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From: "Stephen Howe" <SPAMstephen.howeGUARD@tnsofres.com>
Newsgroups: comp.std.c++
Subject: Re: Proposal: allow code hoisting optimizatoin for "assert"
Date: Wed, 10 Apr 2002 12:36:04 GMT
Organization: Taylor Nelson Sofres
Approved: Fergus Henderson <fjh@cs.mu.oz.au>, moderator of comp.std.c++
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Reply-To: "Stephen Howe" <SPAMstephen.howeGUARD@tnsofres.com>
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"John Nagle" <nagle@animats.com> wrote in message
news:3CB1290F.8000407@animats.com...
>     I'd like to propose that the following optimization
> be allowed.  Consider
>
> for (int i=0; i<n; i++)
> { tab[i] = 0; }
>
> Now suppose that tab[i] is a class with subscript
> checking, implemented as an assert.  I'd like to
> allow the compiler to hoist that assert out of the
> loop.  Rather than performing
>
> assert(i < tab.size());
>
> every time through the loop, the compiler should
> be allowed, if it's smart enough, to do
>
> assert(n <= tab.size());
>
> at the beginning of the loop.

But what if the [] operator changes the size on access? Unusual I know but
possible. The assert() could not be hoisted then.

Stephen Howe




>
>     But right now, compilers can't do that.
> Hoisting the assert out of the loop makes it
> fail "early", which is an illegal optimization.
> I suggest making it a legal one, and allow the
> compiler to understand that "assert" is special.
>
>     Specifically,
>
>      A compiler should be allowed to recognize
> "assert" at compile time.
>
>      An assertion failure may be detected any time
> after assertion failure becomes inevitable.
> The compiler is not required to consider the
> possibility that an exception will be raised
> which prevent the assertion from failing.
>
>      This lays the groundwork for efficient subscript,
> checking.  Old studies of Pascal programs indicated
> that 95% of subscript checks could be hoisted out of
> loops.  Subscript checking has been implemented for
> C++, but it's slow.  This allows implementations
> that do it fast.
>
>      Comments?
>
> John Nagle
> Animats
>
> ---
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