[PATCH v3 7/8] riscv: lib: add strchr implementation

Feng Jiang
2026-01-20T06:59:12Z

Add an assembly implementation of strchr() for RISC-V.

By eliminating stack frame management (prologue/epilogue) and optimizing
the function entries, the assembly version provides significant relative
gains for short strings where the fixed overhead of the C function is
most prominent. As the string length increases, the performance converges
with the byte-oriented scan logic.

Benchmark results (QEMU TCG, rv64):
Length | Original (MB/s) | Optimized (MB/s) | Improvement
-------|-----------------|------------------|------------
1 B | 21 | 23 | +9.5%
7 B | 118 | 126 | +6.7%
16 B | 200 | 208 | +4.0%
512 B | 375 | 399 | +6.4%
4096 B | 395 | 401 | +1.5%

Suggested-by: Andy Shevchenko <andy@kernel.org>
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
---
arch/riscv/include/asm/string.h | 3 +++
arch/riscv/lib/Makefile | 1 +
arch/riscv/lib/strchr.S | 35 +++++++++++++++++++++++++++++++++
arch/riscv/purgatory/Makefile | 5 ++++-
4 files changed, 43 insertions(+), 1 deletion(-)
create mode 100644 arch/riscv/lib/strchr.S

diff --git a/arch/riscv/include/asm/string.h b/arch/riscv/include/asm/string.h
index 16634d67c217..ca3ade82b124 100644
--- a/arch/riscv/include/asm/string.h
+++ b/arch/riscv/include/asm/string.h
@@ -31,6 +31,9 @@ extern asmlinkage int strncmp(const char *cs, const char *ct, size_t count);

#define __HAVE_ARCH_STRNLEN
extern asmlinkage __kernel_size_t strnlen(const char *, size_t);
+
+#define __HAVE_ARCH_STRCHR
+extern asmlinkage char *strchr(const char *, int);
#endif

/* For those files which don't want to check by kasan. */
diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index 0969d8136df0..b7f804dce1c3 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -8,6 +8,7 @@ lib-y += strcmp.o
lib-y += strlen.o
lib-y += strncmp.o
lib-y += strnlen.o
+lib-y += strchr.o
endif
lib-y += csum.o
ifeq ($(CONFIG_MMU), y)
diff --git a/arch/riscv/lib/strchr.S b/arch/riscv/lib/strchr.S
new file mode 100644
index 000000000000..48c3a9da53e3
--- /dev/null
+++ b/arch/riscv/lib/strchr.S
@@ -0,0 +1,35 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+/*
+ * Copyright (C) 2025 Feng Jiang <jiangfeng@kylinos.cn>
+ */
+
+#include <linux/linkage.h>
+#include <asm/asm.h>
+
+/* char *strchr(const char *s, int c) */
+SYM_FUNC_START(strchr)
+ /*
+ * Parameters
+ * a0 - The string to be searched
+ * a1 - The character to search for
+ *
+ * Returns
+ * a0 - Address of first occurrence of 'c' or 0
+ *
+ * Clobbers
+ * t0
+ */
+ andi a1, a1, 0xff
+1:
+ lbu t0, 0(a0)
+ beq t0, a1, 2f
+ addi a0, a0, 1
+ bnez t0, 1b
+ li a0, 0
+2:
+ ret
+SYM_FUNC_END(strchr)
+
+SYM_FUNC_ALIAS_WEAK(__pi_strchr, strchr)
+EXPORT_SYMBOL(strchr)
diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
index d7c0533108be..e7b3d748c913 100644
--- a/arch/riscv/purgatory/Makefile
+++ b/arch/riscv/purgatory/Makefile
@@ -2,7 +2,7 @@

purgatory-y := purgatory.o sha256.o entry.o string.o ctype.o memcpy.o memset.o
ifeq ($(CONFIG_KASAN_GENERIC)$(CONFIG_KASAN_SW_TAGS),)
-purgatory-y += strcmp.o strlen.o strncmp.o strnlen.o
+purgatory-y += strcmp.o strlen.o strncmp.o strnlen.o strchr.o
endif

targets += $(purgatory-y)
@@ -35,6 +35,9 @@ $(obj)/sha256.o: $(srctree)/lib/crypto/sha256.c FORCE
$(obj)/strnlen.o: $(srctree)/arch/riscv/lib/strnlen.S FORCE
$(call if_changed_rule,as_o_S)

+$(obj)/strchr.o: $(srctree)/arch/riscv/lib/strchr.S FORCE
+ $(call if_changed_rule,as_o_S)
+
CFLAGS_sha256.o := -D__DISABLE_EXPORTS -D__NO_FORTIFY
CFLAGS_string.o := -D__DISABLE_EXPORTS
CFLAGS_ctype.o := -D__DISABLE_EXPORTS
--
2.25.1