Commit bef64bcb authored by Feng Jiang's avatar Feng Jiang Committed by Paul Walmsley
Browse files

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: default avatarFeng Jiang <jiangfeng@kylinos.cn>
Tested-by: default avatarJoel Stanley <joel@jms.id.au>
Link: https://patch.msgid.link/20260130025018.172925-9-jiangfeng@kylinos.cn


Signed-off-by: default avatarPaul Walmsley <pjw@kernel.org>
parent adf54213

arch/riscv/include/asm/string.h

+3 −0
Original line number Diff line number Diff line
@@ -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. */

arch/riscv/lib/Makefile

+1 −0
Original line number Diff line number Diff line
@@ -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)

arch/riscv/lib/strrchr.S

0 → 100644
+37 −0
Original line number Diff line number Diff line 
/* 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)

arch/riscv/purgatory/Makefile

+4 −1
Original line number Diff line number Diff line
@@ -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