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SubjectRe: [RFC PATCH 1/3] powerpc/lib: implement strlen() in assembly for PPC64
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Le 05/07/2018 à 10:53, Christophe Leroy a écrit :
> The generic implementation of strlen() reads strings byte per byte.
>
> This patch implements strlen() in assembly based on a read of entire
> words, in the same spirit as what some other arches and glibc do.
>
> strlen() selftest on an XXXXXXXX provides the following values:
>
> Before the patch (ie with the generic strlen() in lib/string.c):
>
> After the patch:


I think we should implement it in C using word-at-a-time

Christophe


>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
> This serie applies on top of the PPC32 strlen optimisation serie
>
> Untested
>
> arch/powerpc/include/asm/string.h | 3 +-
> arch/powerpc/lib/Makefile | 4 +-
> arch/powerpc/lib/strlen_64.S | 88 +++++++++++++++++++++++++++++++++++++++
> 3 files changed, 91 insertions(+), 4 deletions(-)
> create mode 100644 arch/powerpc/lib/strlen_64.S
>
> diff --git a/arch/powerpc/include/asm/string.h b/arch/powerpc/include/asm/string.h
> index 1647de15a31e..8fdcb532de72 100644
> --- a/arch/powerpc/include/asm/string.h
> +++ b/arch/powerpc/include/asm/string.h
> @@ -13,6 +13,7 @@
> #define __HAVE_ARCH_MEMCHR
> #define __HAVE_ARCH_MEMSET16
> #define __HAVE_ARCH_MEMCPY_FLUSHCACHE
> +#define __HAVE_ARCH_STRLEN
>
> extern char * strcpy(char *,const char *);
> extern char * strncpy(char *,const char *, __kernel_size_t);
> @@ -50,8 +51,6 @@ static inline void *memset64(uint64_t *p, uint64_t v, __kernel_size_t n)
> return __memset64(p, v, n * 8);
> }
> #else
> -#define __HAVE_ARCH_STRLEN
> -
> extern void *memset16(uint16_t *, uint16_t, __kernel_size_t);
> #endif
> #endif /* __KERNEL__ */
> diff --git a/arch/powerpc/lib/Makefile b/arch/powerpc/lib/Makefile
> index 670286808928..93706b4cdbde 100644
> --- a/arch/powerpc/lib/Makefile
> +++ b/arch/powerpc/lib/Makefile
> @@ -12,7 +12,7 @@ CFLAGS_REMOVE_feature-fixups.o = $(CC_FLAGS_FTRACE)
>
> obj-y += string.o alloc.o code-patching.o feature-fixups.o
>
> -obj-$(CONFIG_PPC32) += div64.o copy_32.o crtsavres.o strlen_32.o
> +obj-$(CONFIG_PPC32) += div64.o copy_32.o crtsavres.o
>
> # See corresponding test in arch/powerpc/Makefile
> # 64-bit linker creates .sfpr on demand for final link (vmlinux),
> @@ -33,7 +33,7 @@ obj64-$(CONFIG_ALTIVEC) += vmx-helper.o
> obj64-$(CONFIG_KPROBES_SANITY_TEST) += test_emulate_step.o
>
> obj-y += checksum_$(BITS).o checksum_wrappers.o \
> - string_$(BITS).o memcmp_$(BITS).o
> + string_$(BITS).o memcmp_$(BITS).o strlen_$(BITS).o
>
> obj-y += sstep.o ldstfp.o quad.o
> obj64-y += quad.o
> diff --git a/arch/powerpc/lib/strlen_64.S b/arch/powerpc/lib/strlen_64.S
> new file mode 100644
> index 000000000000..c9704f2b697d
> --- /dev/null
> +++ b/arch/powerpc/lib/strlen_64.S
> @@ -0,0 +1,88 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * strlen() for PPC64
> + *
> + * Copyright (C) 2018 Christophe Leroy CS Systemes d'Information.
> + *
> + * Inspired from glibc implementation
> + */
> +#include <asm/ppc_asm.h>
> +#include <asm/export.h>
> +#include <asm/cache.h>
> +
> + .text
> +
> +/*
> + * Algorithm:
> + *
> + * 1) Given a word 'x', we can test to see if it contains any 0 bytes
> + * by subtracting 0x01010101, and seeing if any of the high bits of each
> + * byte changed from 0 to 1. This works because the least significant
> + * 0 byte must have had no incoming carry (otherwise it's not the least
> + * significant), so it is 0x00 - 0x01 == 0xff. For all other
> + * byte values, either they have the high bit set initially, or when
> + * 1 is subtracted you get a value in the range 0x00-0x7f, none of which
> + * have their high bit set. The expression here is
> + * (x - 0x01010101) & ~x & 0x80808080), which gives 0x00000000 when
> + * there were no 0x00 bytes in the word. You get 0x80 in bytes that
> + * match, but possibly false 0x80 matches in the next more significant
> + * byte to a true match due to carries. For little-endian this is
> + * of no consequence since the least significant match is the one
> + * we're interested in, but big-endian needs method 2 to find which
> + * byte matches.
> + * 2) Given a word 'x', we can test to see _which_ byte was zero by
> + * calculating ~(((x & ~0x80808080) - 0x80808080 - 1) | x | ~0x80808080).
> + * This produces 0x80 in each byte that was zero, and 0x00 in all
> + * the other bytes. The '| ~0x80808080' clears the low 7 bits in each
> + * byte, and the '| x' part ensures that bytes with the high bit set
> + * produce 0x00. The addition will carry into the high bit of each byte
> + * iff that byte had one of its low 7 bits set. We can then just see
> + * which was the most significant bit set and divide by 8 to find how
> + * many to add to the index.
> + * This is from the book 'The PowerPC Compiler Writer's Guide',
> + * by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
> + */
> +
> +_GLOBAL(strlen)
> + andi. r0, r3, 7
> + lis r7, 0x0101
> + addi r10, r3, -8
> + addic r7, r7, 0x0101 /* r7 = 0x01010101 (lomagic) & clear XER[CA] */
> + rldimi r7, r7, 32, 0 /* r7 = 0x0101010101010101 (lomagic) */
> + rotldi r6, r7, 31 /* r6 = 0x8080808080808080 (himagic) */
> + bne- 3f
> + .balign IFETCH_ALIGN_BYTES
> +1: ldu r9, 8(r10)
> +2: subf r8, r7, r9
> + andc r11, r6, r9
> + and. r8, r8, r11
> + beq+ 1b
> +#ifdef CONFIG_CPU_BIG_ENDIAN
> + andc r8, r9, r6
> + orc r9, r9, r6
> + subfe r8, r6, r8
> + nor r8, r8, r9
> + cntlzd r8, r8
> + subf r3, r3, r10
> + srdi r8, r8, 3
> + add r3, r3, r8
> +#else
> + addi r9, r8, -1
> + addi r10, r10, 7
> + andc r8, r9, r8
> + cntlzd r8, r8
> + subf r3, r3, r10
> + srdi r8, r8, 3
> + subf r3, r8, r3
> +#endif
> + blr
> +
> + /* Missaligned string: make sure bytes before string are seen not 0 */
> +3: xor r10, r10, r0
> + orc r8, r8, r8
> + ldu r9, 8(r10)
> + slwi r0, r0, 3
> + srw r8, r8, r0
> + orc r9, r9, r8
> + b 2b
> +EXPORT_SYMBOL(strlen)
>

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