[musl] [PATCH 1/1] riscv: add Zacas extension support for atomic CAS

[Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>]

Add compile-time detection for RISC-V Zacas extension and use
amocas.w.aqrl/amocas.d.aqrl instructions when available.

When __riscv_zacas is defined, a_cas() and a_cas_p() use single amocas
instructions instead of lr/sc loops. Falls back to existing lr/sc
implementation when Zacas is not available.

Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
---
 arch/riscv32/atomic_arch.h | 17 +++++++++++++++++
 arch/riscv64/atomic_arch.h | 30 ++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/arch/riscv32/atomic_arch.h b/arch/riscv32/atomic_arch.h
index 4d418f63..64ef05b7 100644
--- a/arch/riscv32/atomic_arch.h
+++ b/arch/riscv32/atomic_arch.h
@@ -3,6 +3,21 @@ static inline void a_barrier()
 {
 	__asm__ __volatile__ ("fence rw,rw" : : : "memory");
 }
+#ifdef __riscv_zacas
+
+#define a_cas a_cas
+static inline int a_cas(volatile int *p, int t, int s)
+{
+	int old = t;
+	__asm__ __volatile__ (
+		"amocas.w.aqrl %0, %2, %1"
+		: "+r"(old), "+A"(*(volatile int *)p)
+		: "r"(s)
+		: "memory");
+	return old;
+}
+
+#else /* Fallback to lr/sc when Zacas is not available */
 
 #define a_cas a_cas
 static inline int a_cas(volatile int *p, int t, int s)
@@ -19,3 +34,5 @@ static inline int a_cas(volatile int *p, int t, int s)
 		: "memory");
 	return old;
 }
+
+#endif /* __riscv_zacas */
\ No newline at end of file
diff --git a/arch/riscv64/atomic_arch.h b/arch/riscv64/atomic_arch.h
index 0c382588..9681505e 100644
--- a/arch/riscv64/atomic_arch.h
+++ b/arch/riscv64/atomic_arch.h
@@ -4,6 +4,34 @@ static inline void a_barrier()
 	__asm__ __volatile__ ("fence rw,rw" : : : "memory");
 }
 
+#ifdef __riscv_zacas
+
+#define a_cas a_cas
+static inline int a_cas(volatile int *p, int t, int s)
+{
+	int old = t;
+	__asm__ __volatile__ (
+		"amocas.w.aqrl %0, %2, %1"
+		: "+r"(old), "+A"(*(volatile int *)p)
+		: "r"(s)
+		: "memory");
+	return old;
+}
+
+#define a_cas_p a_cas_p
+static inline void *a_cas_p(volatile void *p, void *t, void *s)
+{
+	void *old = t;
+	__asm__ __volatile__ (
+		"amocas.d.aqrl %0, %2, %1"
+		: "+r"(old), "+A"(*(void *volatile *)p)
+		: "r"(s)
+		: "memory");
+	return old;
+}
+
+#else /* Fallback to lr/sc when Zacas is not available */
+
 #define a_cas a_cas
 static inline int a_cas(volatile int *p, int t, int s)
 {
@@ -36,3 +64,5 @@ static inline void *a_cas_p(volatile void *p, void *t, void *s)
 		: "memory");
 	return old;
 }
+
+#endif /* __riscv_zacas */
-- 
2.39.5