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Message-ID: <20161105211452.GB1555@brightrain.aerifal.cx>
Date: Sat, 5 Nov 2016 17:14:52 -0400
From: Rich Felker <dalias@...c.org>
To: George Kulakowski <kulakowski@...gle.com>, musl@...ts.openwall.com
Subject: Re: ub fix in magenta
On Sat, Nov 05, 2016 at 09:25:59PM +0100, Szabolcs Nagy wrote:
> why do you think union based type punning is ub?
> are you compiling musl as c++ code?
>
> commit 224516687417d5e9dcbb0ba300c3e34bb47bb12b
> Author: George Kulakowski <kulakowski@...gle.com>
> Date: 2016-10-19 17:11:59 -0700
>
> [musl][malloc] Remove undefined behavior in malloc
>
> This bit of code computes an approximation to log2(x) by extracting the
> exponent from a float. Doing it via a union this way is bad, so memcpy
> instead.
>
> https://fuchsia.googlesource.com/magenta/+/224516687417d5e9dcbb0ba300c3e34bb47bb12b
It's definitely not UB (this usage is explicitly permitted by C), and
the memcpy approach is much slower (requires store/call/load) because
-ffreestanding implies -fno-builtin. I'd like to try overriding that
with -fbuiltin-memcpy or a musl-internal header that defines memcpy to
__builtin_memcpy, etc., for all files but src/string/*, but there are
various subtle issues to be concerned about.
> this makes implementation internals publicly visible, introduce
> paddings and whenever you need to add new fields you will have
> to break the abi again.
>
> note that the initializers are not valid c and thus non-conforming.
>
> commit c751172f029e96a3208b37da91fd9e020a792834
> Author: George Kulakowski <kulakowski@...gle.com>
> Date: 2016-10-13 21:31:24 -0700
>
> [musl] Use a single proper struct definition for pthread types
>
> There is one slight change in layout here made for simplicity's
> sake. Upstream's pthread_barrier_t overlays the _b_count and _b_inst
> fields in the 32 bit case. Since this is so rarely used (in Fuchsia
> outside of libc I pretty much only see tsan, gdb etc. test cases),
> just do the simple thing.
>
> https://fuchsia.googlesource.com/magenta/+/c751172f029e96a3208b37da91fd9e020a792834
Yes, this change looks highly problematic to ABI stability.
> (i don't plan to review all changes i just wanted to see if there
> was anything useful in the magenta repo for musl, havent found much
> yet, but some of the changes could have been discussed upstream)
Thanks.
Rich
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