riscv: lib: add strrchr() implementation

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    | 20              | 21               | +5.0%
  7 B    | 111             | 120              | +8.1%
  16 B   | 189             | 199              | +5.3%
  512 B  | 361             | 382              | +5.8%
  4096 B | 388             | 391              | +0.8%

Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Tested-by: Joel Stanley <joel@jms.id.au>
Link: https://patch.msgid.link/20260130025018.172925-9-jiangfeng@kylinos.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>

diff --git a/arch/riscv/include/asm/string.h b/arch/riscv/include/asm/string.h
index ca3ade82b1249537951c12178aea18325aacb14d..764ffe8f6479736cfeb3150af4abc0e4d3d26e30 100644 (file)
--- 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 b7f804dce1c3418ac9433c373f387037728f1c09..735d0b665536d678392e347241d029d2ebec1d0f 100644 (file)
--- 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 (file)
index 0000000..ac58b20
--- /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 e7b3d748c913f65a390d2d7099c5857367b8eaa8..b0358a78f11a8317c9d7a2d3f28f0544cd1233c1 100644 (file)
--- 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