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digitalmars.D - Dynamic Sized Stack Frames

reply "Jonathan Marler" <johnnymarler gmail.com> writes:
When the life of a buffer is tied to the life of a function, it 
makes sense to allocate it on the stack because it's much easier 
to pop memory off the stack then to garbage collect it later.  
For example if had a function that required a buffer to read from 
some sort of input, it would make sense to allocate the buffer on 
the stack, i.e.

run() {
   byte buffer[BUFF_SIZE];
   while(running) {
     int n = read(buffer, BUFF_SIZE);
     ...

However, what if I want the size of the buffer to be configurable 
at runtime?  Maybe this function is part of some sort of library 
that allows the user to specify a smaller or larger buffer to 
optimize the number of read calls.  As far as I know, in order 
for a library to have a buffer whose length isn't known until 
runtime, it would require allocating it on the heap.  Then we 
have a buffer that is only needed for the life of this function 
but still needs to be garbage collected.  The solution I came up 
with for this type of scenario is what I am calling "dynamic 
sized stack frames".  The buffer could be allocated on the stack, 
and the length of the buffer could be pushed on the stack as well 
so the function can properly clean it when it returns.  I don't 
know of any languages that support this type of functionality, 
does anyone know if there is one?  Maybe this is a new feature 
that D could introduce in a later version?

As far as implementation there's a couple of ways this could be 
done...the caller could allocate the buffer, or the caller could 
just push the buffer length as an extra parameter and the callee 
could be solely responsible for allocating the buffer and 
cleaning it up afterwards (as I'm writing this that sounds like a 
safer option).  Anyway I wanted to put this idea out there and 
see what people's thoughts are.  Thanks in advance for any 
helpful opinions or criticism.
Nov 17 2013
parent reply "Chris Cain" <clcain uncg.edu> writes:
On Monday, 18 November 2013 at 05:46:48 UTC, Jonathan Marler 
wrote:

 However, what if I want the size of the buffer to be 
 configurable at runtime?


core.stdc.stdlib has "alloca" ... it allocates a runtime 
configurable amount of memory on the stack. There's a few threads 
on doing some tricks with it. For instance, if you use it as a 
default parameter, then it'll allocate on the caller's stack so 
you can actually return a ref to it from the function :)
Nov 17 2013
parent reply "Jonathan Marler" <johnnymarler gmail.com> writes:
On Monday, 18 November 2013 at 05:58:25 UTC, Chris Cain wrote:

 On Monday, 18 November 2013 at 05:46:48 UTC, Jonathan Marler 
 wrote:

 However, what if I want the size of the buffer to be 
 configurable at runtime?


 core.stdc.stdlib has "alloca" ... it allocates a runtime 
 configurable amount of memory on the stack. There's a few 
 threads on doing some tricks with it. For instance, if you use 
 it as a default parameter, then it'll allocate on the caller's 
 stack so you can actually return a ref to it from the function 
 :)


Are you serious?  It seems the more I learn about D the more 
impressed I become.  Thanks for the quick response :)
Nov 17 2013
parent reply Mike Parker <aldacron gmail.com> writes:
On 11/18/2013 3:03 PM, Jonathan Marler wrote:

 On Monday, 18 November 2013 at 05:58:25 UTC, Chris Cain wrote:



 core.stdc.stdlib has "alloca" ... it allocates a runtime configurable
 amount of memory on the stack. There's a few threads on doing some
 tricks with it. For instance, if you use it as a default parameter,
 then it'll allocate on the caller's stack so you can actually return a
 ref to it from the function :)


 Are you serious?  It seems the more I learn about D the more impressed I
 become.  Thanks for the quick response :)


While D is awesome, alloca is actually a standard C function, which is 
why it's in the core.stdc.stdlib package. You have full access to the C 
standard library from D.
Nov 17 2013
parent dennis luehring <dl.soluz gmx.net> writes:
Am 18.11.2013 08:31, schrieb Mike Parker:

 On 11/18/2013 3:03 PM, Jonathan Marler wrote:

 On Monday, 18 November 2013 at 05:58:25 UTC, Chris Cain wrote:



 core.stdc.stdlib has "alloca" ... it allocates a runtime configurable
 amount of memory on the stack. There's a few threads on doing some
 tricks with it. For instance, if you use it as a default parameter,
 then it'll allocate on the caller's stack so you can actually return a
 ref to it from the function :)


 Are you serious?  It seems the more I learn about D the more impressed I
 become.  Thanks for the quick response :)


 While D is awesome, alloca is actually a standard C function, which is
 why it's in the core.stdc.stdlib package. You have full access to the C
 standard library from D.


alloca is not in the C standard - but most compilers implement it - and 
its not a regular functions cause its reserves space on stack
Nov 18 2013