Re: [PATCH] PR fortran/31593: Speed up loops with DO variables as procedure arguments

[Richard Henderson <rth at redhat dot com>]

> +      if (gfc_is_do_var (expr->symtree->n.sym)
> +	  && !expr->symtree->n.sym->attr.target)
> +	{
> +	  se->want_pointer = 0;
> +	  gfc_conv_expr (se, expr);
> +	  var = gfc_create_var (TREE_TYPE (se->expr), "arg");
> +	  gfc_add_modify (&se->pre, var, se->expr);
> +	  se->expr = gfc_build_addr_expr (NULL_TREE, var);
> +	  gcc_assert (!se->post.head);
> +	  return;
> +

This appears to create an arg variable for each instance of the do 
variable being passed to a call.  That would be wasteful in terms of 
stack usage.

Better would be to create exactly one arg variable per do variable that 
needs copying.  And further, for those arg variables to be properly 
scoped inside the loop, so that arg variables from different loop nests 
can share stack space (see partition_stack_vars).

Further, since it's illegal for the called function to modify the loop 
variable, it ought to be illegal for it to modify the stack copy of the 
loop variable.  Thus it should be possible to write to the copy exactly 
once, at the beginning of the loop iteration.  Ideally the store would 
be positioned at the nearest block that dominates all of the uses, but 
that's obviously hard to do from the front end, and I don't recall that 
the the optimizers are set up to sink a memory store in that direction.


r~