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

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
......@@ -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__ */
......
......@@ -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),
......
/* 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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