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

[Martin Liška]

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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-Make-__FUNCTION__-a-mergeable-string-and-do-not-gene.patch
Type: text/x-patch
Size: 9349 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20170714/d146c228/attachment.bin>