gold patch committed: Handle DW_EH_PE_datarel in FDE

[Ian Lance Taylor <iant at google dot com>]

This gold patch adds support for DW_EH_PE_datarel entries in an FDE.
gold needs to know how to handle them in order to support generating the
.eh_frame_hdr section correctly.  gcc will generate DW_EH_PE_datarel on
Solaris and SVR4, because the native assemblers allegedly can not handle
an expression representing the difference between two symbols in
different sections.  For DW_EH_PE_datarel the base of the value will be
the global offset table, meaning that gcc can simply use a GOTOFF
relocation.  Committed to mainline.

Ian


2011-07-01  Ian Lance Taylor  <iant@google.com>

	PR gold/12525
	* ehframe.cc (Eh_frame_hdr::get_fde_pc): Handle DW_EH_PE_datarel.
	Assert if we see DW_EH_PE_indirect.
	* target.h (Target::ehframe_datarel_base): New function.
	(Target::do_ehframe_datarel_base): New target function.
	* i386.cc (Target_i386::do_ehframe_datarel_base): New function.
	* x86_64.cc (Target_x86_64::do_ehframe_datarel_base): New
	function.


Index: ehframe.cc
===================================================================
RCS file: /cvs/src/src/gold/ehframe.cc,v
retrieving revision 1.20
diff -u -p -r1.20 ehframe.cc
--- ehframe.cc	1 Jul 2011 22:05:01 -0000	1.20
+++ ehframe.cc	1 Jul 2011 23:58:20 -0000
@@ -266,7 +266,7 @@ Eh_frame_hdr::get_fde_pc(
       gold_unreachable();
     }
 
-  switch (fde_encoding & 0xf0)
+  switch (fde_encoding & 0x70)
     {
     case 0:
       break;
@@ -275,12 +275,18 @@ Eh_frame_hdr::get_fde_pc(
       pc += eh_frame_address + fde_offset + 8;
       break;
 
+    case elfcpp::DW_EH_PE_datarel:
+      pc += parameters->target().ehframe_datarel_base();
+      break;
+
     default:
       // If other cases arise, then we have to handle them, or we have
       // to reject them by returning false in Eh_frame::read_cie.
       gold_unreachable();
     }
 
+  gold_assert((fde_encoding & elfcpp::DW_EH_PE_indirect) == 0);
+
   return pc;
 }
 
Index: i386.cc
===================================================================
RCS file: /cvs/src/src/gold/i386.cc,v
retrieving revision 1.133
diff -u -p -r1.133 i386.cc
--- i386.cc	1 Jul 2011 22:54:04 -0000	1.133
+++ i386.cc	1 Jul 2011 23:58:21 -0000
@@ -295,6 +295,10 @@ class Target_i386 : public Sized_target<
   do_can_check_for_function_pointers() const
   { return true; }
 
+  // Return the base for a DW_EH_PE_datarel encoding.
+  uint64_t
+  do_ehframe_datarel_base() const;
+
   // Return whether SYM is call to a non-split function.
   bool
   do_is_call_to_non_split(const Symbol* sym, unsigned int) const;
@@ -3267,6 +3271,21 @@ Target_i386::do_code_fill(section_size_t
   return std::string(nops[length], length);
 }
 
+// Return the value to use for the base of a DW_EH_PE_datarel offset
+// in an FDE.  Solaris and SVR4 use DW_EH_PE_datarel because their
+// assembler can not write out the difference between two labels in
+// different sections, so instead of using a pc-relative value they
+// use an offset from the GOT.
+
+uint64_t
+Target_i386::do_ehframe_datarel_base() const
+{
+  gold_assert(this->global_offset_table_ != NULL);
+  Symbol* sym = this->global_offset_table_;
+  Sized_symbol<32>* ssym = static_cast<Sized_symbol<32>*>(sym);
+  return ssym->value();
+}
+
 // Return whether SYM should be treated as a call to a non-split
 // function.  We don't want that to be true of a call to a
 // get_pc_thunk function.
Index: target.h
===================================================================
RCS file: /cvs/src/src/gold/target.h,v
retrieving revision 1.60
diff -u -p -r1.60 target.h
--- target.h	28 Jun 2011 22:25:14 -0000	1.60
+++ target.h	1 Jul 2011 23:58:21 -0000
@@ -276,6 +276,15 @@ class Target
   section_may_have_icf_unsafe_pointers(const char* section_name) const
   { return this->do_section_may_have_icf_unsafe_pointers(section_name); }
 
+  // Return the base to use for the PC value in an FDE when it is
+  // encoded using DW_EH_PE_datarel.  This does not appear to be
+  // documented anywhere, but it is target specific.  Any use of
+  // DW_EH_PE_datarel in gcc requires defining a special macro
+  // (ASM_MAYBE_OUTPUT_ENCODED_ADDR_RTX) to output the value.
+  uint64_t
+  ehframe_datarel_base() const
+  { return this->do_ehframe_datarel_base(); }
+
   // Return true if a reference to SYM from a reloc of type R_TYPE
   // means that the current function may call an object compiled
   // without -fsplit-stack.  SYM is known to be defined in an object
@@ -521,6 +530,10 @@ class Target
 	    && !is_prefix_of(".eh_frame", section_name));
   }
 
+  virtual uint64_t
+  do_ehframe_datarel_base() const
+  { gold_unreachable(); }
+
   // Virtual function which may be overridden by the child class.  The
   // default implementation is that any function not defined by the
   // ABI is a call to a non-split function.
Index: x86_64.cc
===================================================================
RCS file: /cvs/src/src/gold/x86_64.cc,v
retrieving revision 1.131
diff -u -p -r1.131 x86_64.cc
--- x86_64.cc	1 Jul 2011 22:05:01 -0000	1.131
+++ x86_64.cc	1 Jul 2011 23:58:21 -0000
@@ -350,6 +350,10 @@ class Target_x86_64 : public Sized_targe
   do_can_check_for_function_pointers() const
   { return !parameters->options().pie(); }
 
+  // Return the base for a DW_EH_PE_datarel encoding.
+  uint64_t
+  do_ehframe_datarel_base() const;
+
   // Adjust -fsplit-stack code which calls non-split-stack code.
   void
   do_calls_non_split(Relobj* object, unsigned int shndx,
@@ -3640,6 +3644,21 @@ Target_x86_64::do_reloc_addend(void* arg
   return psymval->value(ti.object, 0);
 }
 
+// Return the value to use for the base of a DW_EH_PE_datarel offset
+// in an FDE.  Solaris and SVR4 use DW_EH_PE_datarel because their
+// assembler can not write out the difference between two labels in
+// different sections, so instead of using a pc-relative value they
+// use an offset from the GOT.
+
+uint64_t
+Target_x86_64::do_ehframe_datarel_base() const
+{
+  gold_assert(this->global_offset_table_ != NULL);
+  Symbol* sym = this->global_offset_table_;
+  Sized_symbol<64>* ssym = static_cast<Sized_symbol<64>*>(sym);
+  return ssym->value();
+}
+
 // FNOFFSET in section SHNDX in OBJECT is the start of a function
 // compiled with -fsplit-stack.  The function calls non-split-stack
 // code.  We have to change the function so that it always ensures