From: Nicolas Pitre <nicolas.pitre@linaro.org>
Date: Wed, 25 Jul 2018 03:38:54 +0000 (+0100)
Subject: ARM: 8785/1: use compiler built-ins for ffs and fls
X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=001a30c4d0e871b21d296e1bcafc6756a1b10607;p=openwrt%2Fstaging%2Fblogic.git

ARM: 8785/1: use compiler built-ins for ffs and fls

On ARMv5 and above, it is beneficial to use compiler built-ins such as
__builtin_ffs() and __builtin_ctzl() to implement ffs(), __ffs(), fls()
and __fls(). The compiler does inline the clz instruction and even the
rbit instruction when available, or provide a constant value when
possible. On ARMv4 the compiler calls out to helper functions for those
built-ins so it is best to keep the open coded versions in that case.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
---

diff --git a/arch/arm/include/asm/bitops.h b/arch/arm/include/asm/bitops.h
index 4cab9bb823fb..c92e42a5c8f7 100644
--- a/arch/arm/include/asm/bitops.h
+++ b/arch/arm/include/asm/bitops.h
@@ -215,7 +215,6 @@ extern int _find_next_bit_be(const unsigned long *p, int size, int offset);
 
 #if __LINUX_ARM_ARCH__ < 5
 
-#include <asm-generic/bitops/ffz.h>
 #include <asm-generic/bitops/__fls.h>
 #include <asm-generic/bitops/__ffs.h>
 #include <asm-generic/bitops/fls.h>
@@ -223,93 +222,20 @@ extern int _find_next_bit_be(const unsigned long *p, int size, int offset);
 
 #else
 
-static inline int constant_fls(int x)
-{
-	int r = 32;
-
-	if (!x)
-		return 0;
-	if (!(x & 0xffff0000u)) {
-		x <<= 16;
-		r -= 16;
-	}
-	if (!(x & 0xff000000u)) {
-		x <<= 8;
-		r -= 8;
-	}
-	if (!(x & 0xf0000000u)) {
-		x <<= 4;
-		r -= 4;
-	}
-	if (!(x & 0xc0000000u)) {
-		x <<= 2;
-		r -= 2;
-	}
-	if (!(x & 0x80000000u)) {
-		x <<= 1;
-		r -= 1;
-	}
-	return r;
-}
-
-/*
- * On ARMv5 and above those functions can be implemented around the
- * clz instruction for much better code efficiency.  __clz returns
- * the number of leading zeros, zero input will return 32, and
- * 0x80000000 will return 0.
- */
-static inline unsigned int __clz(unsigned int x)
-{
-	unsigned int ret;
-
-	asm("clz\t%0, %1" : "=r" (ret) : "r" (x));
-
-	return ret;
-}
-
-/*
- * fls() returns zero if the input is zero, otherwise returns the bit
- * position of the last set bit, where the LSB is 1 and MSB is 32.
- */
-static inline int fls(int x)
-{
-	if (__builtin_constant_p(x))
-	       return constant_fls(x);
-
-	return 32 - __clz(x);
-}
-
-/*
- * __fls() returns the bit position of the last bit set, where the
- * LSB is 0 and MSB is 31.  Zero input is undefined.
- */
-static inline unsigned long __fls(unsigned long x)
-{
-	return fls(x) - 1;
-}
-
-/*
- * ffs() returns zero if the input was zero, otherwise returns the bit
- * position of the first set bit, where the LSB is 1 and MSB is 32.
- */
-static inline int ffs(int x)
-{
-	return fls(x & -x);
-}
-
 /*
- * __ffs() returns the bit position of the first bit set, where the
- * LSB is 0 and MSB is 31.  Zero input is undefined.
+ * On ARMv5 and above, the gcc built-ins may rely on the clz instruction
+ * and produce optimal inlined code in all cases. On ARMv7 it is even
+ * better by also using the rbit instruction.
  */
-static inline unsigned long __ffs(unsigned long x)
-{
-	return ffs(x) - 1;
-}
-
-#define ffz(x) __ffs( ~(x) )
+#include <asm-generic/bitops/builtin-__fls.h>
+#include <asm-generic/bitops/builtin-__ffs.h>
+#include <asm-generic/bitops/builtin-fls.h>
+#include <asm-generic/bitops/builtin-ffs.h>
 
 #endif
 
+#include <asm-generic/bitops/ffz.h>
+
 #include <asm-generic/bitops/fls64.h>
 
 #include <asm-generic/bitops/sched.h>