arch/riscv/lib/strlen.S

root/arch/riscv/lib/strlen.S
/* SPDX-License-Identifier: GPL-2.0-only */

#include <linux/linkage.h>
#include <asm/asm.h>
#include <asm/alternative-macros.h>
#include <asm/hwcap.h>

/* int strlen(const char *s) */
SYM_FUNC_START(strlen)

        __ALTERNATIVE_CFG("nop", "j strlen_zbb", 0, RISCV_ISA_EXT_ZBB,
                IS_ENABLED(CONFIG_RISCV_ISA_ZBB) && IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZBB))

        /*
         * Returns
         *   a0 - string length
         *
         * Parameters
         *   a0 - String to measure
         *
         * Clobbers:
         *   t0, t1
         */
        addi    t1, a0, -1
1:
        addi    t1, t1, 1
        lbu     t0, 0(t1)
        bnez    t0, 1b
        sub     a0, t1, a0
        ret

/*
 * Variant of strlen using the ZBB extension if available
 */
#if defined(CONFIG_RISCV_ISA_ZBB) && defined(CONFIG_TOOLCHAIN_HAS_ZBB)
strlen_zbb:

#ifdef CONFIG_CPU_BIG_ENDIAN
# define CZ     clz
# define SHIFT  sll
#else
# define CZ     ctz
# define SHIFT  srl
#endif

.option push
.option arch,+zbb

        /*
         * Returns
         *   a0 - string length
         *
         * Parameters
         *   a0 - String to measure
         *
         * Clobbers
         *   t0, t1, t2, t3
         */

        /* Number of irrelevant bytes in the first word. */
        andi    t2, a0, SZREG-1

        /* Align pointer. */
        andi    t0, a0, -SZREG

        li      t3, SZREG
        sub     t3, t3, t2
        slli    t2, t2, 3

        /* Get the first word.  */
        REG_L   t1, 0(t0)

        /*
         * Shift away the partial data we loaded to remove the irrelevant bytes
         * preceding the string with the effect of adding NUL bytes at the
         * end of the string's first word.
         */
        SHIFT   t1, t1, t2

        /* Convert non-NUL into 0xff and NUL into 0x00. */
        orc.b   t1, t1

        /* Convert non-NUL into 0x00 and NUL into 0xff. */
        not     t1, t1

        /*
         * Search for the first set bit (corresponding to a NUL byte in the
         * original chunk).
         */
        CZ      t1, t1

        /*
         * The first chunk is special: compare against the number
         * of valid bytes in this chunk.
         */
        srli    a0, t1, 3
        bgtu    t3, a0, 2f

        /* Prepare for the word comparison loop. */
        addi    t2, t0, SZREG
        li      t3, -1

        /*
         * Our critical loop is 4 instructions and processes data in
         * 4 byte or 8 byte chunks.
         */
        .p2align 3
1:
        REG_L   t1, SZREG(t0)
        addi    t0, t0, SZREG
        orc.b   t1, t1
        beq     t1, t3, 1b

        not     t1, t1
        CZ      t1, t1
        srli    t1, t1, 3

        /* Get number of processed bytes. */
        sub     t2, t0, t2

        /* Add number of characters in the first word.  */
        add     a0, a0, t2

        /* Add number of characters in the last word.  */
        add     a0, a0, t1
2:
        ret

.option pop
#endif
SYM_FUNC_END(strlen)
SYM_FUNC_ALIAS(__pi_strlen, strlen)
EXPORT_SYMBOL(strlen)
Linux
