Date: Mon, 16 Feb 2015 11:20:46 -0500 From: John Baldwin <jhb@freebsd.org> To: Pedro Giffuni <pfg@freebsd.org> Cc: src-committers@freebsd.org, Ian Lepore <ian@freebsd.org>, svn-src-all@freebsd.org, Gleb Smirnoff <glebius@freebsd.org>, Bruce Evans <brde@optusnet.com.au>, svn-src-head@freebsd.org Subject: Re: svn commit: r278737 - head/usr.sbin/flowctl Message-ID: <2515976.62jDXqymxG@ralph.baldwin.cx> In-Reply-To: <54DFA7CC.20305@FreeBSD.org> References: <201502132357.t1DNvKda075915@svn.freebsd.org> <1423938828.80968.148.camel@freebsd.org> <54DFA7CC.20305@FreeBSD.org>
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On Saturday, February 14, 2015 02:53:48 PM Pedro Giffuni wrote:
> On 02/14/15 13:33, Ian Lepore wrote:
> > 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.
>
> Because it's non-standard and the alloca(3) man page discourages it:
> _____
> ...
> BUGS
> The alloca() function is machine and compiler dependent; its use is dis-
> couraged.
>
> ____
>
> It is not disappearing anytime soon though, some even say the man
> page is wrong.
Given all the alternative implementations and concerns, it seems like alloca()
is a lot simpler to use. I suspect it isn't going away anytime soon, either.
--
John Baldwin
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