Re: [PATCH] arm64: lse: fix LSE atomics with LLVM's integrated assembler

[Robin Murphy]

On 2019-10-08 10:03 pm, Ard Biesheuvel wrote:
> On Tue, 8 Oct 2019 at 18:19, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > On 08/10/2019 16:22, Sami Tolvanen wrote:
> > > On Mon, Oct 7, 2019 at 2:46 PM 'Nick Desaulniers' via Clang Built
> > > Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > > > I'm worried that one of these might lower to LSE atomics without
> > > > ALTERNATIVE guards by blanketing all C code with `-march=armv8-a+lse`.
> > >
> > > True, that's a valid concern. I think adding the directive to each
> > > assembly block is the way forward then, assuming the maintainers are
> > > fine with that.
> >
> > It's definitely a valid concern in principle, but in practice note that
> > lse.h ends up included in ~99% of C files, so the extension is enabled
> > more or less everywhere already.
>
> lse.h currently does
>
> __asm__(".arch_extension        lse");
>
> which instructs the assembler to permit the use of LSE opcodes, but it
> does not instruct the compiler to emit them, so this is not quite the
> same thing.

Derp, of course it isn't. And IIRC we can't just pass the option through 
with -Wa either because at least some versions of GCC emit an explicit 
.arch directive at the top of the output. Oh well; sorry for the 
distraction.

Robin.