
>   	  DECL_INITIAL (decl) = NULL_TREE;
>   	}
> ! 
>         prev = t;
>         t = TREE_CHAIN (t);
>       }
> --- 879,889 ----
>   	            post_p);
>   	  DECL_INITIAL (decl) = NULL_TREE;
>   	}
> !       else if (DECL_INITIAL (decl) 
> ! 	      && TREE_CODE_CLASS (TREE_CODE (DECL_INITIAL (decl))) == 's')
> !         {
> ! 	 simplify_stmt (DECL_INITIAL (decl));
> ! 	}
>         prev = t;
>         t = TREE_CHAIN (t);
>       }
Why are you leaving the DECL_INITIAL around?  We only leave
DECL_INITIALs in read-only constants.  Othwerise, we move the
initializer into the function body and simplify it later.  If you
leave the DECL_INITIAL here, you will have temporaries inside the
statement expression that will be referenced before their
DECL_STMT, which leads to the abort in varasm.c.

> *************** simplify_expr (expr, pre_p, post_p, simp
> *** 1049,1055 ****
> --- 1053,1123 ----
>          SIMPLE grammar.  */
>       case STMT_EXPR:
>         simplify_stmt (STMT_EXPR_STMT (expr));
> + 
> +       /* These tests determine if the STMT_EXPR returns a value. If it does, we need
> +          to store that value somewhere.  */
> +       if (TREE_CODE (STMT_EXPR_STMT (expr)) == COMPOUND_STMT
> +  	  && TREE_CODE (COMPOUND_BODY (STMT_EXPR_STMT (expr))) == SCOPE_STMT 
> + 	  && TREE_CODE (TREE_TYPE (expr)) != VOID_TYPE)
> + 	{
>
Please call a separate function.  The body of simplify_expr() is
hairy enough already.

[ ... ]
> + 	  if (TREE_CODE_CLASS (TREE_CODE (last_expr)) == 'e')
> + 	    {
> + 	      temp2 = build_modify_expr (tmp, NOP_EXPR, TREE_OPERAND (last_expr, 0));
> + 	      new_stmt = build_stmt (EXPR_STMT ,temp2);
> + 	      TREE_CHAIN (new_stmt) = TREE_CHAIN (temp);
> + 	      TREE_CHAIN (temp) = new_stmt;
> + 	    }
> + 	  else
> + 	    {
> + 	      temp2 = build_modify_expr (tmp, NOP_EXPR, last_expr);
> + 	      EXPR_STMT_EXPR (temp) = temp2;
> + 	    }

More concisely:

	temp2 = (is_simple_modify_expr (last_expr))
	        ? build_modify_expr (tmp, NOP_EXPR, TREE_OPERAND (last_expr, 0))
		: build_modify_expr (tmp, NOP_EXPR, last_expr);

Thanks.  Diego.