Messages in this thread | | | From | Nick Desaulniers <> | Date | Thu, 10 Nov 2022 11:20:21 -0800 | Subject | Re: [PATCH v1 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation |
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On Mon, Nov 7, 2022 at 1:39 AM Peter Zijlstra <peterz@infradead.org> wrote: > > On Sun, Nov 06, 2022 at 06:51:05PM +0900, Vincent Mailhol wrote: > > The builtin implementation is better for two reasons: > > > > 1/ it saves two instructions on clang (a push and a stack pointer > > decrement) because of a useless tentative to save rax. > > I'm thinking this is the same old clang-sucks-at-"rm" constraints and > *really* should not be a reason to change things. Clang should get fixed > already.
Well messing up constant folding for all compilers absolutely should be a reason!
I did get a chance to speak with some colleagues more about this at the LLVM developer meeting during the past 2 days. We have some ideas on approaches that might work. There's some higher priority features we're working on first, but I suspect we'll be able to visit that issue soon. It's a pretty tricky dance between instruction selection and register allocation.
> > > 2/ when used on constant expressions, the compiler is only able to > > fold the builtin version (c.f. [2]). > > > > For those two reasons, replace the assembly implementation by its > > builtin counterpart. > > > > [1] https://elixir.bootlin.com/linux/v6.0/source/include/asm-generic/bitops/builtin-__fls.h > > > > [2] commit 146034fed6ee ("x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions") > > I would much prefer consistently with 146034fed6ee.
The bottom of this file arch/x86/include/asm/bitops.h is full of #include <asm-generic/bitops/*.h>
-- Thanks, ~Nick Desaulniers
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