Gold linker patch to split unlikely text into a separate segment
Sriraman Tallam via binutils
binutils@sourceware.org
Thu Oct 5 18:41:00 GMT 2017
On Wed, Oct 4, 2017 at 5:12 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On 10/4/17, Sriraman Tallam <tmsriram@google.com> wrote:
>> Hi,
>>
>> This patch adds an option to gold to create a new ELF segment for
>> code determined unlikely by the compiler. Currently, this can be done
>> with a linker plugin but I was wondering if it is fine to have this
>> support in gold with an option for ease of use.
>>
>> The advantages of doing this are:
>>
>> * We could only map the hot segment to huge pages to keep iTLB misses low
>> * We could munlock the unlikely segment
>>
>> Is this alright?
>>
>> ChangeLog entry:
>>
>> * options.h (text_unlikely_segment): New option.
>> * layout.cc (Layout::layout): Create new output section
>> for .text.unlikely sections with the new option.
>> (Layout::segment_precedes): Check for the new option
>> when segment flags match.
>>
>> Patch attached.
>>
>
> This is an interesting approach. Do you have some performace
> numbers?
With function re-ordering of hot code, segment splitting and mapping
only hot code to huge pages, we see a reduction in iTLB misses
translating to performance improvements of 0.5 to 1% on some of our
benchmarks.
>
> 2 Comments:
>
> 1. I'd prefer "-z text-unlikely-segment" with '-', instead of '_'.
> 2. Need a testcase.
Done and patch attached.
Thanks
Sri
>
> --
> H.J.
-------------- next part --------------
diff --git a/gold/ChangeLog b/gold/ChangeLog
index 697a40ade1..6757961e92 100644
--- a/gold/ChangeLog
+++ b/gold/ChangeLog
@@ -1,3 +1,14 @@
+2017-10-04 Sriraman Tallam <tmsriram@google.com>
+
+ * options.h (-z,text_unlikely_segment): New option.
+ * layout.cc (Layout::layout): Create new output section
+ for .text.unlikely sections with the new option.
+ (Layout::segment_precedes): Check for the new option
+ when segment flags match.
+ * testsuite/text_unlikely_segment.cc: New test source.
+ * testsuite/text_unlikely_segment.sh: New test script.
+ * testsuite/Makefile.am (text_unlikely_segment): New test.
+
2017-09-26 Cary Coutant <ccoutant@gmail.com>
PR gold/22213
diff --git a/gold/Makefile.in b/gold/Makefile.in
index a3c8b03bec..f5dbd2e025 100644
--- a/gold/Makefile.in
+++ b/gold/Makefile.in
@@ -87,8 +87,8 @@ subdir = .
DIST_COMMON = NEWS README ChangeLog $(srcdir)/Makefile.in \
$(srcdir)/Makefile.am $(top_srcdir)/configure \
$(am__configure_deps) $(srcdir)/config.in \
- $(srcdir)/../mkinstalldirs $(top_srcdir)/po/Make-in ffsll.c \
- ftruncate.c mremap.c pread.c yyscript.h yyscript.c \
+ $(srcdir)/../mkinstalldirs $(top_srcdir)/po/Make-in mremap.c \
+ ftruncate.c ffsll.c pread.c yyscript.h yyscript.c \
$(srcdir)/../depcomp $(srcdir)/../ylwrap
ACLOCAL_M4 = $(top_srcdir)/aclocal.m4
am__aclocal_m4_deps = $(top_srcdir)/../config/depstand.m4 \
diff --git a/gold/layout.cc b/gold/layout.cc
index 5f25faea55..5d0a885c52 100644
--- a/gold/layout.cc
+++ b/gold/layout.cc
@@ -1180,11 +1180,17 @@ Layout::layout(Sized_relobj_file<size, big_endian>* object, unsigned int shndx,
{
// Plugins can choose to place one or more subsets of sections in
// unique segments and this is done by mapping these section subsets
- // to unique output sections. Check if this section needs to be
- // remapped to a unique output section.
+ // to unique output sections. All ".text.unlikely.*" sections can be
+ // moved to a unique segment with --text-unlikely-segment option.
+ // Check if this section needs to be remapped to a unique output section.
Section_segment_map::iterator it
= this->section_segment_map_.find(Const_section_id(object, shndx));
- if (it == this->section_segment_map_.end())
+ bool text_unlikely_segment
+ = (parameters->options().text_unlikely_segment()
+ && is_prefix_of(".text.unlikely",
+ object->section_name(shndx).c_str()));
+ if (it == this->section_segment_map_.end()
+ && !text_unlikely_segment)
{
os = this->choose_output_section(object, name, sh_type,
shdr.get_sh_flags(), true,
@@ -1198,7 +1204,8 @@ Layout::layout(Sized_relobj_file<size, big_endian>* object, unsigned int shndx,
elfcpp::Elf_Xword flags
= this->get_output_section_flags(shdr.get_sh_flags());
- const char* os_name = it->second->name;
+ const char* os_name = text_unlikely_segment ?
+ ".text.unlikely" : it->second->name;
Stringpool::Key name_key;
os_name = this->namepool_.add(os_name, true, &name_key);
os = this->get_output_section(os_name, name_key, sh_type, flags,
@@ -1206,8 +1213,11 @@ Layout::layout(Sized_relobj_file<size, big_endian>* object, unsigned int shndx,
if (!os->is_unique_segment())
{
os->set_is_unique_segment();
- os->set_extra_segment_flags(it->second->flags);
- os->set_segment_alignment(it->second->align);
+ if (!text_unlikely_segment)
+ {
+ os->set_extra_segment_flags(it->second->flags);
+ os->set_segment_alignment(it->second->align);
+ }
}
}
if (os == NULL)
@@ -3449,7 +3459,8 @@ Layout::segment_precedes(const Output_segment* seg1,
// here if plugins want unique segments for subsets of sections.
gold_assert(this->script_options_->saw_phdrs_clause()
|| parameters->options().any_section_start()
- || this->is_unique_segment_for_sections_specified());
+ || this->is_unique_segment_for_sections_specified()
+ || parameters->options().text_unlikely_segment());
return false;
}
diff --git a/gold/options.h b/gold/options.h
index 93f81b29af..8828c81016 100644
--- a/gold/options.h
+++ b/gold/options.h
@@ -1492,6 +1492,11 @@ class General_options
DEFINE_bool_alias(textoff, text, options::DASH_Z, '\0',
N_("Permit relocations in read-only segments"),
NULL, true);
+ DEFINE_bool(text_unlikely_segment, options::DASH_Z, '\0', false,
+ N_("Move .text.unlikely sections to a separate ELF segment."),
+ N_("Do not move .text.unlikely sections to a separate "
+ "ELF segment."));
+
public:
typedef options::Dir_list Dir_list;
diff --git a/gold/testsuite/Makefile.am b/gold/testsuite/Makefile.am
index cae47f1e09..946f88b0db 100644
--- a/gold/testsuite/Makefile.am
+++ b/gold/testsuite/Makefile.am
@@ -340,6 +340,16 @@ section_sorting_name: section_sorting_name.o gcctestdir/ld
section_sorting_name.stdout: section_sorting_name
$(TEST_NM) -n --synthetic section_sorting_name > section_sorting_name.stdout
+check_SCRIPTS += text_unlikely_segment.sh
+check_DATA += text_unlikely_segment_readelf.stdout
+MOSTLYCLEANFILES += text_unlikely_segment
+text_unlikely_segment.o: text_unlikely_segment.cc
+ $(CXXCOMPILE) -O0 -c -ffunction-sections -g -o $@ $<
+text_unlikely_segment: text_unlikely_segment.o gcctestdir/ld
+ $(CXXLINK) -Bgcctestdir/ -Wl,-z,text-unlikely-segment text_unlikely_segment.o
+text_unlikely_segment_readelf.stdout: text_unlikely_segment
+ $(TEST_READELF) -Wl $< >$@
+
check_PROGRAMS += icf_virtual_function_folding_test
MOSTLYCLEANFILES += icf_virtual_function_folding_test icf_virtual_function_folding_test.map
icf_virtual_function_folding_test.o: icf_virtual_function_folding_test.cc
diff --git a/gold/testsuite/text_unlikely_segment.cc b/gold/testsuite/text_unlikely_segment.cc
index e69de29bb2..f8e6dd7e66 100644
--- a/gold/testsuite/text_unlikely_segment.cc
+++ b/gold/testsuite/text_unlikely_segment.cc
@@ -0,0 +1,30 @@
+/* text_unlikey_segment.cc -- a test case for gold
+
+ Copyright (C) 2017 onwards Free Software Foundation, Inc.
+ Written by Sriraman Tallam <tmsriram@google.com>
+
+ This file is part of gold.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program; if not, write to the Free Software
+ Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
+ MA 02110-1301, USA. */
+
+__attribute__((section(".text.unlikely")))
+int foo(void) {
+ return 0;
+}
+
+int main(void) {
+ return 0;
+}
diff --git a/gold/testsuite/text_unlikely_segment.sh b/gold/testsuite/text_unlikely_segment.sh
index e69de29bb2..b01fe6c715 100755
--- a/gold/testsuite/text_unlikely_segment.sh
+++ b/gold/testsuite/text_unlikely_segment.sh
@@ -0,0 +1,60 @@
+#!/bin/sh
+
+# plugin_final_layout.sh -- test
+
+# Copyright (C) 2011-2017 Free Software Foundation, Inc.
+# Written by Sriraman Tallam <tmsriram@google.com>.
+
+# This file is part of gold.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston,
+# MA 02110-1301, USA.
+
+# The goal of this program is to verify if -z,text-unlikely-segment works as
+# intended. File text_unlikely_segment.cc is part of this test.
+
+
+set -e
+
+# With readelf -l, an ELF Section to Segment mapping is printed as :
+##############################################
+# Section to Segment mapping:
+# Segment Sections...
+# ...
+# 0x .text.unlikely
+# ...
+##############################################
+# Check if .text.unlikely is the only section in the segment.
+check_unique_segment()
+{
+ awk "
+BEGIN { saw_section = 0; saw_unique = 0; }
+/$2/ { saw_section = 1; }
+/[ ]*0[0-9][ ]*$2[ ]*\$/ { saw_unique = 1; }
+END {
+ if (!saw_section)
+ {
+ printf \"Section $2 not seen in output\\n\";
+ exit 1;
+ }
+ else if (!saw_unique)
+ {
+ printf \"Unique segment not seen for: $2\\n\";
+ exit 1;
+ }
+ }" $1
+}
+
+check_unique_segment text_unlikely_segment_readelf.stdout ".text.unlikely"
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