ggml-cpu: add ggml_thread_cpu_relax with Zihintpause support (#17784)

* ggml-cpu: add ggml_thread_cpu_relax with Zihintpause support

Signed-off-by: Wang Yang <yangwang@iscas.ac.cn>

* cmake: enable RISC-V zihintpause extension for Spacemit builds

* readme : add ZIHINTPAUSE support for RISC-V

---------

Signed-off-by: Wang Yang <yangwang@iscas.ac.cn>

ggml/src/ggml-cpu/CMakeLists.txt

@@ -469,6 +469,9 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
             if (GGML_RV_ZICBOP)
                 string(APPEND MARCH_STR "_zicbop")
             endif()
+            if (GGML_RV_ZIHINTPAUSE)
+                string(APPEND MARCH_STR "_zihintpause")
+            endif()
             list(APPEND ARCH_FLAGS "-march=${MARCH_STR}" -mabi=lp64d)
         else()
             # Begin with the lowest baseline

ggml/src/ggml-cpu/ggml-cpu.c

@@ -490,6 +490,15 @@ static inline void ggml_thread_cpu_relax(void) {
 static inline void ggml_thread_cpu_relax(void) {
     _mm_pause();
 }
+#elif defined(__riscv)
+static inline void ggml_thread_cpu_relax(void) {
+    #ifdef __riscv_zihintpause
+        __asm__ __volatile__ ("pause");
+    #else
+        /* Encoding of the pause instruction */
+        __asm__ __volatile__ (".4byte 0x100000F");
+    #endif
+}
 #else
 static inline void ggml_thread_cpu_relax(void) {;}
 #endif