__attibute__((__aligned__)) for RSTRING_PTR()

For instance array.c:rb_ary_product() uses RSTRING_PTR() as an
array of int. So to avoid misaligned memory access RSTRING_PTR()
must at least be sizeof(int)-aligned.  However the type of
RSTRING_PTR() is char*, which of course can expect alignment as
much as 1.  This is a problem.

The reality is, there is no misaligned memory access because the
memory region behind RSTRING_PTR() is allocated using malloc().
Memory regions returned from malloc() are always aligned
appropriately.  So let's tell the compiler about this information.
It seems GCC, clang, and MSVC have such feature.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61827 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

compile.c

@@ -8077,7 +8077,7 @@ struct ibf_dump {
 rb_iseq_t * iseq_alloc(void);
 
 struct ibf_load {
-    const char *buff;
+    const RUBY_ALIGNAS(sizeof(VALUE)) char *buff;
     const struct ibf_header *header;
     ID *id_list;     /* [id0, ...] */
     VALUE iseq_list; /* [iseq0, ...] */

configure.ac

@@ -1743,6 +1743,30 @@ EOH
 ])dnl
 ])dnl
 
+AC_CACHE_CHECK([for alignas() syntax], rb_cv_have_alignas, [
+rb_cv_have_alignas=no
+RUBY_WERROR_FLAG([
+for attr in \
+    "_Alignas(x)" \
+    "alignas(x)" \
+    "@<:@@<:@alignas(x)@:>@@:>@" \
+    "__declspec(aligned(x))" \
+    "__attribute__((__aligned__(x)))" \
+;
+do
+    # C11 _Alignas and GCC __attribute__((__aligned__)) behave
+    # slightly differently.  What we want is GCC's.  Check that
+    # here by something C11 does not allow (`struct ALIGNAS ...`)
+    AC_TRY_COMPILE(
+	[@%:@define ALIGNAS(x) $attr
+	struct ALIGNAS(128) conftest_tag { int foo; } foo; ], [],
+        [rb_cv_have_alignas="$attr"; break], [])
+done
+])])
+AS_IF([test "$rb_cv_have_alignas" != no], [
+    AC_DEFINE_UNQUOTED([RUBY_ALIGNAS(x)], $rb_cv_have_alignas)
+])
+
 dnl RUBY_DECL_ATTRIBUTE(attrib, macroname, cachevar, condition, type, code)
 AC_DEFUN([RUBY_DECL_ATTRIBUTE], [dnl
 m4_ifval([$2], dnl
@@ -1773,6 +1797,7 @@ ${rbcv_cond+[@%:@define ]attrib[](attrib_params)[ x]}
 ${rbcv_cond+[@%:@endif]})
 $6
 @%:@define mesg ("")
+@%:@define n (32768)
     attrib[](attrib_params)[;], [],
     [rbcv="$mac"; break])
 done
@@ -1801,7 +1826,6 @@ AC_DEFUN([RUBY_TYPE_ATTRIBUTE], [dnl
 @%:@define x struct conftest_attribute_check {int i;}
 ])
 ])
-
 RUBY_FUNC_ATTRIBUTE(__const__, CONSTFUNC)
 RUBY_FUNC_ATTRIBUTE(__pure__, PUREFUNC)
 RUBY_FUNC_ATTRIBUTE(__noreturn__, NORETURN)

include/ruby/defines.h

@@ -376,6 +376,10 @@ void rb_ia64_flushrs(void);
 # endif
 #endif
 
+#ifndef RUBY_ALIGNAS
+#define RUBY_ALIGNAS(x) y
+#endif
+
 RUBY_SYMBOL_EXPORT_END
 
 #if defined(__cplusplus)

include/ruby/ruby.h

@@ -853,14 +853,10 @@ enum ruby_fl_type {
     RUBY_FL_SINGLETON = RUBY_FL_USER0
 };
 
-struct RBasic {
+struct RUBY_ALIGNAS(sizeof(VALUE)) RBasic {
     VALUE flags;
     const VALUE klass;
-}
-#ifdef __GNUC__
-    __attribute__((aligned(sizeof(VALUE))))
-#endif
-;
+};
 
 VALUE rb_obj_hide(VALUE obj);
 VALUE rb_obj_reveal(VALUE obj, VALUE klass); /* do not use this API to change klass information */
@@ -953,18 +949,21 @@ enum ruby_rstring_flags {
 
     RSTRING_ENUM_END
 };
+
+typedef RUBY_ALIGNAS(sizeof(VALUE)) char ruby_aligned_char;
+
 struct RString {
     struct RBasic basic;
     union {
 	struct {
 	    long len;
-	    char *ptr;
+	    ruby_aligned_char *ptr;
 	    union {
 		long capa;
 		VALUE shared;
 	    } aux;
 	} heap;
-	char ary[RSTRING_EMBED_LEN_MAX + 1];
+	char RUBY_ALIGNAS(sizeof(VALUE)) ary[RSTRING_EMBED_LEN_MAX + 1];
     } as;
 };
 #define RSTRING_EMBED_LEN(str) \
@@ -976,7 +975,7 @@ struct RString {
      RSTRING(str)->as.heap.len)
 #define RSTRING_PTR(str) \
     (!(RBASIC(str)->flags & RSTRING_NOEMBED) ? \
-     RSTRING(str)->as.ary : \
+     (ruby_aligned_char *)RSTRING(str)->as.ary : \
      RSTRING(str)->as.heap.ptr)
 #define RSTRING_END(str) \
     (!(RBASIC(str)->flags & RSTRING_NOEMBED) ? \

win32/Makefile.sub

@@ -636,6 +636,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define PACKED_STRUCT_UNALIGNED(x) x
 !endif
 #define RUBY_EXTERN extern __declspec(dllimport)
+#define RUBY_ALIGNAS(n) __declspec(align(n))
 #define HAVE_DECL_SYS_NERR 1
 #define HAVE_LIMITS_H 1
 #define HAVE_FCNTL_H 1