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Date: Mon, 31 Aug 2015 14:19:10 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.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
On Mon, Aug 31, 2015 at 09:59:47AM +0200, Ingo Molnar wrote:
>
> * Michael S. Tsirkin <mst@...hat.com> wrote:
>
> > On Sun, Aug 30, 2015 at 11:13:20PM -0700, H. Peter Anvin wrote:
> > > Presumably because gcc can't generate bt... whether or not it is worth it is another matter.
> > >
> > > On August 30, 2015 11:05:49 PM PDT, Ingo Molnar <mingo@...nel.org> wrote:
> > > >
> > > >* 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
> >
> > Exactly:
> >
> >
> > Disassembly of section .text:
> >
> > 00000000 <__variable_test_bit>:
> > __variable_test_bit():
> > 0: 8b 54 24 08 mov 0x8(%esp),%edx
> > 4: 8b 44 24 04 mov 0x4(%esp),%eax
> > 8: 0f a3 02 bt %eax,(%edx)
> > b: 19 c0 sbb %eax,%eax
> > d: c3 ret
> > e: 66 90 xchg %ax,%ax
> >
> > 00000010 <__constant_test_bit>:
> > __constant_test_bit():
> > 10: 8b 4c 24 04 mov 0x4(%esp),%ecx
> > 14: 8b 44 24 08 mov 0x8(%esp),%eax
> > 18: 89 ca mov %ecx,%edx
> > 1a: c1 fa 04 sar $0x4,%edx
> > 1d: 8b 04 90 mov (%eax,%edx,4),%eax
> > 20: d3 e8 shr %cl,%eax
> > 22: 83 e0 01 and $0x1,%eax
> > 25: c3 ret
>
> But that's due to the forced interface of generating a return code. Please compare
> it at an inlined usage site, where GCC is free to do the comparison directly and
> use the result in flags.
>
> Thanks,
>
> Ingo
I applied this patch on top of mine:
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 9229334..2aed985 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -323,24 +323,17 @@ static inline int variable_test_bit(long nr, volatile const unsigned long *addr)
return oldbit;
}
-static __always_inline int __constant_test_bit(long nr, const unsigned long *addr)
+/**
+ * __test_bit - Determine whether a bit is set
+ * @nr: bit number to test
+ * @addr: Address to start counting from
+ */
+static __always_inline int __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;
-}
-
#if 0 /* Fool kernel-doc since it doesn't do macros yet */
/**
* test_bit - Determine whether a bit is set
@@ -348,13 +341,6 @@ static inline int __variable_test_bit(long nr, const unsigned long *addr)
* @addr: Address to start counting from
*/
static int test_bit(int nr, const volatile unsigned long *addr);
-
-/**
- * __test_bit - Determine whether a bit is set
- * @nr: bit number to test
- * @addr: Address to start counting from
- */
-static int __test_bit(int nr, const volatile unsigned long *addr);
#endif
#define test_bit(nr, addr) \
@@ -362,10 +348,6 @@ static int __test_bit(int nr, const volatile unsigned long *addr);
? constant_test_bit((nr), (addr)) \
: variable_test_bit((nr), (addr)))
-#define __test_bit(nr, addr) \
- (__builtin_constant_p((nr)) \
- ? __constant_test_bit((nr), (addr)) \
- : __variable_test_bit((nr), (addr)))
/**
* __ffs - find first set bit in word
And the code size went up:
134836 2997 8372 146205 23b1d arch/x86/kvm/kvm-intel.ko ->
134846 2997 8372 146215 23b27 arch/x86/kvm/kvm-intel.ko
342690 47640 441 390771 5f673 arch/x86/kvm/kvm.ko ->
342738 47640 441 390819 5f6a3 arch/x86/kvm/kvm.ko
I tried removing __always_inline, this had no effect.
--
MST
--
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