[PATCH v3 8/8] riscv: lib: add strrchr implementation

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

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

This implementation minimizes instruction count and avoids unnecessary
memory access to the stack. The performance benefits are most visible
on small workloads (1-16 bytes) where the architectural savings in
function overhead outweigh the execution time of the scan loop.

Benchmark results (QEMU TCG, rv64):
Length | Original (MB/s) | Optimized (MB/s) | Improvement
-------|-----------------|------------------|------------
1 B | 21 | 22 | +4.7%
7 B | 116 | 122 | +5.1%
16 B | 195 | 208 | +6.6%
512 B | 388 | 399 | +2.8%
4096 B | 411 | 411 | +0.0%

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/strrchr.S | 37 +++++++++++++++++++++++++++++++++
arch/riscv/purgatory/Makefile | 5 ++++-
4 files changed, 45 insertions(+), 1 deletion(-)
create mode 100644 arch/riscv/lib/strrchr.S

diff --git a/arch/riscv/include/asm/string.h b/arch/riscv/include/asm/string.h
index ca3ade82b124..764ffe8f6479 100644
--- a/arch/riscv/include/asm/string.h
+++ b/arch/riscv/include/asm/string.h
@@ -34,6 +34,9 @@ extern asmlinkage __kernel_size_t strnlen(const char *, size_t);

#define __HAVE_ARCH_STRCHR
extern asmlinkage char *strchr(const char *, int);
+
+#define __HAVE_ARCH_STRRCHR
+extern asmlinkage char *strrchr(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 b7f804dce1c3..735d0b665536 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -9,6 +9,7 @@ lib-y += strlen.o
lib-y += strncmp.o
lib-y += strnlen.o
lib-y += strchr.o
+lib-y += strrchr.o
endif
lib-y += csum.o
ifeq ($(CONFIG_MMU), y)
diff --git a/arch/riscv/lib/strrchr.S b/arch/riscv/lib/strrchr.S
new file mode 100644
index 000000000000..ac58b20ca21d
--- /dev/null
+++ b/arch/riscv/lib/strrchr.S
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+/*
+ * Copyright (C) 2025 Feng Jiang <jiangfeng@kylinos.cn>
+ */
+
+#include <linux/linkage.h>
+#include <asm/asm.h>
+
+/* char *strrchr(const char *s, int c) */
+SYM_FUNC_START(strrchr)
+ /*
+ * Parameters
+ * a0 - The string to be searched
+ * a1 - The character to seaerch for
+ *
+ * Returns
+ * a0 - Address of last occurrence of 'c' or 0
+ *
+ * Clobbers
+ * t0, t1
+ */
+ andi a1, a1, 0xff
+ mv t1, a0
+ li a0, 0
+1:
+ lbu t0, 0(t1)
+ bne t0, a1, 2f
+ mv a0, t1
+2:
+ addi t1, t1, 1
+ bnez t0, 1b
+ ret
+SYM_FUNC_END(strrchr)
+
+SYM_FUNC_ALIAS_WEAK(__pi_strrchr, strrchr)
+EXPORT_SYMBOL(strrchr)
diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
index e7b3d748c913..b0358a78f11a 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 strchr.o
+purgatory-y += strcmp.o strlen.o strncmp.o strnlen.o strchr.o strrchr.o
endif

targets += $(purgatory-y)
@@ -38,6 +38,9 @@ $(obj)/strnlen.o: $(srctree)/arch/riscv/lib/strnlen.S FORCE
$(obj)/strchr.o: $(srctree)/arch/riscv/lib/strchr.S FORCE
$(call if_changed_rule,as_o_S)

+$(obj)/strrchr.o: $(srctree)/arch/riscv/lib/strrchr.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