TempAlloc: an unusual request

[Andrei Alexandrescu]

On 6/19/11 5:03 AM, Lars T. Kyllingstad wrote:
> On Sat, 18 Jun 2011 09:00:13 -0500, Andrei Alexandrescu wrote:
>
>> I'd like to kindly request the author, the review manager, and the
>> community that TempAlloc skips this round of reviews without inclusion
>> in Phobos, to be reviewed again later.
>
> As review manager, I don't mind postponing the review until some later
> time.
>
> As a community member and Phobos user, I think it would of course be
> preferable if TempAlloc fits into a more general allocator interface.
>
> As an active TempAlloc user, I hope it doesn't take too long before said
> interface is decided upon. ;)
>
> -Lars

I'm thinking of defining a general interface that catches malloc, the 
GC, scoped allocators a la TempAlloc, and possibly compound allocators a 
la reaps.

I'll start with an example:

struct Mallocator
{
     /** Allocates a chunk of size s. */
     static void * allocate(size_t s) {
         return enforce(malloc(s), new OutOfMemory);
     }

     /** Optional: frees a chunk allocated with allocate */
     static void free(void *p) {
         .free(p);
     }

     /** Optional: frees all chunks allocated with allocate */
     // static void freeAll();

     /** Resizes a chunk allocated with allocate without moving.
         Required, but may be implemented to always return false. */
     static bool resize(void* p, size_t newSize) {
         // Can't use realloc here
         return false;
     }

     /** true if calls to free enforce the pointer is valid */
     enum { freeIsChecked = false }

     /** Returns the guaranteed alignment for allocation requests
         of size s. Must be computable during compilation. */
     size_t align(size_t s) { return (void*).sizeof; }

     /** Returns the size effectively allocated if an allocation
         of size s were requested. Must be computable during
         compilation.*/
     static size_t allocSize(size_t s) {
         return ((s + align(s) - 1) / align(s)) * align(s);
     }

     /** true if memory is automatically reclaimed (GC) */
     enum { isAutomatic = false }

     /** true if allocator is scoped, i.e. all memory allocated
         goes away when the original allocator goes out of scope */
     enum { isScoped = false }
}

This is a symbolic interface. Some or all of the functions above may be 
static. I think, without being yet positive, that the allocator should 
have reference semantics, i.e. all copies of a given allocator should 
manipulate the same memory. This should reduce unwitting errors caused 
e.g. by passing an allocator by reference instead of by value.

For TempAlloc that means the primitives frameInit and frameFree are 
replaced with an object, e.g. RegionAllocator. That object and all of 
its copies manage a given frame. (BTW I think the name "region" is 
consecrated and should replace "frame" throughout in TempAlloc. In fact 
TempAlloc should be renamed to RegionAlloc.)

With this interface at hand I think we can tackle a variety of 
heavy-duty allocation tasks in a relatively abstract manner. In 
particular, we can compose several allocation strategies (e.g. a heap on 
top of a region). A simpler task would be e.g. to define an aligned 
allocator on top of an unaligned one.


Andrei