llvm dev - Jun 2015 - [LLVMdev] [cfe-dev] RFC: Adding attribute(nonnull) to things in libc++

On Mon, Jun 01, 2015 at 10:52:20AM -0700, Marshall Clow
wrote:
> P.S.   recent gcc (at least 4.8.x and later) make optimizations based on
> this UB (i.e, if you pass a pointer to memcpy, then it can't be NULL).
BTW, this seems to be more an issue with glibc adding the tagging and
not behavior of GCC itself.

Joerg

> On 01.06.2015, at 23:14, Joerg Sonnenberger <joerg at
britannica.bec.de> wrote:
> 
> On Mon, Jun 01, 2015 at 10:52:20AM -0700, Marshall Clow wrote:
>> P.S.   recent gcc (at least 4.8.x and later) make optimizations based
on
>> this UB (i.e, if you pass a pointer to memcpy, then it can't be
NULL).
> 
> BTW, this seems to be more an issue with glibc adding the tagging and
> not behavior of GCC itself.
GCC also adds nonnull attributes via its builtin functions mechanism. If we want
to follow GCC here we should do the same (Builtins.def has a FIXME about
nonnull), which is imo cleaner than wrapping memcpy in libc++ and has the
advantage of also working with plain C code.

I don't think this has an impact on optimization right now; Clang lowers
memcpy calls to the llvm.memcpy intrinsic and we traditionally have allowed
llvm.memcpy with a nullptr and zero length (i.e. the optimizer has to prove that
the length argument is non-zero to assume the pointer is dereferenceable).

- Ben

On Tue, Jun 02, 2015 at 12:49:11PM +0200, Benjamin Kramer
wrote:
> 
> > On 01.06.2015, at 23:14, Joerg Sonnenberger <joerg at
britannica.bec.de> wrote:
> > 
> > On Mon, Jun 01, 2015 at 10:52:20AM -0700, Marshall Clow wrote:
> >> P.S.   recent gcc (at least 4.8.x and later) make optimizations
based on
> >> this UB (i.e, if you pass a pointer to memcpy, then it can't
be NULL).
> > 
> > BTW, this seems to be more an issue with glibc adding the tagging and
> > not behavior of GCC itself.
> 
> GCC also adds nonnull attributes via its builtin functions mechanism.
I don't see it on NetBSD with GCC 4.8.4, so a plain prototype doesn't
seeem to trigger it.

Joerg