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
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