[PATCH] MIPS specific strcmp function

[Steve Ellcey <sellcey at mips dot com>]

I have created a new MIPS strcmp routine.  I tried rewriting this in C a
while back but it was still slower then the standard version in some
unaligned cases.  I have gone back and rewritten it in assembly language
and this version is faster then the standard version in all cases except
for one byte comparisons and one of the two byte comparison tests from
bench-strcmp.

I have attached old and new bench-strcmp.out files to show the
improvement.

There is an ifdef for using the CLZ instruction but on the machine where
I was doing performance testing using CLZ did not help so I did not turn
this ifdef on by default.  Other MIPS platforms might find it helpful
though so I left the code in.

I ran the glibc testsuite with no regressions on mips32 big and little
endian platforms and also did builds and tests using my own tests on a
mips64 simulator.

OK to checkin?

Steve Ellcey
sellcey@mips.com


2014-10-01  Steve Ellcey  <sellcey@mips.com>

	* sysdeps/mips/strcmp.S: New.

diff --git a/sysdeps/mips/strcmp.S b/sysdeps/mips/strcmp.S
new file mode 100644
index 0000000..e0c8212
--- /dev/null
+++ b/sysdeps/mips/strcmp.S
@@ -0,0 +1,249 @@
+/* Copyright (C) 2014 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library.  If not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifdef ANDROID_CHANGES
+# include "machine/asm.h"
+# include "machine/regdef.h"
+#elif _LIBC
+# include <sysdep.h>
+# include <regdef.h>
+# include <sys/asm.h>
+#elif _COMPILING_NEWLIB
+# include "machine/asm.h"
+# include "machine/regdef.h"
+#else
+# include <regdef.h>
+# include <sys/asm.h>
+#endif
+
+/* Technically strcmp should not read past the end of the strings being
+   compared.  We will read a full word that may contain excess bits beyond
+   the NULL string terminator but unless ENABLE_READAHEAD is set, we will not
+   read the next word after the end of string.  Setting ENABLE_READAHEAD will
+   improve performance but is technically illegal based on the definition of
+   strcmp.  */
+#ifdef ENABLE_READAHEAD
+# define DELAY_READ
+#else
+# define DELAY_READ nop
+#endif
+
+/* Testing on a little endian machine showed using CLZ was a
+   performance loss, so we are not turning it on by default.  */
+#if defined(ENABLE_CLZ) && (__mips_isa_rev > 1)
+# define USE_CLZ
+#endif
+
+/* Some asm.h files do not have the L macro definition.  */
+#ifndef L
+# if _MIPS_SIM == _ABIO32
+#  define L(label) $L ## label
+# else
+#  define L(label) .L ## label
+# endif
+#endif
+
+/* Some asm.h files do not have the PTR_ADDIU macro definition.  */
+#ifndef PTR_ADDIU
+# ifdef USE_DOUBLE
+#  define PTR_ADDIU       daddiu
+# else
+#  define PTR_ADDIU       addiu
+# endif
+#endif
+
+/* Allow the routine to be named something else if desired.  */
+#ifndef STRCMP_NAME
+# define STRCMP_NAME strcmp
+#endif
+
+#ifdef ANDROID_CHANGES
+LEAF(STRCMP_NAME, 0)
+#else
+LEAF(STRCMP_NAME)
+#endif
+	.set	nomips16
+	.set	noreorder
+
+	or	t0, a0, a1
+	andi	t0,0x3
+	bne	t0, zero, L(byteloop)
+
+/* Both strings are 4 byte aligned at this point.  */
+
+	lui	t8, 0x0101
+	ori	t8, t8, 0x0101
+	lui	t9, 0x7f7f
+	ori	t9, 0x7f7f
+
+#define STRCMP32(OFFSET) \
+	lw	v0, OFFSET(a0); \
+	lw	v1, OFFSET(a1); \
+	subu	t0, v0, t8; \
+	bne	v0, v1, L(worddiff); \
+	nor	t1, v0, t9; \
+	and	t0, t0, t1; \
+	bne	t0, zero, L(returnzero)
+
+L(wordloop):
+	STRCMP32(0)
+	DELAY_READ
+	STRCMP32(4)
+	DELAY_READ
+	STRCMP32(8)
+	DELAY_READ
+	STRCMP32(12)
+	DELAY_READ
+	STRCMP32(16)
+	DELAY_READ
+	STRCMP32(20)
+	DELAY_READ
+	STRCMP32(24)
+	DELAY_READ
+	STRCMP32(28)
+	PTR_ADDIU a0, a0, 32
+	b	L(wordloop)
+	PTR_ADDIU a1, a1, 32
+
+L(returnzero):
+	j	ra
+	move	v0, zero
+
+L(worddiff):
+#ifdef USE_CLZ
+	subu	t0, v0, t8
+	nor	t1, v0, t9
+	and	t1, t0, t1
+	xor	t0, v0, v1	
+	or	t0, t0, t1
+# if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	wsbh	t0, t0
+	rotr	t0, t0, 16
+# endif
+	clz	t1, t0
+	and	t1, 0xf8
+# if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+	neg	t1
+	addu	t1, 24
+# endif
+	rotrv	v0, v0, t1
+	rotrv	v1, v1, t1
+	and	v0, v0, 0xff
+	and	v1, v1, 0xff
+	j	ra
+	subu	v0, v0, v1
+#else /* USE_CLZ */
+# if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	andi	t0, v0, 0xff
+	beq	t0, zero, L(wexit01)
+	andi	t1, v1, 0xff
+	bne	t0, t1, L(wexit01)
+
+	srl	t8, v0, 8
+	srl	t9, v1, 8
+	andi	t8, t8, 0xff
+	beq	t8, zero, L(wexit89)
+	andi	t9, t9, 0xff
+	bne	t8, t9, L(wexit89)
+
+	srl	t0, v0, 16
+	srl	t1, v1, 16
+	andi	t0, t0, 0xff
+	beq	t0, zero, L(wexit01)
+	andi	t1, t1, 0xff
+	bne	t0, t1, L(wexit01)
+
+	srl	t8, v0, 24
+	srl	t9, v1, 24
+# else /* __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ */
+	srl	t0, v0, 24
+	beq	t0, zero, L(wexit01)
+	srl	t1, v1, 24
+	bne	t0, t1, L(wexit01)
+
+	srl	t8, v0, 16
+	srl	t9, v1, 16
+	andi	t8, t8, 0xff
+	beq	t8, zero, L(wexit89)
+	andi	t9, t9, 0xff
+	bne	t8, t9, L(wexit89)
+
+	srl	t0, v0, 8
+	srl	t1, v1, 8
+	andi	t0, t0, 0xff
+	beq	t0, zero, L(wexit01)
+	andi	t1, t1, 0xff
+	bne	t0, t1, L(wexit01)
+
+	andi	t8, v0, 0xff
+	andi	t9, v1, 0xff
+# endif /* __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ */
+
+L(wexit89):
+	j	ra
+	subu	v0, t8, t9
+L(wexit01):
+	j	ra
+	subu	v0, t0, t1
+#endif /* USE_CLZ */
+
+/* It might seem better to do the 'beq' instruction between the two 'lbu'
+   instructions so that the nop is not needed but testing showed that this
+   code is actually faster (based on glibc strcmp test).  */
+#define BYTECMP01(OFFSET) \
+	lbu	v0, OFFSET(a0); \
+	lbu	v1, OFFSET(a1); \
+	beq	v0, zero, L(bexit01); \
+	nop; \
+	bne	v0, v1, L(bexit01)
+
+#define BYTECMP89(OFFSET) \
+	lbu	t8, OFFSET(a0); \
+	lbu	t9, OFFSET(a1); \
+	beq	t8, zero, L(bexit89); \
+	nop;	\
+	bne	t8, t9, L(bexit89)
+
+L(byteloop):
+	BYTECMP01(0)
+	BYTECMP89(1)
+	BYTECMP01(2)
+	BYTECMP89(3)
+	BYTECMP01(4)
+	BYTECMP89(5)
+	BYTECMP01(6)
+	BYTECMP89(7)
+	PTR_ADDIU a0, a0, 8
+	b	L(byteloop)
+	PTR_ADDIU a1, a1, 8
+
+L(bexit01):
+	j	ra
+	subu	v0, v0, v1
+L(bexit89):
+	j	ra
+	subu	v0, t8, t9
+
+	.set	at
+	.set	reorder
+
+END(STRCMP_NAME)
+#ifndef ANDROID_CHANGES
+# ifdef _LIBC
+libc_hidden_builtin_def (STRCMP_NAME)
+# endif
+#endif

Attachment: bench-strcmp.out.old
Description: application/trash

Attachment: bench-strcmp.out.new
Description: Text document