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Message-ID: <kpf3dyqurk7abwmwv2o46huc7hdp6jtctnojtkl55tlk2kcssz@h342n2tbhz7h>
Date: Thu, 6 Nov 2025 22:53:11 +0100
From: Alejandro Colomar <alx@...nel.org>
To: Thiago Macieira <thiago@...ieira.org>
Cc: Rich Felker <dalias@...c.org>,
Demi Marie Obenour <demiobenour@...il.com>, musl@...ts.openwall.com,
The 8472 <the8472.rs@...inite-source.de>, Florian Weimer <fw@...eb.enyo.de>, libc-alpha@...rceware.org,
Arthur O'Dwyer <arthur.j.odwyer@...il.com>, Jonathan Wakely <jwakely@...hat.com>
Subject: Re: Re: realloci(): A realloc() variant that works in-place
Hi Thiago,
On Wed, Nov 05, 2025 at 09:38:42AM -0800, Thiago Macieira wrote:
> On Wednesday, 5 November 2025 03:24:14 Pacific Standard Time Alejandro Colomar
> wrote:
> > Agree. Before adding realloci(), I'd like to see numbers. An
> > interesting thing to do would be to see some application that uses
> > realloc(3) currently, and check how often realloc(3) doesn't move the
> > object. (To test that without UB, one needs to do the trick with
> > uintptr_t.)
>
> I can give you some numbers for realloc() success, with the understanding that
> they are NOT AT ALL the cases where realloci() would be used. I can't emit a
> realloc() call that MAY relocate an an array of objects, if the objects don't
> allow relocating. That would crash the application on the first time realloc()
> did relocate. So I can only do it for the cases where the implementation would
> keep using realloc() in the future.
>
> The only way to test how often realloci() would succeed is to have realloci().
>
> Anyway, my test is running qtcreator. Remember that it called the placeholder
> function that would call realloci() 119 times just for running --help.
>
> It also called QArrayData::reallocateUnaligned() with objectSize > 4 a total
> of 1577 times, in 115 of which realloc() returned the same pointer (7.3%). In
> fact, a quick glimpse of the gdb output shows that the same pointer succeed in
> growing more than once in a row: I see a pointer at least 4x twice in a row,
> with an increased capacity parameter, suggesting that it's the same array.
>
> In the full run (without --help), realloc() extended in place 1045 out of
> 10088 calls, increasing to 10.3% extension success rate.
Thanks! 10% sounds useful enough IMO. That's motivation enough for me
to write the proposal for ISO C, and continue working on the musl
patches.
Have a lovely night!
Alex
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
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