On 08/31/2018 11:19 PM, Jason Merrill wrote:
> On Fri, Aug 24, 2018 at 11:38 AM, Tom de Vries <tdevr...@suse.de> wrote:
>> [ was: Re: [PATCH][debug] Add -gdescriptive-dies ]
>> On Fri, Aug 24, 2018 at 12:44:38PM +0200, Richard Biener wrote:
>>> On Wed, 22 Aug 2018, Tom de Vries wrote:
>>>
>>>> [ was: Re: [PATCH][debug] Add -gforce-named-dies ]
>>>>
>>>> On 08/22/2018 11:46 AM, Tom de Vries wrote:
>>>>> On 08/22/2018 08:56 AM, Tom de Vries wrote:
>>>>>> This is an undocumented developer-only option, because using this option
>>>>>> may
>>>>>> change behaviour of dwarf consumers, f.i., gdb shows the artificial
>>>>>> variables:
>>>>>> ...
>>>>>> (gdb) info locals
>>>>>> a = 0x7fffffffda90 "\005"
>>>>>> D.4278 = <optimized out>
>>>>>> ...
>>>>> I just found in the dwarf 5 spec the attribute DW_AT_description
>>>>> (present since version 3):
>>>>> ...
>>>>> 2.20 Entity Descriptions
>>>>> Some debugging information entries may describe entities in the program
>>>>> that are artificial, or which otherwise have a “name” that is not a
>>>>> valid identifier in the programming language.
>>>>>
>>>>> This attribute provides a means for the producer to indicate the purpose
>>>>> or usage of the containing debugging infor
>>>>>
>>>>> Generally, any debugging information entry that has, or may have, a
>>>>> DW_AT_name attribute, may also have a DW_AT_description attribute whose
>>>>> value is a null-terminated string providing a description of the entity.
>>>>>
>>>>> It is expected that a debugger will display these descriptions as part
>>>>> of displaying other properties of an entity.
>>>>> ...
>>>>>
>>>>> AFAICT, gdb currently does not explicitly handle this attribute, which I
>>>>> think means it's ignored.
>>>>>
>>>>> It seems applicable to use DW_AT_description at least for the artificial
>>>>> decls.
>>>>>
>>>>> Perhaps even for all cases that are added by the patch?
>>>>>
>>>>
>>>> I've chosen for this option. Using DW_AT_desciption instead of
>>>> DW_AT_name should minimize difference in gdb behaviour with and without
>>>> -gdescriptive-dies.
>>>
>>> -gdescribe-dies?
>>>
>>
>> Done.
>>
>>>>> I'll rewrite the patch.
>>>>
>>>> OK for trunk?
>>>
>>> Few comments:
>>>
>>> +static void
>>> +add_desc_attribute (dw_die_ref die, tree decl)
>>> +{
>>> + tree decl_name;
>>> +
>>> + if (!flag_descriptive_dies || dwarf_version < 3)
>>> + return;
>>> +
>>>
>>> you said this is a DWARF5 "feature",
>>
>> No, it's a DWARF3 "feature". I copied the text from the DWARF5 spec.
>>
>>> I'd suggest changing the
>>> check to
>>>
>>> if (!flag_desctiprive_dies || (dwarf_version < 5 && dwarf_strict))
>>>
>>> given -gdescribe-dies is enough of a user request to enable the
>>> feature.
>>
>> Done.
>>
>>> Not sure if we should warn about -gstrict-dwarf
>>> combination with it and -gdwarf-N < 5.
>>>
>>
>> Not sure either, left it out.
>>
>>> + else if (TREE_CODE (decl) == VAR_DECL || TREE_CODE (decl) ==
>>> CONST_DECL)
>>> + {
>>> + char buf[32];
>>> + char decl_letter = TREE_CODE (decl) == CONST_DECL ? 'C' : 'D';
>>> + sprintf (buf, "%c.%u", decl_letter, DECL_UID (decl));
>>> + add_desc_attribute (die, buf);
>>> + }
>>>
>>> I wondered before if we can use pretty-printing of decl here. At
>>> least I wouldn't restrict it to VAR_DECLs, FUNCTION_DECLs can
>>> certainly appear here I think.
>>>
>>
>> Done.
>>
>>> +@item -gdescriptive-dies
>>> +@opindex gdescriptive-dies
>>> +Add description attribute to DWARF DIEs that have no name attribute.
>>> +
>>>
>>> Either "description attributes" or "a description attribute", not
>>> 100% sure being a non-native speaker.
>>>
>>
>> Went for "description attributes".
>>
>>> Otherwise the patch looks OK to me but please leave Jason time
>>> to comment.
>>>
>>
>> Will do.
>>
>> Untested patch attached.
>>
Now bootstrapped and reg-tested on x86_64.
>> [debug] Add -gdescribe-dies
>>
>> This patch adds option -gdescribe-dies. It sets the DW_AT_description
>> attribute of dies that do not get a DW_AT_name attribute, to make it easier
>> to
>> figure out what the die is describing.
>>
>> The option exports the names of artificial variables:
>> ...
>> DIE 0: DW_TAG_variable (0x7fa934dd54b0)
>> + DW_AT_description: "D.1922"
>> DW_AT_type: die -> 0 (0x7fa934dd0d70)
>> DW_AT_artificial: 1
>>
>> ...
>> which can be traced back to gimple dumps:
>> ...
>> char a[0:D.1922] [value-expr: *a.0];
>> ...
>>
>> Furthermore, it adds names to external references:
>> ...
>> DIE 0: DW_TAG_subprogram (0x7fa88b9650f0)
>> +DW_AT_description: "main"
>> DW_AT_abstract_origin: die -> label: vla_1.c.6719312a + 29 (0x7fa88b965140)
>
> Please add more of this description to the one-line documentation
> patch you have now;
Done.
> there are many DIEs that have no name because they
> don't need one, and this patch doesn't add names to all of them.
>
I've added the qualification "some" (to both documentation and command
line option description).
> These two cases seem like they have very different uses, but I suppose
> we don't really need two new options.
In both cases we add a description of the DIE. Could you explain what
the difference in usage you refer to is?
Attached patch (which I will merge with the posted patch pre-commit)
fixes up documentation and command line option description.
OK?
Thanks,
- Tom
fixup
---
gcc/common.opt | 2 +-
gcc/doc/invoke.texi | 4 +++-
2 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/gcc/common.opt b/gcc/common.opt
index f41f8f74b30..ef6a63087af 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2978,7 +2978,7 @@ Don't emit DWARF additions beyond selected version.
gdescribe-dies
Common Driver Report Var(flag_describe_dies) Init(0)
-Add description attributes to DWARF DIEs that have no name attribute.
+Add description attributes to some DWARF DIEs that have no name attribute.
gtoggle
Common Driver Report Var(flag_gtoggle)
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index b06189f73ce..d7cf30ad2d1 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -7468,7 +7468,9 @@ files.
@item -gdescribe-dies
@opindex gdescribe-dies
-Add description attributes to DWARF DIEs that have no name attribute.
+Add description attributes to some DWARF DIEs that have no name attribute,
+such as artificial variables, external references and call site
+parameter DIEs.
@item -gpubnames
@opindex gpubnames
