On Wed, Aug 12, 2015 at 10:36 PM, Sriraman Tallam <tmsri...@google.com> wrote:
> On Tue, Aug 4, 2015 at 11:43 AM, Sriraman Tallam <tmsri...@google.com> wrote:
>> On Tue, Jun 16, 2015 at 4:22 PM, Sriraman Tallam <tmsri...@google.com> wrote:
>>> On Tue, May 19, 2015 at 9:11 AM, Xinliang David Li <davi...@google.com>
>>> wrote:
>>>>>
>>>>> Hm. But which options are unsafe? Also wouldn't it be better to simply
>>>>> _not_ have unsafe options produce comdats but always make local clones
>>>>> for them (thus emit the comdat with "unsafe" flags dropped)?
>>>>
>>>> Always localize comdat functions may lead to text size increase. It
>>>> does not work if the comdat function is a virtual function for
>>>> instance.
>>>
>>> Based on Richard's suggestion, I have a patch to localize comdat
>>> functions which seems like a very effective solution to this problem.
>>> The text size increase is limited to the extra comdat copies generated
>>> for the specialized modules (modules with unsafe options) which is
>>> usually only a few. Since -fweak does something similar for
>>> functions, I have called the new option -fweak-comdat-functions.
>>> This does not apply to virtual comdat functions as their addresses can
>>> always be leaked via the vtable. Using this flag with virtual
>>> functions generates a warning.
>>>
>>> To summarize, this is the intended usage of this option. Modules which
>>> use unsafe code options, like -m<isa> for multiversioning, to generate
>>> code that is meant to run only on a subset of CPUs can generate
>>> comdats with specialized instructions which when picked by the linker
>>> can get run unconditionally causing SIGILL on unsupported platforms.
>>> This flag hides these comdats to be local to these modules and not
>>> make them available publicly, with the caveat that it does not apply
>>> to virtual comdats.
>>>
>>> Could you please review?
>>
>> Ping. This patch uses Richard's suggestion to localize comdat
>> functions with option -fno-weak-comdat-functions. Comments?
Ping.
* c-family/c.opt (fweak-comdat-functions): New option.
* cp/decl2.c (comdat_linkage): Implement new option. Warn when
virtual comdat functions are seen.
* doc/invoke.texi: Document new option.
* testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.
>
>
> Ping.
>
> * c-family/c.opt (fweak-comdat-functions): New option.
> * cp/decl2.c (comdat_linkage): Implement new option. Warn when
> virtual comdat functions are seen.
> * doc/invoke.texi: Document new option.
> * testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.
>
>>
>> * c-family/c.opt (fweak-comdat-functions): New option.
>> * cp/decl2.c (comdat_linkage): Implement new option. Warn when
>> virtual comdat functions are seen.
>> * doc/invoke.texi: Document new option.
>> * testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.
>>
>>
>>>
>>> * c-family/c.opt (fweak-comdat-functions): New option.
>>> * cp/decl2.c (comdat_linkage): Implement new option. Warn when
>>> virtual comdat functions are seen.
>>> * doc/invoke.texi: Document new option.
>>> * testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.
>>>
>>>
>>> Thanks
>>> Sri
>>>
>>>
>>>>
>>>> David
>>>>
>>>>
>>>>>
>>>>> Richard.
>>>>>
>>>>>>
>>>>>> Thanks
>>>>>> Sri
* c-family/c.opt (fweak-comdat-functions): New option.
* cp/decl2.c (comdat_linkage): Implement new option. Warn when
virtual comdat functions are seen.
* doc/invoke.texi: Document new option.
* testsuite/g++.dg/no-weak-comdat-functions-1.C: New test.
Index: c-family/c.opt
===================================================================
--- c-family/c.opt (revision 224486)
+++ c-family/c.opt (working copy)
@@ -1236,6 +1236,14 @@ fweak
C++ ObjC++ Var(flag_weak) Init(1)
Emit common-like symbols as weak symbols
+fweak-comdat-functions
+C++ Var(flag_weak_comdat_functions) Init(1)
+Specific to comdat functions(-fno-weak-comdat-functions : Localize Comdat
Functions).
+With -fno-weak-comdat-functions, virtual comdat functions are still linked as
+weak functions. With -fno-weak-comdat-functions, the address of the comdat
+functions that are localized will be unique and this can cause unintended
+behavior when addresses of comdat functions are used.
+
fwide-exec-charset=
C ObjC C++ ObjC++ Joined RejectNegative
-fwide-exec-charset=<cset> Convert all wide strings and character
constants to character set <cset>
Index: cp/decl2.c
===================================================================
--- cp/decl2.c (revision 224486)
+++ cp/decl2.c (working copy)
@@ -1702,8 +1702,19 @@ mark_vtable_entries (tree decl)
void
comdat_linkage (tree decl)
{
- if (flag_weak)
- make_decl_one_only (decl, cxx_comdat_group (decl));
+ if (flag_weak
+ && (flag_weak_comdat_functions
+ || TREE_CODE (decl) != FUNCTION_DECL
+ || DECL_VIRTUAL_P (decl)))
+ {
+ make_decl_one_only (decl, cxx_comdat_group (decl));
+ if (TREE_CODE (decl) == FUNCTION_DECL
+ && DECL_VIRTUAL_P (decl)
+ && !flag_weak_comdat_functions)
+ warning_at (DECL_SOURCE_LOCATION (decl), 0,
+ "fno-weak-comdat-functions: Comdat linkage of virtual "
+ "function %q#D preserved.");
+ }
else if (TREE_CODE (decl) == FUNCTION_DECL
|| (VAR_P (decl) && DECL_ARTIFICIAL (decl)))
/* We can just emit function and compiler-generated variables
Index: doc/invoke.texi
===================================================================
--- doc/invoke.texi (revision 224486)
+++ doc/invoke.texi (working copy)
@@ -189,7 +189,7 @@ in the following sections.
-fno-pretty-templates @gol
-frepo -fno-rtti -fstats -ftemplate-backtrace-limit=@var{n} @gol
-ftemplate-depth=@var{n} @gol
--fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ @gol
+-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -fno-weak-comdat-functions
-nostdinc++ @gol
-fvisibility-inlines-hidden @gol
-fvtable-verify=@var{std|preinit|none} @gol
-fvtv-counts -fvtv-debug @gol
@@ -2445,6 +2445,16 @@ option exists only for testing, and should not be
it results in inferior code and has no benefits. This option may
be removed in a future release of G++.
+@item -fno-weak-comdat-functions
+@opindex fno-weak-comdat-functions
+Do not use weak symbol support for comdat non-virtual functions, even if it
+is provided by the linker. By default, G++ uses weak symbols if they are
+available. This option is useful when comdat functions generated in certain
+compilation units need to be kept local to the respective units and not exposed
+globally. This does not apply to virtual comdat functions as their pointers
+may be taken via virtual tables. This can cause unintended behavior if
+the addresses of comdat functions are used.
+
@item -nostdinc++
@opindex nostdinc++
Do not search for header files in the standard directories specific to
Index: testsuite/g++.dg/no-weak-comdat-functions-1.C
===================================================================
--- testsuite/g++.dg/no-weak-comdat-functions-1.C (revision 0)
+++ testsuite/g++.dg/no-weak-comdat-functions-1.C (working copy)
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-fno-weak-comdat-functions -fkeep-inline-functions
-ffunction-sections" } */
+
+inline int foo () {
+ return 0;
+}
+
+/* { dg-final { scan-assembler "_Z3foov" } } */
+/* { dg-final { scan-assembler-not "_Z3foov.*comdat" } } */
