Messages in this thread | | | From | Arnd Bergmann <> | Date | Tue, 30 Jun 2020 21:25:03 +0200 | Subject | Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y |
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On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will@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
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