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Message-Id: <20220831075742.295-3-mailhol.vincent@wanadoo.fr>
Date: Wed, 31 Aug 2022 16:57:42 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: Borislav Petkov <bp@...en8.de>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
Peter Zijlstra <peterz@...radead.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H . Peter Anvin" <hpa@...or.com>,
Nathan Chancellor <nathan@...nel.org>,
Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>,
Jan Beulich <JBeulich@...e.com>,
Christophe Jaillet <christophe.jaillet@...adoo.fr>,
Joe Perches <joe@...ches.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Vincent Mailhol <mailhol.vincent@...adoo.fr>
Subject: [PATCH v6 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions
__ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x)
is equivalent to (unsigned long)__builtin_ctzl(~x). Because
__builting_ctzl() returns an int, a cast to (unsigned long) is
necessary to avoid potential warnings on implicit casts.
For x86_64, the current __ffs() and ffz() implementations do not
produce optimized code when called with a constant expression. On the
contrary, the __builtin_ctzl() folds into a single instruction.
However, for non constant expressions, the __ffs() and ffz() asm
versions of the kernel remains slightly better than the code produced
by GCC (it produces a useless instruction to clear eax).
Use __builtin_constant_p() to select between the kernel's
__ffs()/ffz() and the __builtin_ctzl() depending on whether the
argument is constant or not.
** Statistics **
On a allyesconfig, before...:
$ objdump -d vmlinux.o | grep tzcnt | wc -l
3607
...and after:
$ objdump -d vmlinux.o | grep tzcnt | wc -l
2600
So, roughly 27.9% of the calls to either __ffs() or ffz() were using
constant expressions and could be optimized out.
(tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)
Note: on x86_64, the asm bsf instruction produces tzcnt when used with
the ret prefix (which explain the use of `grep tzcnt' instead of `grep
bsf' in above benchmark). c.f. [1]
[1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally")
Link: http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
arch/x86/include/asm/bitops.h | 38 ++++++++++++++++++++++-------------
1 file changed, 24 insertions(+), 14 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 6ed979547086..bd49aef87ab6 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -224,13 +224,7 @@ static __always_inline bool variable_test_bit(long nr, volatile const unsigned l
? constant_test_bit((nr), (addr)) \
: variable_test_bit((nr), (addr)))
-/**
- * __ffs - find first set bit in word
- * @word: The word to search
- *
- * Undefined if no bit exists, so code should check against 0 first.
- */
-static __always_inline unsigned long __ffs(unsigned long word)
+static __always_inline unsigned long variable__ffs(unsigned long word)
{
asm("rep; bsf %1,%0"
: "=r" (word)
@@ -238,13 +232,18 @@ static __always_inline unsigned long __ffs(unsigned long word)
return word;
}
-/**
- * ffz - find first zero bit in word
- * @word: The word to search
- *
- * Undefined if no zero exists, so code should check against ~0UL first.
- */
-static __always_inline unsigned long ffz(unsigned long word)
+/**
+ * __ffs - find first set bit in word
+ * @word: The word to search
+ *
+ * Undefined if no bit exists, so code should check against 0 first.
+ */
+#define __ffs(word) \
+ (__builtin_constant_p(word) ? \
+ (unsigned long)__builtin_ctzl(word) : \
+ variable__ffs(word))
+
+static __always_inline unsigned long variable_ffz(unsigned long word)
{
asm("rep; bsf %1,%0"
: "=r" (word)
@@ -252,6 +251,17 @@ static __always_inline unsigned long ffz(unsigned long word)
return word;
}
+/**
+ * ffz - find first zero bit in word
+ * @word: The word to search
+ *
+ * Undefined if no zero exists, so code should check against ~0UL first.
+ */
+#define ffz(word) \
+ (__builtin_constant_p(word) ? \
+ (unsigned long)__builtin_ctzl(~word) : \
+ variable_ffz(word))
+
/*
* __fls: find last set bit in word
* @word: The word to search
--
2.35.1
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