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Re: [PATCH] Use malloca instead alloca

From: Paul Eggert
Subject: Re: [PATCH] Use malloca instead alloca
Date: Sat, 29 Dec 2012 09:38:04 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0

Thanks for looking into this.  Three comments.

First, as far as correctness goes, one cannot simply replace
{ P = alloca (N); use (*P); } with
{ P = malloca (N); use (*P); freea (P); },
as was done in the examples in
This is because malloca might return NULL.  Wouldn't you prefer a
different API, where the allocator cannot possibly return NULL
because it reliably aborts if storage is not available?
(This is what gnulib's xmalloca module does.)

Second, if you do want the malloca API, then it seems to me that,
in the typical case, the gnulib implementation should
perform better than the proposed malloca (with _ALLOCA_MC).
The _ALLOCA_MC approach expands to more machine instructions than
the gnulib approach does, which surely hurts instruction caching.
And in the typical case, where alloca is used, the _ALLOCA_MC
version has more work to do (to store _ALLOCA_MC)
than the gnulib code does (no need to store a magic word).
Have you measured the performance of the _ALLOCA_MC approach,
compared to the gnulib approach, on realistic workloads?

Third, the _ALLOCA_MC version mishandles very large sizes, e.g.,
its malloca (SIZE_MAX) goes greatly awry.

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