[RFC PATCH 18/19] riscv: Add an optimized strncmp routine
Philipp Tomsich
philipp.tomsich@vrull.eu
Wed Feb 8 15:13:44 GMT 2023
On Tue, 7 Feb 2023 at 02:20, Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, Feb 6, 2023 at 6:23 PM Christoph Muellner
> <christoph.muellner@vrull.eu> wrote:
> >
> > 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>
>
> Not necessary, but imo performance patches should have at least some reference
> to the expected speedup versus the existing alternatives.
Given that this is effectively a SWAR-like optimization (orc.b allows
us to test 8 bytes in parallel for a NUL byte), we should be able to
show the benefit through a reduction in dynamic instructions. Would
this be considered reasonable reference data?
> > ---
> > 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
> >
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