Make ARCH_HAS_FAST_MULTIPLIER a real config variable

It used to be an ad-hoc hack defined by the x86 version of
<asm/bitops.h> that enabled a couple of library routines to know whether
an integer multiply is faster than repeated shifts and additions.

This just makes it use the real Kconfig system instead, and makes x86
(which was the only architecture that did this) select the option.

NOTE! Even for x86, this really is kind of wrong.  If we cared, we would
probably not enable this for builds optimized for netburst (P4), where
shifts-and-adds are generally faster than multiplies.  This patch does
*not* change that kind of logic, though, it is purely a syntactic change
with no code changes.

This was triggered by the fact that we have other places that really
want to know "do I want to expand multiples by constants by hand or
not", particularly the hash generation code.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/Kconfig

@@ -23,6 +23,7 @@ config X86
 	def_bool y
 	select ARCH_MIGHT_HAVE_ACPI_PDC if ACPI
 	select ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS
+	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
 	select HAVE_AOUT if X86_32

arch/x86/include/asm/bitops.h

@@ -497,8 +497,6 @@ static __always_inline int fls64(__u64 x)
 
 #include <asm-generic/bitops/sched.h>
 
-#define ARCH_HAS_FAST_MULTIPLIER 1
-
 #include <asm/arch_hweight.h>
 
 #include <asm-generic/bitops/const_hweight.h>

lib/Kconfig

@@ -51,6 +51,9 @@ config PERCPU_RWSEM
 config ARCH_USE_CMPXCHG_LOCKREF
 	bool
 
+config ARCH_HAS_FAST_MULTIPLIER
+	bool
+
 config CRC_CCITT
 	tristate "CRC-CCITT functions"
 	help

lib/hweight.c

@@ -11,7 +11,7 @@
 
 unsigned int __sw_hweight32(unsigned int w)
 {
-#ifdef ARCH_HAS_FAST_MULTIPLIER
+#ifdef CONFIG_ARCH_HAS_FAST_MULTIPLIER
 	w -= (w >> 1) & 0x55555555;
 	w =  (w & 0x33333333) + ((w >> 2) & 0x33333333);
 	w =  (w + (w >> 4)) & 0x0f0f0f0f;
@@ -49,7 +49,7 @@ unsigned long __sw_hweight64(__u64 w)
 	return __sw_hweight32((unsigned int)(w >> 32)) +
 	       __sw_hweight32((unsigned int)w);
 #elif BITS_PER_LONG == 64
-#ifdef ARCH_HAS_FAST_MULTIPLIER
+#ifdef CONFIG_ARCH_HAS_FAST_MULTIPLIER
 	w -= (w >> 1) & 0x5555555555555555ul;
 	w =  (w & 0x3333333333333333ul) + ((w >> 2) & 0x3333333333333333ul);
 	w =  (w + (w >> 4)) & 0x0f0f0f0f0f0f0f0ful;

lib/string.c

@@ -807,9 +807,9 @@ void *memchr_inv(const void *start, int c, size_t bytes)
 		return check_bytes8(start, value, bytes);
 
 	value64 = value;
-#if defined(ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
+#if defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
 	value64 *= 0x0101010101010101;
-#elif defined(ARCH_HAS_FAST_MULTIPLIER)
+#elif defined(CONFIG_ARCH_HAS_FAST_MULTIPLIER)
 	value64 *= 0x01010101;
 	value64 |= value64 << 32;
 #else