Re: [PATCH] string.h: Mark 34 functions with __must_check
From: Dan Carpenter
Date: Wed Oct 09 2019 - 14:48:37 EST
On Wed, Oct 09, 2019 at 09:31:41AM -0700, Nick Desaulniers wrote:
> On Wed, Oct 9, 2019 at 7:30 AM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
> > On Wed, Oct 09, 2019 at 04:21:20PM +0200, Rasmus Villemoes wrote:
> > > On 09/10/2019 15.56, Dan Carpenter wrote:
> > > > That's because glibc strlen is annotated with __attribute_pure__ which
> > > > means it has no side effects.
> > >
> > > I know, except it has nothing to do with glibc headers. Just try the
> > > same thing in the kernel. gcc itself knows this about __builtin_strlen()
> > > etc. If anything, we could annotate some of our non-standard functions
> > > (say, memchr_inv) with __pure - then we'd both get the Wunused-value in
> > > the nonsense cases, and allow gcc to optimize or reorder the calls.
> > Huh. You're right. GCC already knows. So this patch is pointless like
> > you say.
> Is it? None of the functions in include/linux/string.h are currently
> marked __pure today.
I've already embarrassed myself with my ignorance once so I may as well
keep talking now... GCC did complain about the unused result even
though we don't declare them as __pure. So GCC rule must have this rule
We were discussing in a different thread that standard says that
memcpy() pointers can't be NULL (even when we're copying zero bytes) so
GCC will assume that's true. If you have:
memcpy(foo, bar, 0);
*foo = 0;
GCC will sometimes remove the condition. This doesn't affect the kernel
because we use -fno-delete-null-pointer-checks.