Re: [PATCH] Extend store ccp

[Diego Novillo <dnovillo at redhat dot com>]

Revital1 Eres wrote on 03/23/07 01:26:

> This patch enhance store ccp to propagate constants from stores to
> loads when the previous implementation failed due to alias uncertainty.
> The testcase attached demonstrates this optimization:
> 
> S[5].x = 0;
> S[5].y = 0;
> 
> x = S[5].x;
> y = S[5].y;
> 
> Previously x = S[5].x; failed to be replaced with x = 0.  This patch
> fixes this problem by allowing the propagation of constants if all the
> stores in the alias group are initialized with the same constant.
> 
We could do one better.  While following the use-def chain, we could ask if the LHS of the assignment overlaps the RHS that we are trying to replace.  So:

     1	# S_3 = VDEF <S_1>
     2	S[5].x = 0;
     3	
     4	# S_4 = VDEF <S_3>
     5	S[5].y = x_9;
     6	
     7	# VUSE <S_4>
     8	x = S[5].x;
     9	
    10	# VUSE <S_4>
    11	y = S[5].y;

>From statement #8 you go to statement #4, since a store to S[5].y does
not affect S[5].x (we can use nonoverlapping_component_refs_p as a
starting point or access_can_touch_variable, for instance).

This will improve both copy and constant propagation.

Another problem with the patch is that you are doing this too late.
Why not do this *during* propagation?  In this case, you would allow S_4 to assume the lattice value of 0 and use it during propagation.

> +/* Return the defining stmt for the rhs of STMT following the virtual
> +   use-def chains if all the VDEFs of this rhs up until the defining stmt
> +   initialized by the same constant value as CONST_VAL.  Returns the
> +   MODIFY_EXPR stmt which lhs is equal to the rhs of STMT or NULL_TREE
> +   if no such stmt can be found.  */
> +static tree
> +get_def_of_stmt_rhs (tree stmt, tree const_val)

Vertical spacing.

> +static bool
> +do_store_ccp (void)
> +{

Missing documentation.

> +  return (flag_tree_store_ccp != 0 && flag_tree_ccp != 0);
> +}
> +
> +/* Return true if rhs of STMT has a known VUSE and all
> +   it's VDEFs in the virtual use-def chain (up until the insn

s/it's/its/

> +   which created it's SSA_NAME) have the same constant value as

Likewise.

> +   recorded in VAL.  */
> +bool
> +all_vdef_have_same_lattice_const_value (prop_value_t* val, tree stmt)

Vertical spacing after comment.

>  bool
> -substitute_and_fold (prop_value_t *prop_value, bool use_ranges_p)
> +substitute_and_fold (prop_value_t *prop_value, bool use_ranges_p,
> +                     bool aggressive_store_ccp_p)

The aggressive_store_ccp_p argument is not necessary.