[PATCH] arm: Use builtins for ffs/fls
Sean Anderson
sean.anderson at seco.com
Mon Jul 31 23:27:33 CEST 2023
Since ARMv5, the clz instruction allows for efficient implementation of
ffs/fls with builtins. Until ARMv7 (with Thumb-2), this instruction is
only available in ARM mode. LTO makes it difficult to force specific
functions to be in ARM mode, as it is effectively a form of very
aggressive inlining. To work around this, fls/ffs are implemented in
assembly for ARMv5 and ARMv6 when compiling U-Boot in Thumb mode.
Overall, this saves around 75 bytes per call.
Signed-off-by: Sean Anderson <sean.anderson at seco.com>
---
I only tested this on ARMv8. If someone has an ARMv5 or ARMv6 board,
please test this.
arch/arm/include/asm/bitops.h | 27 ++++++++++++-
arch/arm/lib/Makefile | 5 +++
arch/arm/lib/bitops.S | 45 ++++++++++++++++++++++
include/asm-generic/bitops/builtin-__ffs.h | 16 ++++++++
include/asm-generic/bitops/builtin-__fls.h | 16 ++++++++
include/asm-generic/bitops/builtin-ffs.h | 15 ++++++++
include/asm-generic/bitops/builtin-fls.h | 17 ++++++++
7 files changed, 140 insertions(+), 1 deletion(-)
create mode 100644 arch/arm/lib/bitops.S
create mode 100644 include/asm-generic/bitops/builtin-__ffs.h
create mode 100644 include/asm-generic/bitops/builtin-__fls.h
create mode 100644 include/asm-generic/bitops/builtin-ffs.h
create mode 100644 include/asm-generic/bitops/builtin-fls.h
diff --git a/arch/arm/include/asm/bitops.h b/arch/arm/include/asm/bitops.h
index fa8548624a..8e897833bb 100644
--- a/arch/arm/include/asm/bitops.h
+++ b/arch/arm/include/asm/bitops.h
@@ -15,9 +15,34 @@
#ifndef __ASM_ARM_BITOPS_H
#define __ASM_ARM_BITOPS_H
+#if __LINUX_ARM_ARCH__ < 5
+
#include <asm-generic/bitops/__ffs.h>
#include <asm-generic/bitops/__fls.h>
#include <asm-generic/bitops/fls.h>
+
+#else
+
+#define PLATFORM_FFS
+#define PLATFORM_FLS
+
+#if !IS_ENABLED(CONFIG_HAS_THUMB2) && CONFIG_IS_ENABLED(SYS_THUMB_BUILD)
+
+unsigned long __fls(unsigned long word);
+unsigned long __ffs(unsigned long word);
+int fls(unsigned int x);
+int ffs(int x);
+
+#else
+
+#include <asm-generic/bitops/builtin-__fls.h>
+#include <asm-generic/bitops/builtin-__ffs.h>
+#include <asm-generic/bitops/builtin-fls.h>
+#include <asm-generic/bitops/builtin-ffs.h>
+
+#endif
+#endif
+
#include <asm-generic/bitops/fls64.h>
#ifdef __KERNEL__
@@ -113,7 +138,7 @@ static inline int test_bit(int nr, const void * addr)
static inline int __ilog2(unsigned int x)
{
- return generic_fls(x) - 1;
+ return fls(x) - 1;
}
#define ffz(x) __ffs(~(x))
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index 62cf80f373..b1bcd37466 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -113,6 +113,11 @@ AFLAGS_REMOVE_memset.o := -mthumb -mthumb-interwork
AFLAGS_REMOVE_memcpy.o := -mthumb -mthumb-interwork
AFLAGS_memset.o := -DMEMSET_NO_THUMB_BUILD
AFLAGS_memcpy.o := -DMEMCPY_NO_THUMB_BUILD
+
+# This is only necessary to force ARM mode on THUMB1 targets.
+ifneq ($(CONFIG_SYS_ARM_ARCH),4)
+obj-y += bitops.o
+endif
endif
endif
diff --git a/arch/arm/lib/bitops.S b/arch/arm/lib/bitops.S
new file mode 100644
index 0000000000..29d1524634
--- /dev/null
+++ b/arch/arm/lib/bitops.S
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2023 Sean Anderson <sean.anderson at seco.com>
+ *
+ * ARM bitops to call when using THUMB1, which doesn't have these instructions.
+ */
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+.pushsection .text.__fls
+ENTRY(__fls)
+ clz r0, r0
+ rsb r0, r0, #31
+ ret lr
+ENDPROC(__fls)
+.popsection
+
+.pushsection .text.__ffs
+ENTRY(__ffs)
+ rsb r3, r0, #0
+ and r0, r0, r3
+ clz r0, r0
+ rsb r0, r0, #31
+ ret lr
+ENDPROC(__ffs)
+.popsection
+
+.pushsection .text.fls
+ENTRY(fls)
+ cmp r0, #0
+ clzne r0, r0
+ rsbne r0, r0, #32
+ ret lr
+ENDPROC(fls)
+.popsection
+
+.pushsection .text.ffs
+ENTRY(ffs)
+ rsb r3, r0, #0
+ and r0, r0, r3
+ clz r0, r0
+ rsb r0, r0, #32
+ ret lr
+ENDPROC(ffs)
+.popsection
diff --git a/include/asm-generic/bitops/builtin-__ffs.h b/include/asm-generic/bitops/builtin-__ffs.h
new file mode 100644
index 0000000000..87024da44d
--- /dev/null
+++ b/include/asm-generic/bitops/builtin-__ffs.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS_BUILTIN___FFS_H_
+#define _ASM_GENERIC_BITOPS_BUILTIN___FFS_H_
+
+/**
+ * __ffs - find first 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)
+{
+ return __builtin_ctzl(word);
+}
+
+#endif
diff --git a/include/asm-generic/bitops/builtin-__fls.h b/include/asm-generic/bitops/builtin-__fls.h
new file mode 100644
index 0000000000..43a5aa9afb
--- /dev/null
+++ b/include/asm-generic/bitops/builtin-__fls.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS_BUILTIN___FLS_H_
+#define _ASM_GENERIC_BITOPS_BUILTIN___FLS_H_
+
+/**
+ * __fls - find last (most-significant) set bit in a long 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)
+{
+ return (sizeof(word) * 8) - 1 - __builtin_clzl(word);
+}
+
+#endif
diff --git a/include/asm-generic/bitops/builtin-ffs.h b/include/asm-generic/bitops/builtin-ffs.h
new file mode 100644
index 0000000000..7b12932904
--- /dev/null
+++ b/include/asm-generic/bitops/builtin-ffs.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS_BUILTIN_FFS_H_
+#define _ASM_GENERIC_BITOPS_BUILTIN_FFS_H_
+
+/**
+ * ffs - find first bit set
+ * @x: the word to search
+ *
+ * This is defined the same way as
+ * the libc and compiler builtin ffs routines, therefore
+ * differs in spirit from ffz (man ffs).
+ */
+#define ffs(x) __builtin_ffs(x)
+
+#endif
diff --git a/include/asm-generic/bitops/builtin-fls.h b/include/asm-generic/bitops/builtin-fls.h
new file mode 100644
index 0000000000..c8455cc288
--- /dev/null
+++ b/include/asm-generic/bitops/builtin-fls.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_BITOPS_BUILTIN_FLS_H_
+#define _ASM_GENERIC_BITOPS_BUILTIN_FLS_H_
+
+/**
+ * fls - find last (most-significant) bit set
+ * @x: the word to search
+ *
+ * This is defined the same way as ffs.
+ * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
+ */
+static __always_inline int fls(unsigned int x)
+{
+ return x ? sizeof(x) * 8 - __builtin_clz(x) : 0;
+}
+
+#endif
--
2.40.1
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