Add MRB_ALLOCV() for temporary memory allocation

include/mrbconf.h

@@ -120,6 +120,9 @@
 /* fixed size state atexit stack */
 //#define MRB_FIXED_STATE_ATEXIT_STACK
 
+/* maximum size to allocate on the stack in MRB_ALLOCV  */
+//#define MRB_ALLOCV_ON_STACK_MAX 256
+
 /* -DMRB_DISABLE_XXXX to drop following features */
 //#define MRB_DISABLE_STDIO /* use of stdio */
 

include/mruby.h

@@ -1355,6 +1355,45 @@ MRB_API void mrb_show_copyright(mrb_state *mrb);
 
 MRB_API mrb_value mrb_format(mrb_state *mrb, const char *format, ...);
 
+/*
+ * MRB_ALLOCV(mrb, type, var, n)
+ *
+ * Define `type *var` variable and assign temporarily allocated memory for
+ * _n_ objects of type _type_. If the allocation size is less than or equal
+ * to `MRB_ALLOCV_ON_STACK_MAX`, allocate on the stack, otherwise use
+ * `mrb_alloca()`.
+ */
+#ifndef MRB_ALLOCV_ON_STACK_MAX
+# define MRB_ALLOCV_ON_STACK_MAX 256
+#endif
+#ifdef _WIN32
+# include <malloc.h>
+# if MRB_ALLOCV_ON_STACK_MAX > _ALLOCA_S_THRESHOLD
+#  define MRB_ALLOCV_ON_STACK_MAX _ALLOCA_S_THRESHOLD
+# endif
+#endif
+#if defined __clang__ && __clang_major__ >= 4 || \
+    defined __GNUC__ && __GNUC__ >= 4 && __GNUC_MINOR__ >= 7
+# define MRB_ALLOCV(mrb, type, var, n)                                  \
+  _MRB_ALLOCV(mrb, type, var, n,                                        \
+    __builtin_alloca_with_align(sizeof(type)*(n), __alignof__(type)*CHAR_BIT))
+#elif defined _WIN32
+# define MRB_ALLOCV(mrb, type, var, n) \
+  _MRB_ALLOCV(mrb, type, var, n, _alloca(sizeof(type)*(n)))
+#elif defined __STDC_NO_VLA__
+# define MRB_ALLOCV(mrb, type, var, n)                                         \
+  type _MRB_ALLOCV_ON_STACK_BUFFER(var)[MRB_ALLOCV_ON_STACK_MAX/sizeof(type)]; \
+  _MRB_ALLOCV(mrb, type, var, n, _MRB_ALLOCV_ON_STACK_BUFFER(var))
+#else
+# define MRB_ALLOCV(mrb, type, var, n)                                  \
+  type _MRB_ALLOCV_ON_STACK_BUFFER(var)[n];                             \
+  _MRB_ALLOCV(mrb, type, var, n, _MRB_ALLOCV_ON_STACK_BUFFER(var))
+#endif
+#define _MRB_ALLOCV(mrb, type, var, n, on_stack_expr)                   \
+  type *var = (type*)(MRB_ALLOCV_ON_STACK_MAX < sizeof(type)*(n) ?      \
+    mrb_alloca(mrb, sizeof(type)*(n)) : on_stack_expr)
+#define _MRB_ALLOCV_ON_STACK_BUFFER(var) mrb_allocv_on_stack_buffer_##var##__
+
 #if 0
 /* memcpy and memset does not work with gdb reverse-next on my box */
 /* use naive memcpy and memset instead */

mrbgems/mruby-struct/src/struct.c

@@ -123,29 +123,15 @@ mrb_struct_ref(mrb_state *mrb, mrb_value obj)
 static mrb_sym
 mrb_id_attrset(mrb_state *mrb, mrb_sym id)
 {
-#define ONSTACK_ALLOC_MAX 32
-#define ONSTACK_STRLEN_MAX (ONSTACK_ALLOC_MAX - 1) /* '=' character */
-
-  const char *name;
-  char *buf;
   mrb_int len;
   mrb_sym mid;
-  char onstack[ONSTACK_ALLOC_MAX];
+  const char *name = mrb_sym_name_len(mrb, id, &len);
+  MRB_ALLOCV(mrb, char, buf, len+1);
 
-  name = mrb_sym_name_len(mrb, id, &len);
-  if (len > ONSTACK_STRLEN_MAX) {
-    buf = (char *)mrb_malloc(mrb, (size_t)len+1);
-  }
-  else {
-    buf = onstack;
-  }
   memcpy(buf, name, (size_t)len);
   buf[len] = '=';
 
   mid = mrb_intern(mrb, buf, len+1);
-  if (buf != onstack) {
-    mrb_free(mrb, buf);
-  }
   return mid;
 }
 

src/class.c

@@ -1407,18 +1407,15 @@ mrb_method_search(mrb_state *mrb, struct RClass* c, mrb_sym mid)
   return m;
 }
 
-#define ONSTACK_ALLOC_MAX 32
-
 static mrb_sym
 prepare_name_common(mrb_state *mrb, mrb_sym sym, const char *prefix, const char *suffix)
 {
-  char onstack[ONSTACK_ALLOC_MAX];
   mrb_int sym_len;
   const char *sym_str = mrb_sym_name_len(mrb, sym, &sym_len);
   size_t prefix_len = prefix ? strlen(prefix) : 0;
   size_t suffix_len = suffix ? strlen(suffix) : 0;
   size_t name_len = sym_len + prefix_len + suffix_len;
-  char *buf = name_len > sizeof(onstack) ? (char *)mrb_alloca(mrb, name_len) : onstack;
+  MRB_ALLOCV(mrb, char, buf, name_len);
   char *p = buf;
 
   if (prefix_len > 0) {