[PATCH] Make __FUNCTION__ a mergeable string and do not generate symbol entry.

[Martin Liška]

PING^2

On 07/27/2017 02:56 PM, Martin Liška wrote:
> PING^1
> 
> On 07/14/2017 10:35 AM, Martin Liška wrote:
>> On 05/01/2017 09:13 PM, Jason Merrill wrote:
>>> On Wed, Apr 26, 2017 at 6:58 AM, Martin Liška <mliska@suse.cz> wrote:
>>>> On 04/25/2017 01:58 PM, Jakub Jelinek wrote:
>>>>> On Tue, Apr 25, 2017 at 01:48:05PM +0200, Martin Liška wrote:
>>>>>> Hello.
>>>>>>
>>>>>> This is patch that was originally installed by Jason and later reverted due to PR70422.
>>>>>> In the later PR Richi suggested a fix for that and Segher verified that it helped him
>>>>>> to survive regression tests. That's reason why I'm resending that.
>>>>>>
>>>>>> Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.
>>>>>>
>>>>>> Ready to be installed?
>>>>>> Martin
>>>>>
>>>>>> >From a34ce0ef37ae00609c9f3ff98a9cb0b7db6a8bd0 Mon Sep 17 00:00:00 2001
>>>>>> From: marxin <mliska@suse.cz>
>>>>>> Date: Thu, 20 Apr 2017 14:56:30 +0200
>>>>>> Subject: [PATCH] Make __FUNCTION__ a mergeable string and do not generate
>>>>>>  symbol entry.
>>>>>>
>>>>>> gcc/cp/ChangeLog:
>>>>>>
>>>>>> 2017-04-20  Jason Merrill  <jason@redhat.com>
>>>>>>          Martin Liska  <mliska@suse.cz>
>>>>>>          Segher Boessenkool  <segher@kernel.crashing.org>
>>>>>>
>>>>>>      PR c++/64266
>>>>>>      PR c++/70353
>>>>>>      PR bootstrap/70422
>>>>>>      Core issue 1962
>>>>>>      * decl.c (cp_fname_init): Decay the initializer to pointer.
>>>>>>      (cp_make_fname_decl): Set DECL_DECLARED_CONSTEXPR_P,
>>>>>>      * pt.c (tsubst_expr) [DECL_EXPR]: Set DECL_VALUE_EXPR,
>>>>>>      DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P and
>>>>>>      DECL_IGNORED_P.  Don't call cp_finish_decl.
>>>>>
>>>>> If we don't emit those into the debug info, will the debugger be
>>>>> able to handle __FUNCTION__ etc. properly?
>>>>
>>>> No, debugger with the patch can't handled these. Similar to how clang
>>>> behaves currently. Maybe it can be conditionally enabled with -g3, or -g?
>>>>
>>>>> Admittedly, right now we emit it into debug info only if those decls
>>>>> are actually used, say on:
>>>>> const char * foo () { return __FUNCTION__; }
>>>>> const char * bar () { return ""; }
>>>>> we'd emit foo::__FUNCTION__, but not bar::__FUNCTION__, so the debugger
>>>>> has to have some handling of it anyway.  But while in functions
>>>>> that don't refer to __FUNCTION__ it is always the debugger that needs
>>>>> to synthetize those and thus they will be always pointer-equal,
>>>>> if there are some uses of it and for other uses the debugger would
>>>>> synthetize it, there is the possibility that the debugger synthetized
>>>>> string will not be the same object as actually used in the function.
>>>>
>>>> You're right, currently one has to use a special function to be able to
>>>> print it in debugger. I believe we've already discussed that, according
>>>> to spec, the strings don't have to point to a same string.
>>>>
>>>> Suggestions what we should do with the patch?
>>>
>>> We need to emit debug information for these variables.  From Jim's
>>> description in 70422 it seems that the problem is that the reference
>>> to the string from the debug information is breaking
>>> function_mergeable_rodata_prefix, which relies on
>>> current_function_decl.  It seems to me that its callers should pass
>>> along their decl parameter so that f_m_r_p can use the decl's
>>> DECL_CONTEXT rather than rely on current_function_decl being set
>>> properly> 
>>> Jason
>>>
>>
>> Ok, after some time I returned back to it. I followed your advises and
>> changed the function function_mergeable_rodata_prefix. Apart from a small
>> rebase was needed.
>>
>> May I ask Jim to test the patch?
>> Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.
>>
>> Martin
>>
>