[PATCH] Generalize GC support for SHF_LINK_ORDER.

["Evgenii Stepanov via binutils" <binutils at sourceware dot org>]

This change implements new GC semantics of SHF_LINK_ORDER discussed
in the following thread on generic-abi ML:
https://groups.google.com/d/msg/generic-abi/_CbBM6T6WeM/LZnqx1KZAQAJ

It is a generalization of the logic that prevents garbage collection
of .ARM.exidx sections by adding a GC reference from the
corresponding text section (found by the sh_link field of the
.ARM.exidx section).

The new logic is exactly the same, and the code is basically a copy
of the .ARM.exidx code, but now it is keyed off the SHF_LINK_ORDER
flag instead of SHT_ARM_EXIDX section type. The existing ARM behavior
is not changed because Gold already demands that SHT_ARM_EXIDX
sections have SHF_LINK_ORDER flag set.

Another change is not to consider __start_$section and
__stop_$section symbols as GC references when $section has
SHF_LINK_ORDER flag. The idea behind this is that SHF_LINK_ORDER
section can contain metadata for the linked section, and section
start/stop symbols can be used to enumarate that metadata at runtime.
In this case the liveness of the metadata section should be
determined only by the liveness of the linked section, and not be
affected by the presence of start/stop symbols.
---
 ChangeLog      |  6 ++++++
 gold/arm.cc    | 47 -----------------------------------------------
 gold/object.cc |  4 +++-
 gold/reloc.cc  | 26 ++++++++++++++++++++++++++
 4 files changed, 35 insertions(+), 48 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 3904dc134d..082cb0670f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-03-31 Evgenii Stepanov <eugenis@google.com>
+
+	* gold/arm.cc: Remove do_gc_process_relocs
+	* gold/reloc.cc: Handle SHF_LINK_ORDER in do_gc_process_relocs
+	* gold/object.cc: Do not add SHF_LINK_ORDER sections to cident_sections.
+
 2016-03-17  Cary Coutant  <ccoutant@gmail.com>
 
 	* configure.ac: Add mips and s390 to the gold target check.
diff --git a/gold/arm.cc b/gold/arm.cc
index c47b00224c..f83c3bd737 100644
--- a/gold/arm.cc
+++ b/gold/arm.cc
@@ -1686,10 +1686,6 @@ class Arm_relobj : public Sized_relobj_file<32, big_endian>
   void
   do_read_symbols(Read_symbols_data* sd);
 
-  // Process relocs for garbage collection.
-  void
-  do_gc_process_relocs(Symbol_table*, Layout*, Read_relocs_data*);
-
  private:
 
   // Whether a section needs to be scanned for relocation stubs.
@@ -6939,49 +6935,6 @@ Arm_relobj<big_endian>::do_read_symbols(Read_symbols_data* sd)
     }
 }
 
-// Process relocations for garbage collection.  The ARM target uses .ARM.exidx
-// sections for unwinding.  These sections are referenced implicitly by
-// text sections linked in the section headers.  If we ignore these implicit
-// references, the .ARM.exidx sections and any .ARM.extab sections they use
-// will be garbage-collected incorrectly.  Hence we override the same function
-// in the base class to handle these implicit references.
-
-template<bool big_endian>
-void
-Arm_relobj<big_endian>::do_gc_process_relocs(Symbol_table* symtab,
-					     Layout* layout,
-					     Read_relocs_data* rd)
-{
-  // First, call base class method to process relocations in this object.
-  Sized_relobj_file<32, big_endian>::do_gc_process_relocs(symtab, layout, rd);
-
-  // If --gc-sections is not specified, there is nothing more to do.
-  // This happens when --icf is used but --gc-sections is not.
-  if (!parameters->options().gc_sections())
-    return;
-
-  unsigned int shnum = this->shnum();
-  const unsigned int shdr_size = elfcpp::Elf_sizes<32>::shdr_size;
-  const unsigned char* pshdrs = this->get_view(this->elf_file()->shoff(),
-					       shnum * shdr_size,
-					       true, true);
-
-  // Scan section headers for sections of type SHT_ARM_EXIDX.  Add references
-  // to these from the linked text sections.
-  const unsigned char* ps = pshdrs + shdr_size;
-  for (unsigned int i = 1; i < shnum; ++i, ps += shdr_size)
-    {
-      elfcpp::Shdr<32, big_endian> shdr(ps);
-      if (shdr.get_sh_type() == elfcpp::SHT_ARM_EXIDX)
-	{
-	  // Found an .ARM.exidx section, add it to the set of reachable
-	  // sections from its linked text section.
-	  unsigned int text_shndx = this->adjust_shndx(shdr.get_sh_link());
-	  symtab->gc()->add_reference(this, text_shndx, this, i);
-	}
-    }
-}
-
 // Update output local symbol count.  Owing to EXIDX entry merging, some local
 // symbols  will be removed in output.  Adjust output local symbol count
 // accordingly.  We can only changed the static output local symbol count.  It
diff --git a/gold/object.cc b/gold/object.cc
index a631c9937c..160156397d 100644
--- a/gold/object.cc
+++ b/gold/object.cc
@@ -1623,10 +1623,12 @@ Sized_relobj_file<size, big_endian>::do_layout(Symbol_table* symtab,
 	      symtab->gc()->worklist().push_back(Section_id(this, i));
 	    }
 	  // If the section name XXX can be represented as a C identifier
+          // and the section does not have SHF_LINK_ORDER flag
 	  // it cannot be discarded if there are references to
 	  // __start_XXX and __stop_XXX symbols.  These need to be
 	  // specially handled.
-	  if (is_cident(name))
+	  if (!(shdr.get_sh_flags() & elfcpp::SHF_LINK_ORDER)
+	      && is_cident(name))
 	    {
 	      symtab->gc()->add_cident_section(name, Section_id(this, i));
 	    }
diff --git a/gold/reloc.cc b/gold/reloc.cc
index ca54f153a3..f782627f26 100644
--- a/gold/reloc.cc
+++ b/gold/reloc.cc
@@ -35,6 +35,7 @@
 #include "icf.h"
 #include "compressed_output.h"
 #include "incremental.h"
+#include "gc.h"
 
 namespace gold
 {
@@ -415,6 +416,31 @@ Sized_relobj_file<size, big_endian>::do_gc_process_relocs(Symbol_table* symtab,
                                         local_symbols);
         }
     }
+
+  // If --gc-sections is not specified, there is nothing more to do.
+  // This happens when --icf is used but --gc-sections is not.
+  if (!parameters->options().gc_sections())
+    return;
+
+  unsigned int shnum = this->shnum();
+  const unsigned char* pshdrs = this->get_view(this->elf_file_.shoff(),
+					       shnum * This::shdr_size,
+					       true, true);
+
+  // Scan section headers for sections with SHF_LINK_ORDER.  Add references
+  // to these from the linked text sections.
+  const unsigned char* ps = pshdrs + This::shdr_size;
+  for (unsigned int i = 1; i < shnum; ++i, ps += This::shdr_size)
+    {
+      This::Shdr shdr(ps);
+      if (shdr.get_sh_flags() & elfcpp::SHF_LINK_ORDER)
+	{
+	  // Found an SHF_LINK_ORDER section, add it to the set of reachable
+	  // sections from its linked text section.
+	  unsigned int text_shndx = this->adjust_shndx(shdr.get_sh_link());
+	  symtab->gc()->add_reference(this, text_shndx, this, i);
+	}
+    }
 }
 
 
-- 
2.12.2.564.g063fe858b8-goog