[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CA+fZqCX_bxjDcEBtXonJieFE-4xyU5fT+d47_5B=xNGAoE+Pdw@mail.gmail.com>
Date: Sun, 1 Apr 2018 13:19:01 +0200
From: ardi <ardillasdelmonte@...il.com>
To: musl@...ts.openwall.com
Subject: Re: Views on bare metal port
On Wed, Jan 31, 2018 at 7:25 PM, Szabolcs Nagy <nsz@...t70.net> wrote:
>
> note that in particular for math code llvm is problematic.
>
> i'm not aware of any issues in musl libm currently, but
> rounding mode changes can break in unexpected ways because
> there is no support for -frounding-math (last time i checked
> the few cases where it matters clang did not miscompile those,
> but this is not actively tested)
>
> an illustration of the kind of things that can go wrong:
> https://github.com/freebsd/freebsd/commit/e7b0a63c190c7ecb475b09f41387a75952540f49
> we don't have equivalent hacks in musl.
Being a clang user, this comment has worried me, because, according to
this bug report, there's a risk of generating wrong code:
https://bugs.llvm.org/show_bug.cgi?id=8100
However, a participant in the bug comments, argues that this doesn't
really violate standards compliance, just that a C99 feature is not
implemented yet, and that a warning will be triggered:
>> (quoting Richard Smith at bug report above):
>>
>> You are mistaken. Setting the rounding mode
>> without using #pragma STDC FENV_ACCESS ON
>> invokes undefined behavior. See C11 7.6.1/2. (This
>> pragma does not exist in C++, so <cfenv> is
>> unusable, but that's not our fault...)
>>
>> Clang doesn't support that pragma, making this
>> an unimplemented feature from C99, for which we
>> will produce a warning or error. That's the subject of
>> this enhancement request.
Now, and to the point that really interests me (generating safe code
from a clang+musl environment):
- What kind of math operations can be affected by this bug?
- Is there any workaround at this moment?
- Or is gcc the only compiler that can generate valid math code with
musl at this moment and we should better wait until clang gets bug
8100 fixed?
Thanks!
ardi
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
