Re: [google][4.7] Allow function reordering linker plugin to separate hot and cold code into different ELF segments

[Sriraman Tallam <tmsriram@google.com>]

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Hi Rong,

  The following patch modifies the behaviour of the linker plugin to
not create a separate segment for cold sections by default. Separate
segments can be created with the plugin option "segment=cold". Is this
alright to commit?

Thanks,
-Sri.

On Mon, Dec 17, 2012 at 11:14 AM, Sriraman Tallam <tmsriram@google.com> wrote:
> I have committed this patch.
>
> Thanks,
> -Sri.
>
> On Fri, Dec 14, 2012 at 4:16 PM, Rong Xu <xur@google.com> wrote:
>> Looks good to me for google/gcc-4_7 branch.
>>
>> Thanks,
>>
>> -Rong
>>
>>
>> On Fri, Dec 14, 2012 at 3:42 PM, Sriraman Tallam <tmsriram@google.com>
>> wrote:
>>>
>>> Hi Rong,
>>>
>>>     Please review this code. This code allows the function reordering
>>> plugin to separate hot and cold code into different ELF segments.
>>> This would allow optimizations like mapping the hot code alone to huge
>>> pages.
>>>
>>>     With this patch, by default, the plugin maps .text.unlikely
>>> sections into a separate ELF segment.  This can be turned off with
>>> plugin option "--segment=none".
>>>
>>>     The include/plugin-api.h changes are a backport from trunk.
>>>
>>> Thanks,
>>> -Sri.
>>
>>

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Index: function_reordering_plugin/function_reordering_plugin.c
===================================================================
--- function_reordering_plugin/function_reordering_plugin.c	(revision 194878)
+++ function_reordering_plugin/function_reordering_plugin.c	(working copy)
@@ -34,8 +34,12 @@ along with this program; see the file COPYING3.  I
    This plugin dumps the final layout order of the functions in a file
    called "final_layout.txt".  To change the output file, pass the new
    file name with --plugin-opt.  To dump to stderr instead, just pass
-   stderr to --plugin-opt.  */
+   stderr to --plugin-opt.  
 
+   This plugin also allows placing all functions found cold in a separate
+   segment.  This can be enabled with the linker option:
+   --plugin-opt,segment=cold.  */
+
 #if HAVE_STDINT_H
 #include <stdint.h>
 #endif
@@ -89,9 +93,10 @@ static int is_api_exist = 0;
 /* The plugin does nothing when no-op is 1.  */
 static int no_op = 0;
 
-/* The plugin does not create a new segment for unlikely code if
-   no_segment is set.  */
-static int no_segment = 0;
+/* The plugin creates a new segment for unlikely code if unlikely_segment
+   is set.  This can be set with the linker option:
+   "--plugin-opt,segment=cold".  */
+static int unlikely_segment = 0;
 
 /* Copies new output file name out_file  */
 void get_filename (const char *name)
@@ -132,10 +137,11 @@ process_option (const char *name)
   /* Check if options is "segment=none"  */
   if (strncmp (name, option_segment, strlen (option_segment)) == 0)
     {
-      if (strcmp (name + strlen (option_segment), "none") == 0)
-	no_segment = 1;
-      else
-	no_segment = 0;
+      const char *option_val = name + strlen (option_segment);
+      if (strcmp (option_val, "none") == 0)
+	unlikely_segment = 0;
+      else if (strcmp (option_val, "cold") == 0)
+	unlikely_segment = 1;
       return;
     }
 
@@ -244,7 +250,10 @@ claim_file_hook (const struct ld_plugin_input_file
     {
       /* Inform the linker to prepare for section reordering.  */
       (*allow_section_ordering) ();
-      (*allow_unique_segment_for_sections) ();
+      /* Inform the linker to allow certain sections to be placed in
+	 a separate segment.  */
+      if (unlikely_segment == 1)
+        (*allow_unique_segment_for_sections) ();
       is_ordering_specified = 1;
     }
 
@@ -335,15 +344,29 @@ all_symbols_read_hook (void)
   if (out_file != NULL
       && strcmp (out_file, "stderr") != 0)
     fclose (fp);
-  /* Pass the new order of functions to the linker.  */
-  update_section_order (section_list, unlikely_segment_start);
-  assert (num_entries >= unlikely_segment_end);
-  update_section_order (section_list, num_entries - unlikely_segment_end);
-  /* Map all unlikely code into a new segment.  */
-  if (no_segment == 0)
-    unique_segment_for_sections (".text.unlikely_executed", 0, 0x1000,
-				 section_list + unlikely_segment_start,
-				 unlikely_segment_end - unlikely_segment_start);
+
+  if (unlikely_segment == 1)
+    {
+      /* Pass the new order of functions to the linker.  */
+      /* Fix the order of all sections upto the beginning of the
+	 unlikely section.  */
+      update_section_order (section_list, unlikely_segment_start);
+      assert (num_entries >= unlikely_segment_end);
+      /* Fix the order of all sections after the end of the unlikely
+	 section.  */
+      update_section_order (section_list, num_entries - unlikely_segment_end);
+      /* Map all unlikely code into a new segment.  */
+      unique_segment_for_sections (
+	  ".text.unlikely_executed", 0, 0x1000,
+	  section_list + unlikely_segment_start,
+	  unlikely_segment_end - unlikely_segment_start);
+    }
+  else
+    {
+      /* Pass the new order of functions to the linker.  */
+      update_section_order (section_list, num_entries);
+    }
+
   cleanup ();
   return LDPS_OK;
 }