Re: [PATCH] Don't add unnecessary var self-conflicts (PR tree-optimization/86214)

[Jeff Law <law@redhat.com>]

On 1/16/19 3:49 PM, Jakub Jelinek wrote:
> Hi!
> 
> While looking at this PR (I've just started) I've noticed that
> add_stack_var_conflict is quite often called with x == y.
> We don't really need to record that a variable conflicts with itself,
> the only reader of the conflicts bitmaps, stack_var_conflict_p,
> starts with
>   if (x == y)
>     return false;
> conflicts bitmap are set either by this function, or by the
>               EXECUTE_IF_SET_IN_BITMAP (work, 0, i, bi)
>                 {
>                   struct stack_var *a = &stack_vars[i];
>                   if (!a->conflicts)
>                     a->conflicts = BITMAP_ALLOC (&stack_var_bitmap_obstack);
>                   bitmap_ior_into (a->conflicts, work);
>                 }
> code (where work isn't derived from any conflicts bitmap though, so
> doesn't care if we've added those self-conflicts or not).  The above
> bitmap_ior_into stuff actually always sets self-conflicts (if you think
> bitmap_clear_bit is worth it, I can add it afterwards though).
> 
> But I think the following patch is helpful, don't create the conflicts
> bitmaps at all if all we'd record is just self-conflict which we'll ignore.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> 2019-01-16  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR tree-optimization/86214
> 	* cfgexpand.c (add_stack_var_conflict): Don't add any conflicts
> 	if x == y.
Yea.  Seems reasonable.

jeff