Update comments about output offsets and merged input sections.

gold/merge.cc

@@ -49,7 +49,8 @@ class Object_merge_map
   // Add a mapping for MERGE_MAP, for the bytes from OFFSET to OFFSET
   // + LENGTH in the input section SHNDX to OUTPUT_OFFSET in the
   // output section.  An OUTPUT_OFFSET of -1 means that the bytes are
-  // discarded.
+  // discarded.  OUTPUT_OFFSET is relative to the start of the merged
+  // data in the output section.
   void
   add_mapping(const Merge_map*, unsigned int shndx, section_offset_type offset,
 	      section_size_type length, section_offset_type output_offset);
@@ -58,7 +59,8 @@ class Object_merge_map
   // input address is at offset OFFSET in section SHNDX.  This sets
   // *OUTPUT_OFFSET to the offset in the output section; this will be
   // -1 if the bytes are not being copied to the output.  This returns
-  // true if the mapping is known, false otherwise.
+  // true if the mapping is known, false otherwise.  *OUTPUT_OFFSET is
+  // relative to the start of the merged data in the output section.
   bool
   get_output_offset(const Merge_map*, unsigned int shndx,
 		    section_offset_type offset,
@@ -86,10 +88,28 @@ class Object_merge_map
     { return i1.input_offset < i2.input_offset; }
   };
 
-  // A list of entries for a particular section.
+  // A list of entries for a particular input section.
   struct Input_merge_map
   {
-    // The Merge_map for this section.
+    // We store these with the Relobj, and we look them up by input
+    // section.  It is possible to have two different merge maps
+    // associated with a single output section.  For example, this
+    // happens routinely with .rodata, when merged string constants
+    // and merged fixed size constants are both put into .rodata.  The
+    // output offset that we store is not the offset from the start of
+    // the output section; it is the offset from the start of the
+    // merged data in the output section.  That means that the caller
+    // is going to add the offset of the merged data within the output
+    // section, which means that the caller needs to know which set of
+    // merged data it found the entry in.  So it's not enough to find
+    // this data based on the input section and the output section; we
+    // also have to find it based on a set of merged data in the
+    // output section.  In order to verify that we are looking at the
+    // right data, we store a pointer to the Merge_map here, and we
+    // pass in a pointer when looking at the data.  If we are asked to
+    // look up information for a different Merge_map, we report that
+    // we don't have it, rather than trying a lookup and returning an
+    // answer which will receive the wrong offset.
     const Merge_map* merge_map;
     // The list of mappings.
     std::vector<Input_merge_entry> entries;
@@ -162,7 +182,7 @@ Object_merge_map::get_or_make_input_merge_map(const Merge_map* merge_map,
   if (map != NULL)
     {
       // For a given input section in a given object, every mapping
-      // must be donw with the same Merge_map.
+      // must be done with the same Merge_map.
       gold_assert(map->merge_map == merge_map);
       return map;
     }
@@ -277,8 +297,8 @@ Object_merge_map::get_output_offset(const Merge_map* merge_map,
 // Class Merge_map.
 
 // Add a mapping for the bytes from OFFSET to OFFSET + LENGTH in input
-// section SHNDX in object OBJECT to an OUTPUT_OFFSET in a merged
+// section SHNDX in object OBJECT to an OUTPUT_OFFSET in merged data
-// output section.
+// in an output section.
 
 void
 Merge_map::add_mapping(Relobj* object, unsigned int shndx,
@@ -297,8 +317,9 @@ Merge_map::add_mapping(Relobj* object, unsigned int shndx,
 
 // Return the output offset for an input address.  The input address
 // is at offset OFFSET in section SHNDX in OBJECT.  This sets
-// *OUTPUT_OFFSET to the offset in the output section.  This returns
+// *OUTPUT_OFFSET to the offset in the merged data in the output
-// true if the mapping is known, false otherwise.
+// section.  This returns true if the mapping is known, false
+// otherwise.
 
 bool
 Merge_map::get_output_offset(const Relobj* object, unsigned int shndx,

gold/merge.h

@@ -44,7 +44,9 @@ class Merge_map
   // Add a mapping for the bytes from OFFSET to OFFSET + LENGTH in the
   // input section SHNDX in object OBJECT to OUTPUT_OFFSET in the
   // output section.  An OUTPUT_OFFSET of -1 means that the bytes are
-  // discarded.
+  // discarded.  OUTPUT_OFFSET is not the offset from the start of the
+  // output section, it is the offset from the start of the merged
+  // data within the output section.
   void
   add_mapping(Relobj* object, unsigned int shndx,
 	      section_offset_type offset, section_size_type length,
@@ -54,7 +56,9 @@ class Merge_map
   // is at offset OFFSET in section SHNDX in OBJECT.  This sets
   // *OUTPUT_OFFSET to the offset in the output section; this will be
   // -1 if the bytes are not being copied to the output.  This returns
-  // true if the mapping is known, false otherwise.
+  // true if the mapping is known, false otherwise.  This returns the
+  // value stored by add_mapping, namely the offset from the start of
+  // the merged data within the output section.
   bool
   get_output_offset(const Relobj* object, unsigned int shndx,
 		    section_offset_type offset,
@@ -84,7 +88,9 @@ class Output_merge_base : public Output_section_data
   { return this->entsize_; }
 
   // Add a mapping from an OFFSET in input section SHNDX in object
-  // OBJECT to an OUTPUT_OFFSET in the output section.
+  // OBJECT to an OUTPUT_OFFSET in the output section.  OUTPUT_OFFSET
+  // is the offset from the start of the merged data in the output
+  // section.
   void
   add_mapping(Relobj* object, unsigned int shndx, section_offset_type offset,
 	      section_size_type length, section_offset_type output_offset)

gold/output.cc

@@ -1347,7 +1347,9 @@ Output_section::Input_section::finalize_data_size()
     this->u2_.posd->finalize_data_size();
 }
 
-// Try to turn an input offset into an output offset.
+// Try to turn an input offset into an output offset.  We want to
+// return the output offset relative to the start of this
+// Input_section in the output section.
 
 inline bool
 Output_section::Input_section::output_offset(
@@ -1362,11 +1364,7 @@ Output_section::Input_section::output_offset(
     {
       if (this->shndx_ != shndx || this->u2_.object != object)
 	return false;
-      section_offset_type output_offset;
+      *poutput = offset;
-      Output_section* os = object->output_section(shndx, &output_offset);
-      gold_assert(os != NULL);
-      gold_assert(output_offset != -1);
-      *poutput = output_offset + offset;
       return true;
     }
 }
@@ -1655,8 +1653,8 @@ Output_section::is_input_address_mapped(const Relobj* object,
 
 // Given an address OFFSET relative to the start of input section
 // SHNDX in object OBJECT, return the output offset relative to the
-// start of the section.  This should only be called if SHNDX in
+// start of the input section in the output section.  This should only
-// OBJECT has a special mapping.
+// be called if SHNDX in OBJECT has a special mapping.
 
 section_offset_type
 Output_section::output_offset(const Relobj* object, unsigned int shndx,

gold/output.h

@@ -1895,8 +1895,10 @@ class Output_section : public Output_data
     // Given an input OBJECT, an input section index SHNDX within that
     // object, and an OFFSET relative to the start of that input
     // section, return whether or not the output offset is known.  If
-    // this function returns true, it sets *POUTPUT to the output
+    // this function returns true, it sets *POUTPUT to the offset in
-    // offset.
+    // the output section, relative to the start of the input section
+    // in the output section.  *POUTPUT may be different from OFFSET
+    // for a merged section.
     bool
     output_offset(const Relobj* object, unsigned int shndx,
 		  section_offset_type offset,