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Message-Id: <20221106095106.849154-2-mailhol.vincent@wanadoo.fr>
Date: Sun, 6 Nov 2022 18:51:05 +0900
From: Vincent Mailhol <mailhol.vincent@...adoo.fr>
To: x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org, Yury Norov <yury.norov@...il.com>,
llvm@...ts.linux.dev, Vincent Mailhol <mailhol.vincent@...adoo.fr>,
Borislav Petkov <bp@...e.de>
Subject: [PATCH v1 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation
Below snippet:
#include <linux/bitops.h>
unsigned int foo(unsigned long word)
{
return __fls(word);
}
produces this on GCC 12.1.0:
0000000000000000 <foo>:
0: f3 0f 1e fa endbr64
4: e8 00 00 00 00 call 9 <foo+0x9>
9: 53 push %rbx
a: 48 89 fb mov %rdi,%rbx
d: e8 00 00 00 00 call 12 <foo+0x12>
12: 48 0f bd c3 bsr %rbx,%rax
16: 5b pop %rbx
17: 31 ff xor %edi,%edi
19: e9 00 00 00 00 jmp 1e <foo+0x1e>
and that on clang 14.0.6:
0000000000000000 <foo>:
0: f3 0f 1e fa endbr64
4: e8 00 00 00 00 call 9 <foo+0x9>
9: 53 push %rbx
a: 50 push %rax
b: 48 89 fb mov %rdi,%rbx
e: e8 00 00 00 00 call 13 <foo+0x13>
13: 48 89 1c 24 mov %rbx,(%rsp)
17: 48 0f bd 04 24 bsr (%rsp),%rax
1c: 48 83 c4 08 add $0x8,%rsp
20: 5b pop %rbx
21: c3 ret
The implementation from <asm-generic/bitops/builtin-__fls.h> [1]
produces the exact same code on GCC and below code on clang:
0000000000000000 <foo>:
0: f3 0f 1e fa endbr64
4: e8 00 00 00 00 call 9 <foo+0x9>
9: 53 push %rbx
a: 48 89 fb mov %rdi,%rbx
d: e8 00 00 00 00 call 12 <foo+0x12>
12: 48 0f bd c3 bsr %rbx,%rax
16: 5b pop %rbx
17: c3 ret
The builtin implementation is better for two reasons:
1/ it saves two instructions on clang (a push and a stack pointer
decrement) because of a useless tentative to save rax.
2/ when used on constant expressions, the compiler is only able to
fold the builtin version (c.f. [2]).
For those two reasons, replace the assembly implementation by its
builtin counterpart.
[1] https://elixir.bootlin.com/linux/v6.0/source/include/asm-generic/bitops/builtin-__fls.h
[2] commit 146034fed6ee ("x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions")
CC: Borislav Petkov <bp@...e.de>
CC: Nick Desaulniers <ndesaulniers@...gle.com>
CC: Yury Norov <yury.norov@...il.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>
---
arch/x86/include/asm/bitops.h | 14 +-------------
1 file changed, 1 insertion(+), 13 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 2edf68475fec..a31453d7686d 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -285,19 +285,7 @@ static __always_inline unsigned long variable_ffz(unsigned long word)
(unsigned long)__builtin_ctzl(~word) : \
variable_ffz(word))
-/*
- * __fls: find last set bit in word
- * @word: The word to search
- *
- * Undefined if no set bit exists, so code should check against 0 first.
- */
-static __always_inline unsigned long __fls(unsigned long word)
-{
- asm("bsr %1,%0"
- : "=r" (word)
- : "rm" (word));
- return word;
-}
+#include <asm-generic/bitops/builtin-__fls.h>
#undef ADDR
--
2.37.4
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