Re: [PATCH] irange_pool class

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2020-09-17 at 12:36 +0200, Aldy Hernandez via Gcc-patches
wrote:
> This is the irange storage class.  It is used to allocate the
> minimum 
> amount of storage needed for a given irange.  Storage is
> automatically 
> freed at destruction.
> 
> It is meant for long term storage, as opposed to int_range_max which
> is 
> meant for intermediate temporary results on the stack.
> 
> The general gist is:
> 
> 	irange_pool pool;
> 
> 	// Allocate an irange of 5 sub-ranges.
> 	irange *p = pool.allocate (5);
> 
> 	// Allocate an irange of 3 sub-ranges.
> 	irange *q = pool.allocate (3);
> 
> 	// Allocate an irange with as many sub-ranges as are currently
> 	// used in "some_other_range".
> 	irange *r = pool.allocate (some_other_range);

FWIW my first thoughts reading this example were - "how do I deallocate
these iranges?" and "do I need to call pool.deallocate on them, or is
that done for me by the irange dtor?"

I think of a "pool allocator" as something that makes a small number of
large allocation under the covers, and then uses that to serve large
numbers of fixed sized small allocations and deallocations with O(1)
using a free list.

[...]

> +// This is the irange storage class.  It is used to allocate the
> +// minimum amount of storage needed for a given irange.  Storage is
> +// automatically freed at destruction.

"at destruction" of what object - the irange or the irange_pool? 
Reading the code, it turns out to be "at destruction of the
irange_pool", and it turns out that irange_pool is an obstack under the
covers (also called a "bump allocator") and thus, I believe, the
lifetime of the irange instances is that of the storage instance.

I think it would be clearer to name this "irange_obstack", or somesuch.

> +//
> +// It is meant for long term storage, as opposed to int_range_max
> +// which is meant for intermediate temporary results on the stack.
> +
> +class irange_pool
> +{
> +public:
> +  irange_pool ();
> +  ~irange_pool ();
> +  // Return a new range with NUM_PAIRS.
> +  irange *allocate (unsigned num_pairs);
> +  // Return a copy of SRC with the minimum amount of sub-ranges
> needed
> +  // to represent it.
> +  irange *allocate (const irange &src);
> +private:
> +  struct obstack irange_obstack;

...and thus to rename this field to "m_obstack" or similar.

[...]

Hope this is constructive
Dave