From: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
To: Borislav Petkov <bp@alien8.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H . Peter Anvin" <hpa@zytor.com>,
Nathan Chancellor <nathan@kernel.org>, Tom Rix <trix@redhat.com>,
linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
David Howells <dhowells@redhat.com>,
Jan Beulich <JBeulich@suse.com>,
Christophe Jaillet <christophe.jaillet@wanadoo.fr>,
Joe Perches <joe@perches.com>,
Josh Poimboeuf <jpoimboe@kernel.org>,
Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Subject: [PATCH v5 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions
Date: Fri, 12 Aug 2022 20:44:37 +0900 [thread overview]
Message-ID: <20220812114438.1574-2-mailhol.vincent@wanadoo.fr> (raw)
In-Reply-To: <20220812114438.1574-1-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 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>
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
next prev parent reply other threads:[~2022-08-12 11:53 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-11 16:03 [PATCH v2 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions Vincent Mailhol
2022-05-11 16:03 ` [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-05-11 20:56 ` Christophe JAILLET
2022-05-11 23:30 ` Vincent MAILHOL
2022-05-11 21:35 ` Nick Desaulniers
2022-05-11 23:48 ` Vincent MAILHOL
2022-05-11 16:03 ` [PATCH v2 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-05-11 22:20 ` Nick Desaulniers
2022-05-11 23:23 ` Vincent MAILHOL
2022-05-12 0:03 ` [PATCH v3 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-05-12 0:03 ` [PATCH v3 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-05-12 0:28 ` Nick Desaulniers
2022-05-12 1:18 ` Vincent MAILHOL
2022-05-12 0:03 ` [PATCH v3 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-05-12 0:19 ` Nick Desaulniers
2022-05-12 1:18 ` [PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-05-12 1:18 ` [PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-05-12 3:02 ` Joe Perches
2022-05-12 4:29 ` Vincent MAILHOL
2022-05-12 1:18 ` [PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-05-23 9:22 ` [PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent MAILHOL
2022-06-25 7:26 ` [RESEND PATCH " Vincent Mailhol
2022-06-25 7:26 ` [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-06-25 7:26 ` [RESEND PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-07-23 15:15 ` [RESEND PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-07-23 15:15 ` [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-08-11 14:59 ` Borislav Petkov
2022-08-12 11:55 ` Vincent MAILHOL
2022-07-23 15:15 ` [RESEND PATCH v4 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-07-29 11:24 ` [RESEND PATCH v4 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent MAILHOL
2022-07-29 12:22 ` Borislav Petkov
2022-07-29 13:50 ` Vincent MAILHOL
2022-08-12 11:44 ` [PATCH v5 " Vincent Mailhol
2022-08-12 11:44 ` Vincent Mailhol [this message]
2022-08-12 11:44 ` [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate " Vincent Mailhol
2022-08-23 16:23 ` Borislav Petkov
2022-08-23 17:12 ` Nick Desaulniers
2022-08-23 17:43 ` Borislav Petkov
2022-08-23 20:31 ` Vincent MAILHOL
2022-08-24 8:43 ` Borislav Petkov
2022-08-24 12:10 ` Vincent MAILHOL
2022-08-24 13:24 ` Borislav Petkov
2022-08-26 21:32 ` Vincent MAILHOL
2022-09-07 4:06 ` Borislav Petkov
2022-09-07 5:35 ` Vincent MAILHOL
2022-09-07 8:50 ` Borislav Petkov
2022-08-31 7:57 ` [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Vincent Mailhol
2022-08-31 7:57 ` [PATCH v6 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-08-31 7:57 ` [PATCH v6 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-08-31 8:51 ` [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Yury Norov
2022-09-01 3:49 ` Yury Norov
2022-09-01 10:30 ` Vincent MAILHOL
2022-09-01 14:19 ` Yury Norov
2022-09-01 17:06 ` Nick Desaulniers
2022-09-02 5:34 ` Borislav Petkov
2022-09-02 0:41 ` Vincent MAILHOL
2022-09-02 1:19 ` Vincent MAILHOL
2022-09-05 0:37 ` [PATCH v7 " Vincent Mailhol
2022-09-05 0:37 ` [PATCH v7 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-09-05 0:37 ` [PATCH v7 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-09-06 18:26 ` [PATCH v7 0/2] x86/asm/bitops: optimize ff{s,z} functions for " Nick Desaulniers
2022-09-07 7:04 ` Nick Desaulniers
2022-09-07 7:49 ` Vincent MAILHOL
2022-09-07 9:09 ` [PATCH v8 " Vincent Mailhol
2022-09-07 9:09 ` [PATCH v8 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate " Vincent Mailhol
2022-09-07 9:09 ` [PATCH v8 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl " Vincent Mailhol
2022-09-20 20:00 ` [tip: x86/asm] x86/asm/bitops: Use __builtin_ctzl() " tip-bot2 for Vincent Mailhol
2022-09-20 20:00 ` [tip: x86/asm] x86/asm/bitops: Use __builtin_ffs() " tip-bot2 for Vincent Mailhol
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