On Tue, Nov 17, 2020 at 09:10:53PM +0000, Will Deacon wrote: > On Tue, Nov 17, 2020 at 11:31:57AM -0800, Linus Torvalds wrote: > > On Tue, Nov 17, 2020 at 11:25 AM Jakub Jelinek <ja...@redhat.com> wrote: > > > > > > It would need to be typeof( (typeof(type)) (type) ) to not be that > > > constrained on what kind of expressions it accepts as arguments. > > > > Yup. > > > > > Anyway, it won't work with array types at least, > > > int a[10]; > > > typeof ((typeof (a)) (a)) b; > > > is an error (in both gcc and clang), while typeof (a) b; will work > > > (but not drop the qualifiers). Don't know if the kernel cares or not. > > > > Well, the kernel already doesn't allow that, because our existing > > horror only handles simple integer scalar types. > > > > So that macro is a clear improvement - if it actually works (local > > testing says it does, but who knows about random compiler versions > > etc) > > I'll give it a go now, although if it works I honestly won't know whether > to laugh or cry.
GCC 9 and Clang 11 both seem to generate decent code for aarch64 defconfig with: #define __unqual_scalar_typeof(x) typeof( (typeof(x)) (x)) replacing the current monstrosity. allnoconfig and allmodconfig build fine too. However, GCC 4.9.0 goes mad and starts spilling to the stack when dealing with a pointer to volatile, as though we were just using typeof(). I tried GCC 5.4.0 and that looks ok, so I think if anybody cares about the potential performance regression with 4.9 then perhaps they should consider upgrading their toolchain. In other words, let's do it. Will
