TempAlloc review starts now

[Lars T. Kyllingstad]

All right, folks, it's time to get the review queue started again.  First 
up is David Simcha's TempAlloc, which, if accepted, is to be included in 
the core.memory module in druntime.

If there are no objections, I suggest we use 3 weeks (until 27 June) for 
the review, followed by one week for voting.  Please post reviews in this 
thread.

Here follows a description of TempAlloc's purpose and functionality.  
Links to the code and documentation are at the bottom.

-Lars


David Simcha wrote:

TempAlloc is a thread-local segmented stack memory allocator
(defined/detailed in the docs) for efficiently allocating temporary
buffers, matrices, etc.  It has the following advantages compared to
allocation on the call stack:

1. Pointers to memory allocated on the TempAlloc stack are still valid
when the function they were allocated from returns. Functions can be
written to create and return data structures on the TempAlloc stack.

2. Since it is a segmented stack, large allocations can be performed with
no danger of stack overflow errors.

It has the following advantages compared to heap allocation:

1. Both allocation and deallocation are extremely fast. Most allocations
consist of verifying enough space is available, incrementing a pointer and
a performing a few cheap bookkeeping operations. Most deallocations
consist decrementing a pointer and performing a few cheap bookkeeping
operations.

2. The segmented stack is thread-local, so synchronization is only needed
when a segment needs to be allocated or freed.

3. Fragmentation is not an issue when allocating memory on the TempAlloc
stack, though it can be an issue when trying to allocate a new segment.

It'd be nice to get this in the next release b/c SciD, which is being
worked on extensively for GSoC, uses it and Don said he wanted to use it
in BigInt.

Code:

https://github.com/dsimcha/TempAlloc/blob/master/tempalloc.d

Docs:

http://cis.jhu.edu/~dsimcha/d/phobos/core_tempalloc.html