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. (Side note, I'm surprised that any function that accepts a pointer could be considered pure. I could reassign pointed to value without changing the pointers value. I can see strlen being "pure" for string literals, but not for char[]. This is something I'll play with more, I've already spotted one missed optimization in LLVM: https://bugs.llvm.org/show_bug.cgi?id=43624). I think it would be an interesting study to see how often functions that have return codes are ok to not check vs aren't ok (in a large production codebase like the Linux kernel), similar to how 97% of cases fallthrough is unintentional (which to me sounds like maybe the default behavior of the language is incorrect). -- Thanks, ~Nick Desaulniers
