Re: [PATCH 2/2] strbuf: allow to use preallocated memory

From: Mike Hommey <mh@glandium.org>
To: William Duclot <william.duclot@ensimag.grenoble-inp.fr>
Cc: git@vger.kernel.org, simon.rabourg@ensimag.grenoble-inp.fr,
	francois.beutin@ensimag.grenoble-inp.fr,
	antoine.queru@ensimag.grenoble-inp.fr,
	matthieu.moy@grenoble-inp.fr, mhagger@alum.mit.edu
Subject: Re: [PATCH 2/2] strbuf: allow to use preallocated memory
Date: Tue, 31 May 2016 06:56:52 +0900	[thread overview]
Message-ID: <20160530215652.GA6456@glandium.org> (raw)
In-Reply-To: <20160530103642.7213-3-william.duclot@ensimag.grenoble-inp.fr>

>  struct strbuf {
> +	unsigned int flags;
>  	size_t alloc;
>  	size_t len;
>  	char *buf;
>  };

Depending whether the size of strbuf matters, it /might/ be worth
considering some packing here. malloc() usually returns buffers that can
contain more data than what is requested. Which means allocation sizes
could be rounded and that wouldn't change the amount of allocated
memory. On glibc malloc_usable_size(malloc(1)) apparently returns 24.
On jemalloc, it's 4 or 8. It's in the same ballbark with many
allocators.

So, it would be possible to round alloc such that it's always a multiple
of, say, 4, and stick flags in the low, unused bits.

Whether it's worth doing is another question.

Mike