Commit 9412b234 authored by Christophe Leroy's avatar Christophe Leroy Committed by Michael Ellerman
Browse files

powerpc/lib: Implement strlen() in assembly for PPC32 



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.

On a 8xx the time spent in strlen is reduced by 3/4 for long strings.

strlen() selftest on an 8xx provides the following values:

Before the patch (ie with the generic strlen() in lib/string.c):

  len 256 : time = 1.195055
  len 016 : time = 0.083745
  len 008 : time = 0.046828
  len 004 : time = 0.028390

After the patch:

  len 256 : time = 0.272185 ==> 78% improvment
  len 016 : time = 0.040632 ==> 51% improvment
  len 008 : time = 0.033060 ==> 29% improvment
  len 004 : time = 0.029149 ==> 2% degradation

On a 832x:

Before the patch:

  len 256 : time = 0.236125
  len 016 : time = 0.018136
  len 008 : time = 0.011000
  len 004 : time = 0.007229

After the patch:

  len 256 : time = 0.094950 ==> 60% improvment
  len 016 : time = 0.013357 ==> 26% improvment
  len 008 : time = 0.010586 ==> 4% improvment
  len 004 : time = 0.008784

Signed-off-by: default avatarChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
parent f0abbfd8

arch/powerpc/include/asm/string.h

+2 −0
Original line number Diff line number Diff line
@@ -50,6 +50,8 @@ 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__ */

arch/powerpc/lib/Makefile

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

obj-$(CONFIG_PPC32)	+= div64.o copy_32.o crtsavres.o strlen_32.o



# See corresponding test in arch/powerpc/Makefile

# 64-bit linker creates .sfpr on demand for final link (vmlinux),

arch/powerpc/lib/strlen_32.S

0 → 100644
+78 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0 */

/*

 * strlen() for PPC32

 *

 * 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, 3

	lis	r7, 0x0101

	addi	r10, r3, -4

	addic	r7, r7, 0x0101	/* r7 = 0x01010101 (lomagic) & clear XER[CA] */

	rotlwi	r6, r7, 31 	/* r6 = 0x80808080 (himagic) */

	bne-	3f

	.balign IFETCH_ALIGN_BYTES

1:	lwzu	r9, 4(r10)

2:	subf	r8, r7, r9

	and.	r8, r8, r6

	beq+	1b

	andc.	r8, r8, r9

	beq+	1b

	andc	r8, r9, r6

	orc	r9, r9, r6

	subfe	r8, r6, r8

	nor	r8, r8, r9

	cntlzw	r8, r8

	subf	r3, r3, r10

	srwi	r8, r8, 3

	add	r3, r3, r8

	blr



	/* Missaligned string: make sure bytes before string are seen not 0 */

3:	xor	r10, r10, r0

	orc	r8, r8, r8

	lwzu	r9, 4(r10)

	slwi	r0, r0, 3

	srw	r8, r8, r0

	orc	r9, r9, r8

	b	2b

EXPORT_SYMBOL(strlen)

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