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Date: Mon, 31 Aug 2015 08:05:49 +0200 From: Ingo Molnar <mingo@...nel.org> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, Rusty Russell <rusty@...tcorp.com.au>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH 1/2] x86/bitops: implement __test_bit * Michael S. Tsirkin <mst@...hat.com> wrote: > +static __always_inline int __constant_test_bit(long nr, const unsigned long *addr) > +{ > + return ((1UL << (nr & (BITS_PER_LONG-1))) & > + (addr[nr >> _BITOPS_LONG_SHIFT])) != 0; > +} > + > +static inline int __variable_test_bit(long nr, const unsigned long *addr) > +{ > + int oldbit; > + > + asm volatile("bt %2,%1\n\t" > + "sbb %0,%0" > + : "=r" (oldbit) > + : "m" (*addr), "Ir" (nr)); > + > + return oldbit; > +} Color me confused, why use assembly for this at all? Why not just use C for testing the bit (i.e. turn __constant_test_bit() into __test_bit()) - that would also allow the compiler to propagate the result, potentially more optimally than we can do it via SBB... Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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