[PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions

[Vincent Mailhol <mailhol.vincent@wanadoo.fr>]

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 simplify 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

For both example, we clearly see 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).

This patch uses the __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, this patch 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 applying this patch...:

| $ 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()")
http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@warthog.procyon.org.uk

Reviewed-by: Nick Desaulniers <ndesaulniers@google.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 a288ecd230ab..6ed979547086 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -269,18 +269,7 @@ 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;
 
@@ -310,6 +299,19 @@ 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