[RFC PATCH 18/19] riscv: Add an optimized strncmp routine

[Christoph Muellner]

From: Christoph Müllner <christoph.muellner@vrull.eu>

The implementation of strncmp() can be accelerated using Zbb's orc.b
instruction. Let's add an optimized implementation that makes use
of this instruction.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
---
 sysdeps/riscv/multiarch/Makefile          |   3 +-
 sysdeps/riscv/multiarch/ifunc-impl-list.c |   1 +
 sysdeps/riscv/multiarch/strncmp.c         |   6 +-
 sysdeps/riscv/multiarch/strncmp_zbb.S     | 119 ++++++++++++++++++++++
 4 files changed, 127 insertions(+), 2 deletions(-)
 create mode 100644 sysdeps/riscv/multiarch/strncmp_zbb.S

diff --git a/sysdeps/riscv/multiarch/Makefile b/sysdeps/riscv/multiarch/Makefile
index 056ce2ffc0..9f22e31b99 100644
--- a/sysdeps/riscv/multiarch/Makefile
+++ b/sysdeps/riscv/multiarch/Makefile
@@ -14,5 +14,6 @@ sysdep_routines += \
 	strcmp_generic \
 	strcmp_zbb \
 	strcmp_zbb_unaligned \
-	strncmp_generic
+	strncmp_generic \
+	strncmp_zbb
 endif
diff --git a/sysdeps/riscv/multiarch/ifunc-impl-list.c b/sysdeps/riscv/multiarch/ifunc-impl-list.c
index eb37ed6017..82fd34d010 100644
--- a/sysdeps/riscv/multiarch/ifunc-impl-list.c
+++ b/sysdeps/riscv/multiarch/ifunc-impl-list.c
@@ -64,6 +64,7 @@ __libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
 	      IFUNC_IMPL_ADD (array, i, strcmp, 1, __strcmp_generic))
 
   IFUNC_IMPL (i, name, strncmp,
+	      IFUNC_IMPL_ADD (array, i, strncmp, 1, __strncmp_zbb)
 	      IFUNC_IMPL_ADD (array, i, strncmp, 1, __strncmp_generic))
   return i;
 }
diff --git a/sysdeps/riscv/multiarch/strncmp.c b/sysdeps/riscv/multiarch/strncmp.c
index 970aeb8b85..5b0fe08e98 100644
--- a/sysdeps/riscv/multiarch/strncmp.c
+++ b/sysdeps/riscv/multiarch/strncmp.c
@@ -30,8 +30,12 @@
 
 extern __typeof (__redirect_strncmp) __libc_strncmp;
 extern __typeof (__redirect_strncmp) __strncmp_generic attribute_hidden;
+extern __typeof (__redirect_strncmp) __strncmp_zbb attribute_hidden;
 
-libc_ifunc (__libc_strncmp, __strncmp_generic);
+libc_ifunc (__libc_strncmp,
+	    HAVE_RV(zbb)
+	    ? __strncmp_zbb
+	    : __strncmp_generic);
 
 # undef strncmp
 strong_alias (__libc_strncmp, strncmp);
diff --git a/sysdeps/riscv/multiarch/strncmp_zbb.S b/sysdeps/riscv/multiarch/strncmp_zbb.S
new file mode 100644
index 0000000000..29cff30def
--- /dev/null
+++ b/sysdeps/riscv/multiarch/strncmp_zbb.S
@@ -0,0 +1,119 @@
+/* Copyright (C) 2022 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
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sysdep.h>
+#include <sys/asm.h>
+
+/* Assumptions: rvi_zbb.  */
+
+#define src1		a0
+#define result		a0
+#define src2		a1
+#define len		a2
+#define data1		a2
+#define data2		a3
+#define align		a4
+#define data1_orcb	t0
+#define limit		t1
+#define fast_limit      t2
+#define m1		t3
+
+#if __riscv_xlen == 64
+# define REG_L	ld
+# define SZREG	8
+# define PTRLOG	3
+#else
+# define REG_L	lw
+# define SZREG	4
+# define PTRLOG	2
+#endif
+
+#ifndef STRNCMP
+# define STRNCMP __strncmp_zbb
+#endif
+
+.option push
+.option arch,+zbb
+
+ENTRY_ALIGN (STRNCMP, 6)
+	beqz	len, L(equal)
+	or	align, src1, src2
+	and	align, align, SZREG-1
+	add	limit, src1, len
+	bnez	align, L(simpleloop)
+	li	m1, -1
+
+	/* Adjust limit for fast-path.  */
+	andi fast_limit, limit, -SZREG
+
+	/* Main loop for aligned string.  */
+	.p2align 3
+L(loop):
+	bge	src1, fast_limit, L(simpleloop)
+	REG_L	data1, 0(src1)
+	REG_L	data2, 0(src2)
+	orc.b	data1_orcb, data1
+	bne	data1_orcb, m1, L(foundnull)
+	addi	src1, src1, SZREG
+	addi	src2, src2, SZREG
+	beq	data1, data2, L(loop)
+
+	/* Words don't match, and no null byte in the first
+	 * word. Get bytes in big-endian order and compare.  */
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+	rev8	data1, data1
+	rev8	data2, data2
+#endif
+	/* Synthesize (data1 >= data2) ? 1 : -1 in a branchless sequence.  */
+	sltu	result, data1, data2
+	neg	result, result
+	ori	result, result, 1
+	ret
+
+L(foundnull):
+	/* Found a null byte.
+	 * If words don't match, fall back to simple loop.  */
+	bne	data1, data2, L(simpleloop)
+
+	/* Otherwise, strings are equal.  */
+	li	result, 0
+	ret
+
+	/* Simple loop for misaligned strings.  */
+	.p2align 3
+L(simpleloop):
+	bge	src1, limit, L(equal)
+	lbu	data1, 0(src1)
+	addi	src1, src1, 1
+	lbu	data2, 0(src2)
+	addi	src2, src2, 1
+	bne	data1, data2, L(sub)
+	bnez	data1, L(simpleloop)
+
+L(sub):
+	sub	result, data1, data2
+	ret
+
+L(equal):
+	li	result, 0
+	ret
+
+.option pop
+
+END (STRNCMP)
+libc_hidden_builtin_def (STRNCMP)
-- 
2.39.1