Re: [Tinycc-devel] Buiding binutils 2.17 (needs dynamic arrays).

[Daniel Glöckner]

On Wed, Oct 03, 2007 at 01:59:47AM -0500, Rob Landley wrote:
> It boils down to a funky call to alloca()...

One thing to note is that memory for VLAs is freed when the block ends.
GCC does this by saving the stack pointer before allocating the VLA and
restoring it at the end of the block.

This approach doesn't work if we mix VLAs with alloca.

#include <stdlib.h>

void f(char *,char *);

void g(int n)
{
  while(n--) {
    char p[n];
    char *q=alloca(n*7);
    f(p,q);
  }
}

void h(int n)
{
  while(n--) {
    char *q=alloca(n*7);
    f(q,q);
  }
}

In g GCC will free the alloca'ed memory at the end of each iteration while
in h it will free the memory at the end of the function.

The GCC approach has the benefit that it won't force you to have 0,5TB of
memory if one calls 

void e(int n) {
  while(n--) {
    char p[n];
    f(p,p);
  }
}

with n=1048576.

  Daniel