[musl] [PATCH v3 1/1] riscv64: add runtime-detected vector optimized memset

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

Add a RISC-V vector extension optimized memset implementation with
runtime CPU capability detection via HW_CAP.

The implementation provides both vector and scalar variants in a single
binary. At process startup, __init_riscv_string_optimizations() queries
AT_HWCAP to detect RVV support and selects the appropriate
implementation via function pointer dispatch. This allows the same libc
to run correctly on both vector-capable and non-vector RISC-V CPUs.

The vector implementation uses m8 register grouping and processes data
in vector-length chunks, providing significant performance improvements
on RVV-capable hardware while maintaining compatibility with non-vector
RISC-V systems.

Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
---
 src/env/__libc_start_main.c          |  3 +++
 src/internal/libc.h                  |  3 +++
 src/string/riscv64/memset.c          |  4 ++++
 src/string/riscv64/memset_dispatch.c | 36 ++++++++++++++++++++++++++++
 src/string/riscv64/memset_vector.S   | 28 ++++++++++++++++++++++
 5 files changed, 74 insertions(+)
 create mode 100644 src/string/riscv64/memset.c
 create mode 100644 src/string/riscv64/memset_dispatch.c
 create mode 100644 src/string/riscv64/memset_vector.S

diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c
index c5b277bd..c23e63f4 100644
--- a/src/env/__libc_start_main.c
+++ b/src/env/__libc_start_main.c
@@ -38,6 +38,9 @@ void __init_libc(char **envp, char *pn)
 
 	__init_tls(aux);
 	__init_ssp((void *)aux[AT_RANDOM]);
+#ifdef __riscv
+	__init_riscv_string_optimizations();
+#endif
 
 	if (aux[AT_UID]==aux[AT_EUID] && aux[AT_GID]==aux[AT_EGID]
 		&& !aux[AT_SECURE]) return;
diff --git a/src/internal/libc.h b/src/internal/libc.h
index 619bba86..28860b06 100644
--- a/src/internal/libc.h
+++ b/src/internal/libc.h
@@ -40,6 +40,9 @@ extern hidden struct __libc __libc;
 hidden void __init_libc(char **, char *);
 hidden void __init_tls(size_t *);
 hidden void __init_ssp(void *);
+#ifdef __riscv
+hidden void __init_riscv_string_optimizations(void);
+#endif
 hidden void __libc_start_init(void);
 hidden void __funcs_on_exit(void);
 hidden void __funcs_on_quick_exit(void);
diff --git a/src/string/riscv64/memset.c b/src/string/riscv64/memset.c
new file mode 100644
index 00000000..11fa3032
--- /dev/null
+++ b/src/string/riscv64/memset.c
@@ -0,0 +1,4 @@
+/* Rename the generic memset to __memset_scalar and include it */
+#define memset __memset_scalar
+#include "../memset.c"
+#undef memset
diff --git a/src/string/riscv64/memset_dispatch.c b/src/string/riscv64/memset_dispatch.c
new file mode 100644
index 00000000..8e36d0f9
--- /dev/null
+++ b/src/string/riscv64/memset_dispatch.c
@@ -0,0 +1,36 @@
+#include <stddef.h>
+#include <stdint.h>
+#include <sys/auxv.h>
+#include "libc.h"
+
+void *__memset_scalar(void *s, int c, size_t n);
+void *__memset_vect(void *s, int c, size_t n);
+
+/* memset function pointer, runtime-dispatched based on RVV support */
+__attribute__((visibility("hidden")))
+#ifndef __riscv_vector
+void *(*__memset_ptr)(void *, int, size_t) = __memset_scalar;
+#else
+void *(*__memset_ptr)(void *, int, size_t) = __memset_vect;
+#endif
+
+void *memset(void *s, int c, size_t n)
+{
+	return __memset_ptr(s, c, n);
+}
+
+static __inline int __has_rvv_via_hwcap(void)
+{
+	const unsigned long V_bit = (1ul << ('V' - 'A'));
+	unsigned long hwcap = __getauxval(AT_HWCAP);
+	return (hwcap & V_bit) != 0;
+}
+
+__attribute__((visibility("hidden")))
+void __init_riscv_string_optimizations(void)
+{
+	if (__has_rvv_via_hwcap())
+		__memset_ptr = __memset_vect;
+	else
+		__memset_ptr = __memset_scalar;
+}
diff --git a/src/string/riscv64/memset_vector.S b/src/string/riscv64/memset_vector.S
new file mode 100644
index 00000000..513645d3
--- /dev/null
+++ b/src/string/riscv64/memset_vector.S
@@ -0,0 +1,28 @@
+    .text
+    .global __memset_vect
+    .option push
+    .option arch, +v
+/* void *__memset_vect(void *s, int c, size_t n)
+ * a0 = s (dest), a1 = c (fill byte), a2 = n (size)
+ * Returns a0.
+ */
+__memset_vect:
+    mv      t0, a0                    /* running dst; keep a0 as return */
+    beqz    a2, .Ldone_vect           /* n == 0 then return */
+
+    /* Broadcast fill byte once. */
+    vsetvli t1, zero, e8, m8, ta, ma
+    vmv.v.x v0, a1
+
+.Lbulk_vect:
+    vsetvli t1, a2, e8, m8, ta, ma    /* t1 = vl (bytes) */
+    vse8.v  v0, (t0)
+    add     t0, t0, t1
+    sub     a2, a2, t1
+    bnez    a2, .Lbulk_vect
+    /* fallthrough */
+
+.Ldone_vect:
+    ret
+.size __memset_vect, .-__memset_vect
+.option pop
-- 
2.39.5