svn commit: r278737 - head/usr.sbin/flowctl

[Ian Lepore]

On Sat, 2015-02-14 at 21:15 +0300, Gleb Smirnoff wrote:
>   Bruce,
> 
> On Sat, Feb 14, 2015 at 08:46:58PM +1100, Bruce Evans wrote:
> B> Using VLAs and also the C99 feature of declarations anwhere, and extensions
> B> like __aligned(), we can almost implement a full alloca() using the fixed
> B> version of this change:
> B> 
> B> /*
> B>   * XXX need extended statement-expression so that __buf doesn't go out
> B>   * of scope after the right brace.
> B>   */
> B> #define	my_alloca(n) __extension__ ({
> B>  	/* XXX need unique name. */				\
> B>  	char __buf[__roundup2((n), MUMBLE)] __aligned(MUMBLE);	\
> B>  								\
> B>  	(void *)__buf;						\
> B> })
> 
> I like this idea. But would this exact code work? The life of
> __buf is limited by the code block, and we exit the block
> immediately. Wouldn't the allocation be overwritten if we
> enter any function or block later?
> 

Why put any effort into avoiding alloca() in the first place?  Is it
inefficient on some platforms?  On arm it's like 5 instructions, it just
adjusts the size to keep the stack dword-aligned and subtracts the
result from sp, done.

-- Ian