On 3/6/2011 9:07 PM, Chad Nelson wrote:
On Sun, 6 Mar 2011 17:43:15 -0800 "Simonson, Lucanus J"<lucanus.j.simonson@intel.com> wrote: [...]
This whole argument misses the point of optimizing code that uses a bigint class. You want to reuse the memory allocated in a bigint variable over and over instead of reallocating every time.
That requires writing at least a minimal memory manager. As I understand it, the managers that come with compiler libraries are optimized for that sort of thing these days, since C++ is geared toward so many small heap-allocated objects. I'd rather let the compiler's library deal with it than write more non-core code for something that the built-in code will almost certainly be able to do better most of the time.
I don't think that's what Luke's talking about. He's saying, if x has some value, say [x0, x1, x2], and you assign some expression to it, you want the result of that expression to directly replace the digits [x0, x1, x2], rather than allocating a new block, compute the result in this new block, and swap this block in place of the block holding [x0, x1, x2].
Whether you make one unneeded allocation or two is a less important consideration than whether you make one or zero since 2/1 is a lot smaller than 1/0. If a temporary is generated by the user's code they have already pessimized their code. I am sorry, but you cannot avoid allocation with a return by value semantic, only expression templates allows you to reuse the storage on the left hand side of the assignment operator. [...]
I'm not familiar with a use of expression templates that would allow that. Can you point me to something explaining it?
Expression templates would allow you to tell the compiler, when it sees x = a + b, to generate code that replaces x's digits with the sum of a's and b's digits, one at a time. Under ideal circumstances, if x already has a large enough block, no new memory needs to be allocated. With no expression templates, with or without COW or move emulation, a temporary result holding the sum of a and b would be created, necessitating an allocation, which would then (at best) be moved into x. google "c++ expression templates" and/or look at Boost.Proto and libraries that use Boost.Proto. - Jeff