TempAlloc Overhaul

[dsimcha]

On 6/11/2011 7:04 PM, bearophile wrote:
> dsimcha:
>
>> I've overhauled my TempAlloc proposal based on some of the suggestions I've received.
>
> It's improved :-)
>
>
>> 3.  Make alignedMalloc and friends private for now, again to make the proposal more tightly focused.<
>
> They are useful so I'd like such alignedCMalloc / free to be generally visible, with a template argument to specify the desire alignment, that defaults to 16. But I agree it's better to not put it in std.tempalloc.

Eventually, I may make them visible.  For now, I want to make this 
proposal tightly focused.

>
>
>> The first segment of the TempAlloc stack is allocated lazily, so no space is allocated in any thread that does not use TempAlloc.<
>
> Is this causing any slowdown in the usage of the memory?

Technically yes, because every call to anything TempAlloc-related has to 
compare a pointer to null.  In practice, I don't think this is ever a 
significant slowdown.

>
>
>> static void* malloc(size_t nbytes);<
>
> I think the docs have to say this is aligned memory (and there is no way to specify a smaller alignment).

I guess I could put in the docs that it's 16-byte aligned, but there are 
two reasons why specifying smaller alignment isn't allowed:

1.  It would clutter the API with a feature that that is very 
infrequently used.

2.  It would probably have a performance penalty, whereas rounding up 
all allocations to a hard-coded power of two is dirt-cheap.

>
>
>> In this case sizes may be specified for any number of dimensions from 1 to the number in T.<
>
> Isn't it bug prone to allow to specify a number of sizes less than the number of dimensions?


I don't see why it is, though perhaps I should include this as an 
example, too:

// Allocate an array of 100 double[]'s and don't initialize them.
double[][] matrix = TempAlloc.newArray!(double[][])(100);

// Use it.
matrix[0] = TempAlloc.newArray!(double[])(100);

>
>
>> double[] foo = TempAlloc.newArray!(double[])(100);
>
> A little usability problem: an array of array of double or just array of double?

Yeah, you might be getting a little confused because I actually changed 
this in the overhaul.  Used to be that you would write:

double[] foo = newStack!double(100);

The reason I did this was to make the multidimensional syntax sane.
>
> Bye,
> bearophile