[cfe-dev] RFC: do not optimize on basis of __attribute__((nonnull)) in glibc headers

[Chandler Carruth via cfe-dev]

I've sent a patch to implement this here: https://reviews.llvm.org/D30806

It turns out to be both easier and I hope less controversial than I had
imagined. Neither Clang nor LLVM ever actually got nonnull onto these
declarations even when they were suitable annotated. We didn't carry that
annotation across to the declaration. Oops.

So I've fixed that, but *also* disabled it for the libc functions that we
have library builtin recognition for. We can extend this to cover any set
of library functions folks want, just let me know. The use of it will even
be correctly controlled by -fno-builtin and friends.

The net result is that this should not regress optimizations for *anyone*,
and actually enable a bunch of optimizations outside of libc functions,
while preserving safety on those libc functions.

We still have the nullability attributes which can be used to annotate more
interfaces in a way that will provide warnings and static analysis aid, but
not optimize on and so not run the risk of changing behavior of existing
code. Those would be good candidates (IMO) for libc++ to use on any
relevant interfaces which match the criteria outlined in this thread:
pointers paired with a size should allow null+zero.

Hopefully this makes everyone happy, please feel free to chime in on the
review thread if more discussion is needed.

On Wed, Jan 4, 2017 at 11:24 AM Aaron Ballman via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Wed, Jan 4, 2017 at 2:01 PM, James Y Knight <jyknight at google.com>
> wrote:
> >
> >
> > On Wed, Jan 4, 2017 at 12:54 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> >>
> >>
> >> On 01/04/2017 11:43 AM, James Y Knight via cfe-dev wrote:
> >>
> >> On Wed, Jan 4, 2017 at 11:12 AM, Aaron Ballman via cfe-dev
> >> <cfe-dev at lists.llvm.org> wrote:
> >>>
> >>> So I would be opposed to ignoring those attributes in
> >>>
> >>> Sema (I think we should still warn when users do nonportable things),
> >>> but in favor of not changing the optimizer to capitalize on this
> >>> "opportunity."
> >>
> >>
> >> I'd be opposed to ignoring the attributes only in some places and not in
> >> others. It should be ignored totally, as if it wasn't present on those
> >> functions. Doing anything else sends the wrong message -- that libc
> authors
> >> should continue to use nonnull on these functions because they might be
> >> helpful, and won't do anything bad.
> >>
> >>
> >> I think that we have a responsibility to our users to continue to warn
> >> (statically, in ubsan, etc.) on non-portable behavior, which this is and
> >> will continue to be in practice for at least a decade or two,
> regardless of
> >> the message we'd like to send libc authors. We cannot undo history here
> and
> >> this will be relevant to production systems for at least a decade. We
> can
> >> talk to libc developers directly -- they're a much smaller set -- and
> we can
> >> pursue change at the standards level while still providing the most
> useful
> >> set of tools to our users in the mean time.
> >
> >
> > But, this is an entirely different question.
> >
> > - Should clang warn about non-portable usage of passing NULL to
> memcpy/etc?
> >
> > Sounds like a fine warning to add.
> >
> > - Should that warning be dependent on the libc headers having nonnull
> > annotations on these functions, which will be used only for warnings, and
> > ignored for semantics, on this given list of hardcoded functions?
> >
> > No.
> >
> > Firstly: I'd note that nearly all libc implementations don't use these
> > attributes today. In some cases, because they've simply not thought about
> > it, but in others because they explicitly decided to NOT break their
> users'
> > code by introducing this problem! Glibc is the outlier, here.
> >
> > So: what portability do you want to warn for? Portability assuming the
> same
> > libc, but a different compiler which might fail to ignore the nonnull
> > attribute? Or portability to other libc? If the latter, depending on the
> > nonnull attribute being present doesn't and can't work.
>
> My preference is for portability for both, but you bring up a good
> point about other libc implementations not being annotated. So long as
> we retain the ability to tell users their code is not portable *and*
> we get rid of the dangerous optimizations, I'm happy. I just don't
> want to lose the diagnostics because of the optimizations.
>
> ~Aaron
>
> > Secondly: if we already have a hardcoded list of functions to special
> case,
> > that could just as well be used to generate a nonportable-stringfunc-null
> > warning, as well.
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