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 <tmsri...@google.com> wrote:
> I have committed this patch.
>
> Thanks,
> -Sri.
>
> On Fri, Dec 14, 2012 at 4:16 PM, Rong Xu <x...@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 <tmsri...@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.
>>
>>
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;
}
