[PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

[Arnd Bergmann]

On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will at kernel.org> wrote:
> +#define __READ_ONCE(x)                                                 \
> +({                                                                     \
> +       int atomic = 1;                                                 \
> +       union { __unqual_scalar_typeof(x) __val; char __c[1]; } __u;    \
> +       typeof(&(x)) __x = &(x);                                        \
> +       switch (sizeof(x)) {                                            \
...
> +       atomic ? (typeof(x))__u.__val : (*(volatile typeof(x) *)__x);   \
> +})

This expands (x) nine times (five in __unqual_scala_typeof()), which can
lead to significant code bloat after preprocessing if something passes a
compound expression into READ_ONCE().
The compiler works it out eventually, but we've seen an actual slowdown
in compile speed from this recently, especially on clang.

I think if you move the

        typeof(&(x)) __x = &(x);

line first, all other instances can use typeof(*__x) instead of typeof(x)
and avoid this problem. Once we make gcc-4.9 the minimum version,
this could be further improved to

       __auto_type __x = &(x);

       Arnd