The patch titled
Subject: lib/bch.c: use __builtin_parity() when available
has been added to the -mm mm-nonmm-unstable branch. Its filename is
lib-bchc-use-__builtin_parity-when-available.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-bchc-use-__builtin_parity-when-available.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: lib/bch.c: use __builtin_parity() when available
Date: Wed, 29 Jan 2025 21:57:31 +0100
Compilers (GCC and clang) provide optimized __builtin_parity() function
that returns the parity of X, i.e. the number of 1-bits in X modulo 2.
Use __builtin_parity() built-in function to optimize parity(). This
improves generated code on x86_64 from:
movl %edi, %edx
shrl %edx
xorl %edi, %edx
movl %edx, %eax
shrl $2, %eax
xorl %edx, %eax
andl $286331153, %eax
imull $286331153, %eax, %eax
shrl $28, %eax
andl $1, %eax
to an optimized:
movl %edi, %ecx
shrl $16, %ecx
xorl %edi, %ecx
xorl %eax, %eax
xorb %ch, %cl
setnp %al
Please note SETNP instruction that exercises hardware parity
calculation of x86 processors. When POPCNT instruction is
available, the generated code gets optimized even further:
popcntl %edi, %eax
andl $1, %eax
Compile-tested only.
Link: https://lkml.kernel.org/r/20250129205746.10963-1-ubizjak@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/bch.c | 4 ++++
1 file changed, 4 insertions(+)
--- a/lib/bch.c~lib-bchc-use-__builtin_parity-when-available
+++ a/lib/bch.c
@@ -313,6 +313,9 @@ static inline int deg(unsigned int poly)
static inline int parity(unsigned int x)
{
+#if __has_builtin(__builtin_parity)
+ return __builtin_parity(x);
+#else
/*
* public domain code snippet, lifted from
* http://www-graphics.stanford.edu/~seander/bithacks.html
@@ -321,6 +324,7 @@ static inline int parity(unsigned int x)
x ^= x >> 2;
x = (x & 0x11111111U) * 0x11111111U;
return (x >> 28) & 1;
+#endif
}
/* Galois field basic operations: multiply, divide, inverse, etc. */
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
x86-kgdb-use-is_err_pcpu-macro.patch
compilerh-introduce-typeof_unqual-macro.patch
percpu-use-typeof_unqual-in-variable-declarations.patch
percpu-use-typeof_unqual-in-_cpu_ptr-accessors.patch
percpu-repurpose-__percpu-tag-as-a-named-address-space-qualifier.patch
percpu-x86-enable-strict-percpu-checks-via-named-as-qualifiers.patch
lib-bchc-use-__builtin_parity-when-available.patch
