Making alloca more safe

[Andrei Alexandrescu]

Yigal Chripun wrote:
> Andrei Alexandrescu wrote:
>> Denis Koroskin wrote:
>>> On Mon, 16 Nov 2009 19:27:41 +0300, Andrei Alexandrescu 
>>> <SeeWebsiteForEmail at erdani.org> wrote:
>>>
>>>> bearophile wrote:
>>>>> Walter Bright:
>>>>>
>>>>>> A person using alloca is expecting stack allocation, and that it 
>>>>>> goes away after the function exits. Switching arbitrarily to the 
>>>>>> gc will not be detected and may hide a programming error (asking 
>>>>>> for a gigantic piece of memory is not anticipated for alloca, and 
>>>>>> could be caused by an overflow or logic error in calculating its 
>>>>>> size).
>>>>>  There's another solution, that I'd like to see more often used in 
>>>>> Phobos: you can add another function to Phobos, let's call it 
>>>>> salloca (safe alloca) that does what Denis Koroskin asks for (it's 
>>>>> a very simple function).
>>>>
>>>> Can't be written. Try it.
>>>>
>>>> Andrei
>>>
>>> It's tricky. It can't be written *without a compiler support*, 
>>> because it is considered special for a compiler (it always inlines 
>>> the call to it). It could be written otherwise.
>>>
>>> I was thinking about proposing either an inline keyword in a language 
>>> (one that would enforce function inlining, rather than suggesting it 
>>> to compiler), or allways inline all the functions that make use of 
>>> alloca. Without either of them, it is impossible to create wrappers 
>>> around alloca (for example, one that create arrays on stack 
>>> type-safely and without casts):
>>>
>>> T[] array_alloca(T)(size_t size) { ... }
>>>
>>> or one that would return GC-allocated memory when stack allocation 
>>> fails:
>>>
>>> void* salloca(size_t size) {
>>>     void* ptr = alloca(size);
>>>     if (ptr is null) return (new void[size]).ptr;
>>>
>>>     return ptr;
>>> }
>>
>> The problem of salloca is that alloca's memory gets released when 
>> salloca returns.
>>
>> Andrei
> 
> template salloca(alias ptr, alias size) { // horrible name, btw
>   ptr = alloca(size);
>   if (ptr is null) ptr = (new void[size]).ptr;
> }
> 
> // use:
> void foo() {
>   int size = 50;
>   void* ptr;
>   mixin salloca!(ptr, size);
>   //...
> }
> 
> wouldn't that work?

mixin? Interesting. Probably it works.

Andrei