Rename MRB_USE_ETEXT_EDATA to MRB_USE_LINK_TIME_RO_DATA_P and support lld linked programs

In lld linked programs, .rodata comes before .text, thus mrb_ro_data_p
will return false for strings in .rodata. Change the lower bound from
_etext to __ehdr_start to catch these cases. This works for ld.bfd, gold
and lld, and it does not have false positives even if .init_array does
not exist.

Remove the branch that uses _edata: strings in .data can be modified so
this is semantically incorrect. Delete the __APPLE__ branch (its
manpages say get_etext() and get_edata() are strongly discouraged).
.init_array has been adopted by most ELF platforms to supersede .ctors.

Neither _etext nor _edata is used, so rename MRB_USE_ETEXT_EDATA to
MRB_USE_EHDR_START.

doc/guides/mrbconf.md

@@ -134,21 +134,16 @@ largest value of required alignment.
 
 ## Reduce heap memory configuration.
 
-`MRB_USE_ETEXT_EDATA`
+`MRB_USE_LINK_TIME_RO_DATA_P`
+* Only available on ELF platforms.
 * If you specify the address of a read-only section when creating a symbol or string, that string will be used as it is.
 * Heap memory can be saved.
-* Uses `_etext` and `__init_array_start`.
-* It must be `_etext < data_addr < &__init_array_start`.
-
-`MRB_NO_INIT_ARRAY_START`
-* Ignored if `MRB_USE_ETEXT_EDATA` is not defined.
-* Please try if `__init_array_start` is not available.
-* Uses `_etext` and `_edata`.
-* It must be `_etext < data_addr < _edata`.
+* Uses `__ehdr_start` and `__init_array_start`.
+* It must be `__ehdr_start < data_addr < __init_array_start`.
 
 `MRB_USE_CUSTOM_RO_DATA_P`
-* Takes precedence over `MRB_USE_ETEXT_EDATA`.
-* Please try if both `MRB_USE_ETEXT_EDATA` and `MRB_NO_INIT_ARRAY_START` are not available.
+* Takes precedence over `MRB_USE_LINK_TIME_RO_DATA_P`.
+* Please try if `MRB_USE_LINK_TIME_RO_DATA_P` is not available.
 * The `mrb_ro_data_p()` function is implemented by the user in an arbitrary file.
 * The prototype declaration is `mrb_bool mrb_ro_data_p(const char *ptr)`.
 * Return `TRUE` if `ptr` is in read-only section, otherwise return `FALSE`.

include/mrbconf.h

@@ -88,14 +88,10 @@
 /* number of object per heap page */
 //#define MRB_HEAP_PAGE_SIZE 1024
 
-/* if _etext and _edata available, mruby can reduce memory used by symbols */
-//#define MRB_USE_ETEXT_EDATA
+/* if __ehdr_start is available, mruby can reduce memory used by symbols */
+//#define MRB_USE_LINK_TIME_RO_DATA_P
 
-/* do not use __init_array_start to determine readonly data section;
-   effective only when MRB_USE_ETEXT_EDATA is defined */
-//#define MRB_NO_INIT_ARRAY_START
-
-/* if do not works both MRB_USE_ETEXT_EDATA and MRB_NO_INIT_ARRAY_START,
+/* if MRB_USE_LINK_TIME_RO_DATA_P does not work,
    you can try mrb_ro_data_p() that you have implemented yourself in any file;
    prototype is `mrb_bool mrb_ro_data_p(const char *ptr)` */
 //#define MRB_USE_CUSTOM_RO_DATA_P

include/mruby/value.h

@@ -309,37 +309,25 @@ mrb_undef_value(void)
   return v;
 }
 
+#if defined(MRB_USE_ETEXT_EDATA) && !defined(MRB_USE_LINK_TIME_RO_DATA_P)
+# ifdef __GNUC__
+#  warning MRB_USE_ETEXT_EDATA is deprecated. Define MRB_USE_LINK_TIME_RO_DATA_P instead.
+# endif
+# define MRB_USE_LINK_TIME_RO_DATA_P
+#endif
+
 #if defined(MRB_USE_CUSTOM_RO_DATA_P)
 /* If you define `MRB_USE_CUSTOM_RO_DATA_P`, you must implement `mrb_ro_data_p()`. */
 mrb_bool mrb_ro_data_p(const char *p);
-#elif defined(MRB_USE_ETEXT_EDATA)
-#if (defined(__APPLE__) && defined(__MACH__))
-#include <mach-o/getsect.h>
-static inline mrb_bool
-mrb_ro_data_p(const char *p)
-{
-  return (const char*)get_etext() < p && p < (const char*)get_edata();
-}
-#else
-extern char _etext[];
-#ifdef MRB_NO_INIT_ARRAY_START
-extern char _edata[];
-
-static inline mrb_bool
-mrb_ro_data_p(const char *p)
-{
-  return _etext < p && p < _edata;
-}
-#else
+#elif defined(MRB_USE_LINK_TIME_RO_DATA_P)
+extern char __ehdr_start[];
 extern char __init_array_start[];
 
 static inline mrb_bool
 mrb_ro_data_p(const char *p)
 {
-  return _etext < p && p < (char*)&__init_array_start;
+  return __ehdr_start < p && p < __init_array_start;
 }
-#endif
-#endif
 #else
 # define mrb_ro_data_p(p) FALSE
 #endif

src/load.c

@@ -554,7 +554,7 @@ read_irep(mrb_state *mrb, const uint8_t *bin, size_t bufsize, uint8_t flags)
 mrb_irep*
 mrb_read_irep(mrb_state *mrb, const uint8_t *bin)
 {
-#if defined(MRB_USE_ETEXT_EDATA) || defined(MRB_USE_CUSTOM_RO_DATA_P)
+#if defined(MRB_USE_LINK_TIME_RO_DATA_P) || defined(MRB_USE_CUSTOM_RO_DATA_P)
   uint8_t flags = mrb_ro_data_p((char*)bin) ? FLAG_SRC_STATIC : FLAG_SRC_MALLOC;
 #else
   uint8_t flags = FLAG_SRC_STATIC;