[RFC PATCH 19/19] riscv: Add __riscv_cpu_relax() to allow yielding in busy loops

[Christoph Muellner]

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

The spinning loop of PTHREAD_MUTEX_ADAPTIVE_NP provides the hook
atomic_spin_nop() that can be used by architectures.

On RISC-V we have two instructions that can be used here:
* WRS.STO from the Zawrs extension
* PAUSE from the Zihintpause extension

Let's use these instructions and prefer WRS.STO over PAUSE
(based on availability of the corresponding ISA extension
at runtime).

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
---
 sysdeps/riscv/multiarch/Makefile              |  5 +++
 sysdeps/riscv/multiarch/cpu_relax.c           | 36 +++++++++++++++++
 sysdeps/riscv/multiarch/cpu_relax_impl.S      | 40 +++++++++++++++++++
 .../unix/sysv/linux/riscv/atomic-machine.h    |  3 ++
 4 files changed, 84 insertions(+)
 create mode 100644 sysdeps/riscv/multiarch/cpu_relax.c
 create mode 100644 sysdeps/riscv/multiarch/cpu_relax_impl.S

diff --git a/sysdeps/riscv/multiarch/Makefile b/sysdeps/riscv/multiarch/Makefile
index 9f22e31b99..b5b9fcf986 100644
--- a/sysdeps/riscv/multiarch/Makefile
+++ b/sysdeps/riscv/multiarch/Makefile
@@ -17,3 +17,8 @@ sysdep_routines += \
 	strncmp_generic \
 	strncmp_zbb
 endif
+
+# nscd uses atomic_spin_nop which in turn requires cpu_relax
+ifeq ($(subdir),nscd)
+routines += cpu_relax cpu_relax_impl
+endif
diff --git a/sysdeps/riscv/multiarch/cpu_relax.c b/sysdeps/riscv/multiarch/cpu_relax.c
new file mode 100644
index 0000000000..4e6825ca50
--- /dev/null
+++ b/sysdeps/riscv/multiarch/cpu_relax.c
@@ -0,0 +1,36 @@
+/* CPU strand yielding for busy loops.  RISC-V version.
+   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 <ldsodefs.h>
+#include <init-arch.h>
+
+void __cpu_relax (void);
+extern void __cpu_relax_zawrs (void);
+extern void __cpu_relax_zihintpause (void);
+
+static void
+__cpu_relax_generic (void)
+{
+}
+
+libc_ifunc (__cpu_relax,
+	    HAVE_RV(zawrs)
+	    ? __cpu_relax_zawrs
+	    : HAVE_RV(zihintpause)
+	      ? __cpu_relax_zihintpause
+	      : __cpu_relax_generic);
diff --git a/sysdeps/riscv/multiarch/cpu_relax_impl.S b/sysdeps/riscv/multiarch/cpu_relax_impl.S
new file mode 100644
index 0000000000..5d349c351f
--- /dev/null
+++ b/sysdeps/riscv/multiarch/cpu_relax_impl.S
@@ -0,0 +1,40 @@
+/* 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>
+
+.option push
+.option arch,+zawrs
+
+ENTRY_ALIGN (__cpu_relax_zawrs, 4)
+	wrs.sto
+	ret
+END (__cpu_relax_zawrs)
+
+.option pop
+
+.option push
+.option arch,+zihintpause
+
+ENTRY_ALIGN (__cpu_relax_zihintpause, 4)
+	pause
+	ret
+END (__cpu_relax_zihintpause)
+
+.option pop
diff --git a/sysdeps/unix/sysv/linux/riscv/atomic-machine.h b/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
index dbf70d8d57..88aa58ef95 100644
--- a/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
+++ b/sysdeps/unix/sysv/linux/riscv/atomic-machine.h
@@ -178,4 +178,7 @@
 # error "ISAs that do not subsume the A extension are not supported"
 #endif /* !__riscv_atomic */
 
+extern void __cpu_relax (void);
+#define atomic_spin_nop() __cpu_relax()
+
 #endif /* bits/atomic.h */
-- 
2.39.1