Commit adf54213 authored by Feng Jiang's avatar Feng Jiang Committed by Paul Walmsley
Browse files

riscv: lib: add strchr() implementation 



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 string length increases, performance converges with
the generic C implementation.

Benchmark results (QEMU TCG, rv64):
  Length | Original (MB/s) | Optimized (MB/s) | Improvement
  -------|-----------------|------------------|------------
  1 B    | 21              | 22               | +4.8%
  7 B    | 113             | 121              | +7.1%
  16 B   | 195             | 202              | +3.6%
  512 B  | 376             | 389              | +3.5%
  4096 B | 394             | 393              | -0.3%

Signed-off-by: default avatarFeng Jiang <jiangfeng@kylinos.cn>
Tested-by: default avatarJoel Stanley <joel@jms.id.au>
Link: https://patch.msgid.link/20260130025018.172925-8-jiangfeng@kylinos.cn


Signed-off-by: default avatarPaul Walmsley <pjw@kernel.org>
parent 5ba15d41

arch/riscv/include/asm/string.h

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

arch/riscv/lib/Makefile

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

arch/riscv/lib/strchr.S

0 → 100644
+35 −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 *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)

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

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