On Wed, Sep 30, 2020 at 10:13 AM Peter Zijlstra <pet...@infradead.org> wrote: > > On Wed, Sep 30, 2020 at 11:10:36AM -0500, Segher Boessenkool wrote: > > > Since this variable is a local register asm, on entry to the asm the > > compiler guarantees that the value lives in the assigned register (the > > "r8" hardware register in this case). This all works completely fine. > > This is the only guaranteed behaviour for local register asm (well, > > together with analogous behaviour for outputs). > > Right, that's what they're trying to achieve. The hypervisor calling > convention needs that variable in %r8 (which is somewhat unfortunate). > > AFAIK this is the first such use in the kernel, but at least the gcc-4.9 > (our oldest supported version) claims to support this. > > So now we need to know if clang will actually do this too..
Does clang support register local storage? Let's use godbolt.org to find out: https://godbolt.org/z/YM45W5 Looks like yes. You can even check different GCC versions via the dropdown in the top right. The -ffixed-* flags are less well supported in Clang; they need to be reimplemented on a per-backend basis. aarch64 is relatively well supported, but other arches not so much IME. Do we need register local storage here? static inline long bar(unsigned long hcall_id) { long result; asm volatile("movl %1, %%r8d\n\t" "vmcall\n\t" : "=a" (result) : "ir" (hcall_id) : ); return result; } -- Thanks, ~Nick Desaulniers
