[PATCH v7 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions

Vincent Mailhol
2022-09-05T00:38:15Z

For x86_64, the current ffs() implementation does not produce
optimized code when called with a constant expression. On the
contrary, the __builtin_ffs() function of both GCC and clang is able
to fold the expression into a single instruction.

** Example **

Let's consider two dummy functions foo() and bar() as below:

#include <linux/bitops.h>
#define CONST 0x01000000

unsigned int foo(void)
{
return ffs(CONST);
}

unsigned int bar(void)
{
return __builtin_ffs(CONST);
}

GCC would produce below assembly code:

0000000000000000 <foo>:
0: ba 00 00 00 01 mov $0x1000000,%edx
5: b8 ff ff ff ff mov $0xffffffff,%eax
a: 0f bc c2 bsf %edx,%eax
d: 83 c0 01 add $0x1,%eax
10: c3 ret
<Instructions after ret and before next function were redacted>

0000000000000020 <bar>:
20: b8 19 00 00 00 mov $0x19,%eax
25: c3 ret

And clang would produce:

0000000000000000 <foo>:
0: b8 ff ff ff ff mov $0xffffffff,%eax
5: 0f bc 05 00 00 00 00 bsf 0x0(%rip),%eax # c <foo+0xc>
c: 83 c0 01 add $0x1,%eax
f: c3 ret

0000000000000010 <bar>:
10: b8 19 00 00 00 mov $0x19,%eax
15: c3 ret

Both examples clearly demonstrate the benefit of using __builtin_ffs()
instead of the kernel's asm implementation for constant expressions.

However, for non constant expressions, the ffs() asm version of the
kernel remains better for x86_64 because, contrary to GCC, it doesn't
emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is
able optimize out the CMOV call).

Use __builtin_constant_p() to select between the kernel's ffs() and
the __builtin_ffs() depending on whether the argument is constant or
not.

As a side benefit, replacing the ffs() function declaration by a macro
also removes below -Wshadow warning:

./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]
283 | static __always_inline int ffs(int x)

** Statistics **

On a allyesconfig, before...:

$ objdump -d vmlinux.o | grep bsf | wc -l
1081

...and after:

$ objdump -d vmlinux.o | grep bsf | wc -l
792

So, roughly 26.7% of the calls to ffs() were using constant
expressions and could be optimized out.

(tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)

[1] commit ca3d30cc02f7 ("x86_64, asm: Optimise fls(), ffs() and fls64()")
Link: http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@warthog.procyon.org.uk

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
arch/x86/include/asm/bitops.h | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 0fe9de58af31..879238e5a6a0 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -292,18 +292,7 @@ static __always_inline unsigned long __fls(unsigned long word)
#undef ADDR

#ifdef __KERNEL__
-/**
- * ffs - find first set bit in word
- * @x: the word to search
- *
- * This is defined the same way as the libc and compiler builtin ffs
- * routines, therefore differs in spirit from the other bitops.
- *
- * ffs(value) returns 0 if value is 0 or the position of the first
- * set bit if value is nonzero. The first (least significant) bit
- * is at position 1.
- */
-static __always_inline int ffs(int x)
+static __always_inline int variable_ffs(int x)
{
int r;

@@ -333,6 +322,19 @@ static __always_inline int ffs(int x)
return r + 1;
}

+/**
+ * ffs - find first set bit in word
+ * @x: the word to search
+ *
+ * This is defined the same way as the libc and compiler builtin ffs
+ * routines, therefore differs in spirit from the other bitops.
+ *
+ * ffs(value) returns 0 if value is 0 or the position of the first
+ * set bit if value is nonzero. The first (least significant) bit
+ * is at position 1.
+ */
+#define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : variable_ffs(x))
+
/**
* fls - find last set bit in word
* @x: the word to search
--
2.35.1