On Friday, 24 April 2015 at 12:34:19 UTC, ponce wrote:
On Friday, 24 April 2015 at 08:16:40 UTC, Walter Bright wrote:
On 4/24/2015 12:23 AM, John Colvin wrote:
Except of course that alloca is a lot cheaper than
malloc/free.
That's not necessarily true. But in any case, go ahead and use
it if you like. Just prepare to benchmark and be disappointed
:-)
Do you have a guess for why and when it could not be faster
than malloc in times?
I have some difficulty imagining a reason (yet I have sometimes
found malloc faster than aligned_malloc which is another odd
thing).
one reason why it might be faster is that e.g. gcc can produce
code like this:
#include<alloca.h>
void bar(char* a);
void foo(unsigned int n)
{
char *a = (char*)alloca(n);
bar(a);
}
foo:
movl %edi, %eax
pushq %rbp
addq $46, %rax
movq %rsp, %rbp
shrq $4, %rax
salq $4, %rax
subq %rax, %rsp
leaq 31(%rsp), %rdi
andq $-32, %rdi
call bar
leave
ret
which is neat. Now of course a push-the-pointer malloc/free
implementation could perhaps be (in theory) optimised to be as
small as this, but is that ever actually the case?
