Re: [PATCH 2/5] bitops: compile time optimization forhweight_long(CONSTANT)

From: Borislav Petkov
Date: Fri Feb 12 2010 - 12:06:58 EST

On Thu, Feb 11, 2010 at 09:33:49AM -0800, H. Peter Anvin wrote:
> You don't do the push/pop inline -- if you're going to take the hit of
> pushing this into the caller, it's better to list them as explicit
> clobbers and let the compiler figure out how to do it. The point of
> doing an explicit push/pop is that it can be pushed into the out-of-line
> subroutine.

Doh! Of course, this was wrong. See below.

> Furthermore, you're still putting "volatile" on there... this is a pure
> computation -- no side effects -- so it is exactly when you *shouldn't*
> declare your asm statement volatile.

Also gone.

> Note: given how simple and regular a popcnt actually is, it might be
> preferrable to have the out-of-line implementation either in assembly,
> or using gcc's -fcall-saved-* options to reduce the number of registers
> that is clobbered by the routine.

Yep, this is actually a very good idea. And it seems to work, I've
tested it on an F10, K8 and even on an Atom and no blow ups so far...

---
arch/alpha/include/asm/bitops.h | 4 ++
arch/ia64/include/asm/bitops.h | 1 +
arch/sparc/include/asm/bitops_64.h | 4 ++
arch/x86/include/asm/bitops.h | 10 ++++
arch/x86/lib/Makefile | 8 +++-
arch/x86/lib/hweight.c | 72 ++++++++++++++++++++++++++++
include/asm-generic/bitops/arch_hweight.h | 8 ++--
include/asm-generic/bitops/const_hweight.h | 43 +++++++++++++++--
lib/hweight.c | 20 ++++----
9 files changed, 151 insertions(+), 19 deletions(-)
create mode 100644 arch/x86/lib/hweight.c

diff --git a/arch/alpha/include/asm/bitops.h b/arch/alpha/include/asm/bitops.h
index 296da1d..7fe6943 100644
--- a/arch/alpha/include/asm/bitops.h
+++ b/arch/alpha/include/asm/bitops.h
@@ -409,21 +409,25 @@ static inline unsigned long __arch_hweight64(unsigned long w)
{
return __kernel_ctpop(w);
}
+#define __arch_hweight64 __arch_hweight64

static inline unsigned int __arch_weight32(unsigned int w)
{
return __arch_hweight64(w);
}
+#define __arch_hweight32 __arch_hweight32

static inline unsigned int __arch_hweight16(unsigned int w)
{
return __arch_hweight64(w & 0xffff);
}
+#define __arch_hweight16 __arch_hweight16

static inline unsigned int __arch_hweight8(unsigned int w)
{
return __arch_hweight64(w & 0xff);
}
+#define __arch_hweight8 __arch_hweight8
#else
#include <asm-generic/bitops/arch_hweight.h>
#endif
diff --git a/arch/ia64/include/asm/bitops.h b/arch/ia64/include/asm/bitops.h
index 9da3df6..9b64f59 100644
--- a/arch/ia64/include/asm/bitops.h
+++ b/arch/ia64/include/asm/bitops.h
@@ -443,6 +443,7 @@ static __inline__ unsigned long __arch_hweight64(unsigned long x)
result = ia64_popcnt(x);
return result;
}
+#define __arch_hweight64 __arch_hweight64

#define __arch_hweight32(x) ((unsigned int) __arch_hweight64((x) & 0xfffffffful))
#define __arch_hweight16(x) ((unsigned int) __arch_hweight64((x) & 0xfffful))
diff --git a/arch/sparc/include/asm/bitops_64.h b/arch/sparc/include/asm/bitops_64.h
index 766121a..4982bc4 100644
--- a/arch/sparc/include/asm/bitops_64.h
+++ b/arch/sparc/include/asm/bitops_64.h
@@ -51,6 +51,7 @@ static inline unsigned int __arch_hweight64(unsigned long w)
__asm__ ("popc %1,%0" : "=r" (res) : "r" (w));
return res;
}
+#define __arch_hweight64 __arch_hweight64

static inline unsigned int __arch_hweight32(unsigned int w)
{
@@ -59,6 +60,7 @@ static inline unsigned int __arch_hweight32(unsigned int w)
__asm__ ("popc %1,%0" : "=r" (res) : "r" (w & 0xffffffff));
return res;
}
+#define __arch_hweight32 __arch_hweight32

static inline unsigned int __arch_hweight16(unsigned int w)
{
@@ -67,6 +69,7 @@ static inline unsigned int __arch_hweight16(unsigned int w)
__asm__ ("popc %1,%0" : "=r" (res) : "r" (w & 0xffff));
return res;
}
+#define __arch_hweight16 __arch_hweight16

static inline unsigned int __arch_hweight8(unsigned int w)
{
@@ -75,6 +78,7 @@ static inline unsigned int __arch_hweight8(unsigned int w)
__asm__ ("popc %1,%0" : "=r" (res) : "r" (w & 0xff));
return res;
}
+#define __arch_hweight8 __arch_hweight8

#else

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 02b47a6..187defe 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -444,6 +444,16 @@ static inline int fls(int x)

#define ARCH_HAS_FAST_MULTIPLIER 1

+extern unsigned int __arch_hweight32(unsigned int w);
+extern unsigned int __arch_hweight16(unsigned int w);
+extern unsigned int __arch_hweight8(unsigned int w);
+extern unsigned long __arch_hweight64(__u64 w);
+
+#define __arch_hweight32 __arch_hweight32
+#define __arch_hweight16 __arch_hweight16
+#define __arch_hweight8 __arch_hweight8
+#define __arch_hweight64 __arch_hweight64
+
#include <asm-generic/bitops/hweight.h>

#endif /* __KERNEL__ */
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index cffd754..ddc32eb 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -22,9 +22,11 @@ lib-y += usercopy_$(BITS).o getuser.o putuser.o
lib-y += memcpy_$(BITS).o
lib-$(CONFIG_KPROBES) += insn.o inat.o

-obj-y += msr.o msr-reg.o msr-reg-export.o
+obj-y += msr.o msr-reg.o msr-reg-export.o hweight.o

ifeq ($(CONFIG_X86_32),y)
+ CFLAGS_hweight.o = -fcall-saved-ecx -fcall-saved-edx
+
obj-y += atomic64_32.o
lib-y += checksum_32.o
lib-y += strstr_32.o
@@ -34,6 +36,10 @@ ifneq ($(CONFIG_X86_CMPXCHG64),y)
endif
lib-$(CONFIG_X86_USE_3DNOW) += mmx_32.o
else
+ CFLAGS_hweight.o = -fcall-saved-rsi -fcall-saved-rdx -fcall-saved-rcx \
+ -fcall-saved-r8 -fcall-saved-r9 -fcall-saved-r10 \
+ -fcall-saved-r11
+
obj-y += io_64.o iomap_copy_64.o
lib-y += csum-partial_64.o csum-copy_64.o csum-wrappers_64.o
lib-y += thunk_64.o clear_page_64.o copy_page_64.o
diff --git a/arch/x86/lib/hweight.c b/arch/x86/lib/hweight.c
new file mode 100644
index 0000000..9b4bfa5
--- /dev/null
+++ b/arch/x86/lib/hweight.c
@@ -0,0 +1,72 @@
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/bitops.h>
+
+#ifdef CONFIG_64BIT
+/* popcnt %rdi, %rax */
+#define POPCNT ".byte 0xf3\n\t.byte 0x48\n\t.byte 0x0f\n\t.byte 0xb8\n\t.byte 0xc7"
+#define REG_IN "D"
+#define REG_OUT "a"
+#else
+/* popcnt %eax, %eax */
+#define POPCNT ".byte 0xf3\n\t.byte 0x0f\n\t.byte 0xb8\n\t.byte 0xc0"
+#define REG_IN "a"
+#define REG_OUT "a"
+#endif
+
+/*
+ * Those are called out-of-line in the alternative below and are added here only
+ * so that gcc is able to figure out which registers have been clobbered by
+ * __sw_hweightXX so that it could restore their values before returning from
+ * the __arch_hweightXX versions. See also <arch/x86/lib/Makefile>.
+ */
+unsigned int _sw_hweight32(unsigned int w)
+{
+ return __sw_hweight32(w);
+}
+
+unsigned long _sw_hweight64(__u64 w)
+{
+ return __sw_hweight64(w);
+}
+
+unsigned int __arch_hweight32(unsigned int w)
+{
+ unsigned int res = 0;
+
+ asm (ALTERNATIVE("call _sw_hweight32", POPCNT, X86_FEATURE_POPCNT)
+ : "="REG_OUT (res)
+ : REG_IN (w));
+
+ return res;
+}
+EXPORT_SYMBOL(__arch_hweight32);
+
+unsigned int __arch_hweight16(unsigned int w)
+{
+ return __arch_hweight32(w & 0xffff);
+}
+EXPORT_SYMBOL(__arch_hweight16);
+
+unsigned int __arch_hweight8(unsigned int w)
+{
+ return __arch_hweight32(w & 0xff);
+}
+EXPORT_SYMBOL(__arch_hweight8);
+
+unsigned long __arch_hweight64(__u64 w)
+{
+ unsigned long res = 0;
+
+#ifdef CONFIG_X86_32
+ return __arch_hweight32((u32)w) +
+ __arch_hweight32((u32)(w >> 32));
+#else
+ asm (ALTERNATIVE("call _sw_hweight64", POPCNT, X86_FEATURE_POPCNT)
+ : "="REG_OUT (res)
+ : REG_IN (w));
+#endif /* CONFIG_X86_32 */
+
+ return res;
+}
+EXPORT_SYMBOL(__arch_hweight64);
diff --git a/include/asm-generic/bitops/arch_hweight.h b/include/asm-generic/bitops/arch_hweight.h
index 3a7be84..c72de8b 100644
--- a/include/asm-generic/bitops/arch_hweight.h
+++ b/include/asm-generic/bitops/arch_hweight.h
@@ -3,9 +3,9 @@

#include <asm/types.h>

-extern unsigned int __arch_hweight32(unsigned int w);
-extern unsigned int __arch_hweight16(unsigned int w);
-extern unsigned int __arch_hweight8(unsigned int w);
-extern unsigned long __arch_hweight64(__u64 w);
+extern unsigned int __sw_hweight32(unsigned int w);
+extern unsigned int __sw_hweight16(unsigned int w);
+extern unsigned int __sw_hweight8(unsigned int w);
+extern unsigned long __sw_hweight64(__u64 w);

#endif /* _ASM_GENERIC_BITOPS_HWEIGHT_H_ */
diff --git a/include/asm-generic/bitops/const_hweight.h b/include/asm-generic/bitops/const_hweight.h
index fa2a50b..8cf8169 100644
--- a/include/asm-generic/bitops/const_hweight.h
+++ b/include/asm-generic/bitops/const_hweight.h
@@ -18,13 +18,48 @@
#define __const_hweight32(w) (__const_hweight16(w) + __const_hweight16((w) >> 16))
#define __const_hweight64(w) (__const_hweight32(w) + __const_hweight32((w) >> 32))

+static inline unsigned int _hweight8(unsigned int w)
+{
+#ifdef __arch_hweight8
+ return __arch_hweight8(w);
+#else
+ return __sw_hweight8(w);
+#endif
+}
+
+static inline unsigned int _hweight16(unsigned int w)
+{
+#ifdef __arch_hweight16
+ return __arch_hweight16(w);
+#else
+ return __sw_hweight16(w);
+#endif
+}
+
+static inline unsigned int _hweight32(unsigned int w)
+{
+#ifdef __arch_hweight32
+ return __arch_hweight32(w);
+#else
+ return __sw_hweight32(w);
+#endif
+}
+
+static inline unsigned long _hweight64(__u64 w)
+{
+#ifdef __arch_hweight64
+ return __arch_hweight64(w);
+#else
+ return __sw_hweight64(w);
+#endif
+}
/*
* Generic interface.
*/
-#define hweight8(w) (__builtin_constant_p(w) ? __const_hweight8(w) : __arch_hweight8(w))
-#define hweight16(w) (__builtin_constant_p(w) ? __const_hweight16(w) : __arch_hweight16(w))
-#define hweight32(w) (__builtin_constant_p(w) ? __const_hweight32(w) : __arch_hweight32(w))
-#define hweight64(w) (__builtin_constant_p(w) ? __const_hweight64(w) : __arch_hweight64(w))
+#define hweight8(w) (__builtin_constant_p(w) ? __const_hweight8(w) : _hweight8(w))
+#define hweight16(w) (__builtin_constant_p(w) ? __const_hweight16(w) : _hweight16(w))
+#define hweight32(w) (__builtin_constant_p(w) ? __const_hweight32(w) : _hweight32(w))
+#define hweight64(w) (__builtin_constant_p(w) ? __const_hweight64(w) : _hweight64(w))

/*
* Interface for known constant arguments
diff --git a/lib/hweight.c b/lib/hweight.c
index 9ff86df..f9ce440 100644
--- a/lib/hweight.c
+++ b/lib/hweight.c
@@ -9,7 +9,7 @@
* The Hamming Weight of a number is the total number of bits set in it.
*/

-unsigned int __arch_hweight32(unsigned int w)
+unsigned int __sw_hweight32(unsigned int w)
{
unsigned int res = w - ((w >> 1) & 0x55555555);
res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
@@ -17,30 +17,30 @@ unsigned int __arch_hweight32(unsigned int w)
res = res + (res >> 8);
return (res + (res >> 16)) & 0x000000FF;
}
-EXPORT_SYMBOL(__arch_hweight32);
+EXPORT_SYMBOL(__sw_hweight32);

-unsigned int __arch_hweight16(unsigned int w)
+unsigned int __sw_hweight16(unsigned int w)
{
unsigned int res = w - ((w >> 1) & 0x5555);
res = (res & 0x3333) + ((res >> 2) & 0x3333);
res = (res + (res >> 4)) & 0x0F0F;
return (res + (res >> 8)) & 0x00FF;
}
-EXPORT_SYMBOL(__arch_hweight16);
+EXPORT_SYMBOL(__sw_hweight16);

-unsigned int __arch_hweight8(unsigned int w)
+unsigned int __sw_hweight8(unsigned int w)
{
unsigned int res = w - ((w >> 1) & 0x55);
res = (res & 0x33) + ((res >> 2) & 0x33);
return (res + (res >> 4)) & 0x0F;
}
-EXPORT_SYMBOL(__arch_hweight8);
+EXPORT_SYMBOL(__sw_hweight8);

-unsigned long __arch_hweight64(__u64 w)
+unsigned long __sw_hweight64(__u64 w)
{
#if BITS_PER_LONG == 32
- return __arch_hweight32((unsigned int)(w >> 32)) +
- __arch_hweight32((unsigned int)w);
+ return __sw_hweight32((unsigned int)(w >> 32)) +
+ __sw_hweight32((unsigned int)w);
#elif BITS_PER_LONG == 64
#ifdef ARCH_HAS_FAST_MULTIPLIER
w -= (w >> 1) & 0x5555555555555555ul;
@@ -57,4 +57,4 @@ unsigned long __arch_hweight64(__u64 w)
#endif
#endif
}
-EXPORT_SYMBOL(__arch_hweight64);
+EXPORT_SYMBOL(__sw_hweight64);
--
1.6.6.1


--
Regards/Gruss,
Boris.

--
Advanced Micro Devices, Inc.
Operating Systems Research Center
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