riscv: Fix alignment-ignorant memcpy implementation

The memcpy optimization (commit 587a1290a1) has a series
of mistakes:

  - The implementation is wrong: the chunk size calculation is wrong
    leading to invalid memory access.

  - It adds ifunc supports as default, so --disable-multi-arch does
    not work as expected for riscv.

  - It mixes Linux files (memcpy ifunc selection which requires the
    vDSO/syscall mechanism)  with generic support (the memcpy
    optimization itself).

  - There is no __libc_ifunc_impl_list, which makes testing only
    check the selected implementation instead of all supported
    by the system.

This patch also simplifies the required bits to enable ifunc: there
is no need to memcopy.h; nor to add Linux-specific files.

The __memcpy_noalignment tail handling now uses a branchless strategy
similar to aarch64 (overlap 32-bits copies for sizes 4..7 and byte
copies for size 1..3).

Checked on riscv64 and riscv32 by explicitly enabling the function
on __libc_ifunc_impl_list on qemu-system.

Changes from v1:
* Implement the memcpy in assembly to correctly handle RISCV
  strict-alignment.
Reviewed-by: Evan Green <evan@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>

sysdeps/unix/sysv/linux/riscv/Makefile

@@ -15,15 +15,6 @@ ifeq ($(subdir),stdlib)
 gen-as-const-headers += ucontext_i.sym
 endif
 
-ifeq ($(subdir),string)
-sysdep_routines += \
-  memcpy \
-  memcpy-generic \
-  memcpy_noalignment \
-  # sysdep_routines
-
-endif
-
 abi-variants := ilp32 ilp32d lp64 lp64d
 
 ifeq (,$(filter $(default-abi),$(abi-variants)))

sysdeps/unix/sysv/linux/riscv/hwprobe.c

@@ -34,3 +34,4 @@ int __riscv_hwprobe (struct riscv_hwprobe *pairs, size_t pair_count,
   /* Negate negative errno values to match pthreads API. */
   return -r;
 }
+libc_hidden_def (__riscv_hwprobe)

sysdeps/unix/sysv/linux/riscv/include/sys/hwprobe.h

@@ -0,0 +1,8 @@
+#ifndef _SYS_HWPROBE_H
+# include_next <sys/hwprobe.h>
+
+#ifndef _ISOMAC
+libc_hidden_proto (__riscv_hwprobe)
+#endif
+
+#endif

sysdeps/unix/sysv/linux/riscv/multiarch/Makefile

@@ -0,0 +1,9 @@
+ifeq ($(subdir),string)
+sysdep_routines += \
+  memcpy \
+  memcpy-generic \
+  memcpy_noalignment \
+  # sysdep_routines
+
+CFLAGS-memcpy_noalignment.c += -mno-strict-align
+endif

sysdeps/unix/sysv/linux/riscv/multiarch/ifunc-impl-list.c

@@ -1,4 +1,4 @@
-/* Re-include the default memcpy implementation.
+/* Enumerate available IFUNC implementations of a function.  RISCV version.
    Copyright (C) 2024 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
 
@@ -16,9 +16,28 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <ifunc-impl-list.h>
 #include <string.h>
+#include <sys/hwprobe.h>
 
-extern __typeof (memcpy) __memcpy_generic;
-hidden_proto (__memcpy_generic)
+size_t
+__libc_ifunc_impl_list (const char *name, struct libc_ifunc_impl *array,
+			size_t max)
+{
+  size_t i = max;
 
-#include <string/memcpy.c>
+  bool fast_unaligned = false;
+
+  struct riscv_hwprobe pair = { .key = RISCV_HWPROBE_KEY_CPUPERF_0 };
+  if (__riscv_hwprobe (&pair, 1, 0, NULL, 0) == 0
+      && (pair.value & RISCV_HWPROBE_MISALIGNED_MASK)
+          == RISCV_HWPROBE_MISALIGNED_FAST)
+    fast_unaligned = true;
+
+  IFUNC_IMPL (i, name, memcpy,
+	      IFUNC_IMPL_ADD (array, i, memcpy, fast_unaligned,
+			      __memcpy_noalignment)
+	      IFUNC_IMPL_ADD (array, i, memcpy, 1, __memcpy_generic))
+
+  return 0;
+}

sysdeps/unix/sysv/linux/riscv/multiarch/memcpy.c

@@ -0,0 +1,57 @@
+/* Multiple versions of memcpy.
+   All versions must be listed in ifunc-impl-list.c.
+   Copyright (C) 2017-2024 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/>.  */
+
+#if IS_IN (libc)
+/* Redefine memcpy so that the compiler won't complain about the type
+   mismatch with the IFUNC selector in strong_alias, below.  */
+# undef memcpy
+# define memcpy __redirect_memcpy
+# include <stdint.h>
+# include <string.h>
+# include <ifunc-init.h>
+# include <riscv-ifunc.h>
+# include <sys/hwprobe.h>
+
+extern __typeof (__redirect_memcpy) __libc_memcpy;
+
+extern __typeof (__redirect_memcpy) __memcpy_generic attribute_hidden;
+extern __typeof (__redirect_memcpy) __memcpy_noalignment attribute_hidden;
+
+static inline __typeof (__redirect_memcpy) *
+select_memcpy_ifunc (uint64_t dl_hwcap, __riscv_hwprobe_t hwprobe_func)
+{
+  unsigned long long int v;
+  if (__riscv_hwprobe_one (hwprobe_func, RISCV_HWPROBE_KEY_CPUPERF_0, &v) == 0
+      && (v & RISCV_HWPROBE_MISALIGNED_MASK) == RISCV_HWPROBE_MISALIGNED_FAST)
+    return __memcpy_noalignment;
+
+  return __memcpy_generic;
+}
+
+riscv_libc_ifunc (__libc_memcpy, select_memcpy_ifunc);
+
+# undef memcpy
+strong_alias (__libc_memcpy, memcpy);
+# ifdef SHARED
+__hidden_ver1 (memcpy, __GI_memcpy, __redirect_memcpy)
+  __attribute__ ((visibility ("hidden"))) __attribute_copy__ (memcpy);
+# endif
+#else
+# include <string/memcpy.c>
+#endif