Re: [PATCH PR43513, 1/3] Replace vla with array - Implementation.

[Richard Guenther <rguenther at suse dot de>]

On Wed, 27 Jul 2011, Tom de Vries wrote:

> On 07/27/2011 01:50 PM, Tom de Vries wrote:
> > Hi Richard,
> > 
> > I have a patch set for bug 43513 - The stack pointer is adjusted twice.
> > 
> > 01_pr43513.3.patch
> > 02_pr43513.3.test.patch
> > 03_pr43513.3.mudflap.patch
> > 
> > The patch set has been bootstrapped and reg-tested on x86_64.
> > 
> > I will sent out the patches individually.
> > 
> 
> The patch replaces a vla __builtin_alloca that has a constant argument with an
> array declaration.
> 
> OK for trunk?

I don't think it is safe to try to get at the VLA type the way you do.
In fact I would simply do sth like

  elem_type = build_nonstandard_integer_type (BITS_PER_UNIT, 1);
  n_elem = size * 8 / BITS_PER_UNIT;
  array_type = build_array_type_nelts (elem_type, n_elem);
  var = create_tmp_var (array_type, NULL);
  return fold_convert (TREE_TYPE (lhs), build_fold_addr_expr (var));

And obviously you lose the optimization we arrange with inserting
__builtin_stack_save/restore pairs that way - stack space will no
longer be shared for subsequent VLAs.  Which means that you'd
better limit the size you allow this promotion.

Alternatively this promotion could happen alongsize 
optimize_stack_restore using more global knowledge of the effects
on the maximum stack size this folding produces.

Thanks,
Richard.