[PATCH fix PR71767 1/4 : ld64 atoms] Make PIC indirections and constant labels linker-visible.

[Iain Sandoe <iain_sandoe at mentor dot com>]

Hi Folks,

(in Darwin)  If we have a situation like:

global_weak_symbol:
  ....
Lnon_weak_local:
  ....

  ld64 will be unable to split this into two atoms (because the "L" makes
  the second symbol 'invisible').  This means that legitimate direct accesses
  to the second symbol will appear to be non-allowed direct accesses to an
  atom of type weak, global which are not allowed.

  To avoid this, we make the indirections have a leading 'l' (lower-case L)
  which has a special meaning: linker can see this and use it to determine
  atoms, but it is not placed into the final symbol table.

  The implementation here is somewhat heavy-handed in that it will also mark
  indirections to the __IMPORT,__pointers section the same way which is
  really unnecessary, since ld64 _can_ split those into atoms as they are
  fixed size.  FIXME: determine if this is a penalty worth extra code to
  figure out.

Similarly, with:

  .const
weak_global_constant:
  ....

LCxx:
  ...

 ld64 can't split the second, causing a warning when it's directly
accessed.  We solve this by making the two cases emit a linker-visible symbol (where the L becomes ‘l’).

TODO: there are also some cases in Objective-C meta-data that need similar treatment, but those need to be caught in the objc runtime lowering.

OK for trunk?
OK for open branches?
Iain

gcc/
2016-11-06  Iain Sandoe  <iain@codesourcery.com>

       PR target/71767
	* config/darwin.c (imachopic_indirection_name): Make data section indirections
	linker-visible.
	* config/darwin.h (ASM_GENERATE_INTERNAL_LABEL): Make local constant
	labels linker-visible.



---
 gcc/config/darwin.c | 37 +++++++++++++++++++++++++++++++++----
 gcc/config/darwin.h |  9 ++++++++-
 2 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/gcc/config/darwin.c b/gcc/config/darwin.c
index ff8600c..210c2da 100644
--- a/gcc/config/darwin.c
+++ b/gcc/config/darwin.c
@@ -474,7 +474,31 @@ indirection_hasher::equal (machopic_indirection *s, const char *k)
 }
 
 /* Return the name of the non-lazy pointer (if STUB_P is false) or
-   stub (if STUB_B is true) corresponding to the given name.  */
+   stub (if STUB_B is true) corresponding to the given name.
+
+  If we have a situation like:
+
+global_weak_symbol:
+  ....
+Lnon_weak_local:
+  ....
+
+  ld64 will be unable to split this into two atoms (because the "L" makes
+  the second symbol 'invisible').  This means that legitimate direct accesses
+  to the second symbol will appear to be non-allowed direct accesses to an
+  atom of type weak, global which are not allowed.
+
+  To avoid this, we make the indirections have a leading 'l' (lower-case L)
+  which has a special meaning: linker can see this and use it to determine
+  atoms, but it is not placed into the final symbol table.
+
+  The implementation here is somewhat heavy-handed in that it will also mark
+  indirections to the __IMPORT,__pointers section the same way which is
+  really unnecessary, since ld64 _can_ split those into atoms as they are
+  fixed size.  FIXME: determine if this is a penalty worth extra code to
+  fix.
+
+*/
 
 const char *
 machopic_indirection_name (rtx sym_ref, bool stub_p)
@@ -485,6 +509,7 @@ machopic_indirection_name (rtx sym_ref, bool stub_p)
   machopic_indirection *p;
   bool needs_quotes;
   const char *suffix;
+  char L_or_l = 'L';
   const char *prefix = user_label_prefix;
   const char *quote = "";
   tree id;
@@ -519,9 +544,13 @@ machopic_indirection_name (rtx sym_ref, bool stub_p)
   if (stub_p)
     suffix = STUB_SUFFIX;
   else
-    suffix = NON_LAZY_POINTER_SUFFIX;
+    {
+      suffix = NON_LAZY_POINTER_SUFFIX;
+      /* Let the linker see this.  */
+      L_or_l = 'l';
+    }
 
-  buffer = XALLOCAVEC (char, strlen ("&L")
+  buffer = XALLOCAVEC (char, 2  /* strlen ("&L") or ("&l") */
 		   + strlen (prefix)
 		   + namelen
 		   + strlen (suffix)
@@ -529,7 +558,7 @@ machopic_indirection_name (rtx sym_ref, bool stub_p)
 		   + 1 /* '\0' */);
 
   /* Construct the name of the non-lazy pointer or stub.  */
-  sprintf (buffer, "&%sL%s%s%s%s", quote, prefix, name, suffix, quote);
+  sprintf (buffer, "&%s%c%s%s%s%s", quote, L_or_l, prefix, name, suffix, quote);
 
   if (!machopic_indirections)
     machopic_indirections = hash_table<indirection_hasher>::create_ggc (37);
diff --git a/gcc/config/darwin.h b/gcc/config/darwin.h
index 98d2382..045f70b 100644
--- a/gcc/config/darwin.h
+++ b/gcc/config/darwin.h
@@ -716,9 +716,16 @@ extern GTY(()) section * darwin_sections[NUM_DARWIN_SECTIONS];
   { "weak_import", 0, 0, true, false, false,				     \
     darwin_handle_weak_import_attribute, false }
 
+/* Make local constant labels linker-visible, so that if one follows a
+   weak_global constant, ld64 will be able to separate the atoms.  */
 #undef ASM_GENERATE_INTERNAL_LABEL
 #define ASM_GENERATE_INTERNAL_LABEL(LABEL,PREFIX,NUM)	\
-  sprintf (LABEL, "*%s%ld", PREFIX, (long)(NUM))
+  do {							\
+    if (strcmp ("LC", PREFIX) == 0)			\
+      sprintf (LABEL, "*%s%ld", "lC", (long)(NUM));	\
+    else						\
+      sprintf (LABEL, "*%s%ld", PREFIX, (long)(NUM));	\
+  } while (0)
 
 #undef TARGET_ASM_MARK_DECL_PRESERVED
 #define TARGET_ASM_MARK_DECL_PRESERVED darwin_mark_decl_preserved
-- 
2.8.1