Optimize syscall_write() to avoid redundant checks and FILE* retrievals

There is redundant checking of map_at_end within a loop, along with
repetitive calls to map_iter_value() to retrieve the file pointer. This
results in unnecessary overhead.

Address the issue by optimizing the code to perform the map_at_end
check and retrieve the file pointer only once at the beginning.

Author: visitorckw

src/syscall.c

@@ -93,32 +93,28 @@ static void syscall_write(riscv_t *rv)
     /* lookup the file descriptor */
     map_iter_t it;
     map_find(attr->fd_map, &it, &fd);
+    if (map_at_end(attr->fd_map, &it))
+        goto error_handler;
+
     uint32_t total_write = 0;
+    FILE *handle = map_iter_value(&it, FILE *);
 
     while (count > PREALLOC_SIZE) {
         memory_read(attr->mem, tmp, buffer + total_write, PREALLOC_SIZE);
-        if (!map_at_end(attr->fd_map, &it)) {
-            /* write out the data */
-            FILE *handle = map_iter_value(&it, FILE *);
-            size_t written = fwrite(tmp, 1, PREALLOC_SIZE, handle);
-            if (written != PREALLOC_SIZE && ferror(handle))
-                goto error_handler;
-            total_write += written;
-            count -= PREALLOC_SIZE;
-        } else
+        /* write out the data */
+        size_t written = fwrite(tmp, 1, PREALLOC_SIZE, handle);
+        if (written != PREALLOC_SIZE && ferror(handle))
             goto error_handler;
+        total_write += written;
+        count -= PREALLOC_SIZE;
     }
 
     memory_read(attr->mem, tmp, buffer + total_write, count);
-    if (!map_at_end(attr->fd_map, &it)) {
-        /* write out the data */
-        FILE *handle = map_iter_value(&it, FILE *);
-        size_t written = fwrite(tmp, 1, count, handle);
-        if (written != count && ferror(handle))
-            goto error_handler;
-        total_write += written;
-    } else
+    /* write out the data */
+    size_t written = fwrite(tmp, 1, count, handle);
+    if (written != count && ferror(handle))
         goto error_handler;
+    total_write += written;
     assert(total_write == rv_get_reg(rv, rv_reg_a2));
 
     /* return number of bytes written */