Messages in this thread | | | From | Nick Desaulniers <> | Date | Wed, 11 May 2022 17:28:06 -0700 | Subject | Re: [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions |
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On Wed, May 11, 2022 at 5:04 PM Vincent Mailhol <mailhol.vincent@wanadoo.fr> wrote: > > And finally, Nick Desaulniers pointed out in [2] that this also fixes > a constant propagation missed-optimization in clang. > > [2] https://lore.kernel.org/all/CAKwvOdnH_gYv4qRN9pKY7jNTQK95xNeH1w1KZJJmvCkh8xJLBg@mail.gmail.com/
Regarding https://github.com/llvm/llvm-project/issues/55394 it seems that functions with static linkage cannot be considered library functions, so libcall optimization will not run on calls to them. So the compiler might be able to do a better job for constants if ffs() and friends indeed were not defined in a header as static inline. But that relies on the compiler knowing these tricks; I think the kernel's approach is just fine (better in fact, because we should inline these tiny functions, regardless of LTO), but like this series shows, there may be room for improvement for other functions within the kernel that are defined as static inline in headers that are normally found in a libc.
So I no longer think there's a missed optimization here, but at this point, it's not worth a respin of the series IMO to just let sleeping dogs lie.
Unless the x86 maintainers wouldn't mind dropping that line and link when applying? -- Thanks, ~Nick Desaulniers
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