llvm dev - Sep 2016 - DebugInfo: purpose of align field

Hello all,

I am currently implementing support for DWARFv5 DW_AT_alignment attr and 
I got a question about align field in debug info section of IR/Bitcode. 
Currently it is being dumped almost in any case, however according to 
code we use align from DI* objects only when dealing with 
class/structure bitfields: DwarfUnit::constructMemberDIE.

Dumping align information everywhere only for 1 case looks like overhead 
to me.

Consider the following code:

struct S {
   char c;
} s;

When compiled with debug enabled in IR we get smth like that:

!6 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", 
file: !5, line: 1, size: 8, align: 8, elements: !7)

Am I missing something? What is the purpose of "align: 8" here? Maybe
we
could include alignment information into DI* objects only when required 
(and thus dump it only when required): for types with bitfields and when 
alignas() was specified in code.

Please advise.

-- 
Best Regards,
Victor

We don't really need to emit alignment information for bitfields anymore,
we emit enough information in the MD to exactly describe the offset of the
storage unit the bitfield is within (DIFlagBitField implies that the
extraData field encodes this information).

On Sat, Sep 10, 2016 at 12:50 PM, Victor Leschuk via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello all,
>
> I am currently implementing support for DWARFv5 DW_AT_alignment attr and I
> got a question about align field in debug info section of IR/Bitcode.
> Currently it is being dumped almost in any case, however according to code
> we use align from DI* objects only when dealing with class/structure
> bitfields: DwarfUnit::constructMemberDIE.
>
> Dumping align information everywhere only for 1 case looks like overhead
> to me.
>
> Consider the following code:
>
> struct S {
>   char c;
> } s;
>
> When compiled with debug enabled in IR we get smth like that:
>
> !6 = distinct !DICompositeType(tag: DW_TAG_structure_type, name:
"S",
> file: !5, line: 1, size: 8, align: 8, elements: !7)
>
> Am I missing something? What is the purpose of "align: 8" here?
Maybe we
> could include alignment information into DI* objects only when required
> (and thus dump it only when required): for types with bitfields and when
> alignas() was specified in code.
>
> Please advise.
>
> --
> Best Regards,
> Victor
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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> On Sep 10, 2016, at 12:50 PM, Victor Leschuk via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Hello all,
> 
> I am currently implementing support for DWARFv5 DW_AT_alignment attr and I
got a question about align field in debug info section of IR/Bitcode.
Thanks for looking into this!
> Currently it is being dumped almost in any case, however according to code
we use align from DI* objects only when dealing with class/structure bitfields:
DwarfUnit::constructMemberDIE.
> 
> Dumping align information everywhere only for 1 case looks like overhead to
me.
One your patch is finished it will be used in more cases, right?
:-)
> 
> Consider the following code:
> 
> struct S {
>  char c;
> } s;
> 
> When compiled with debug enabled in IR we get smth like that:
> 
> !6 = distinct !DICompositeType(tag: DW_TAG_structure_type, name:
"S", file: !5, line: 1, size: 8, align: 8, elements: !7)
> 
> Am I missing something? What is the purpose of "align: 8" here?
Maybe we could include alignment information into DI* objects only when required
(and thus dump it only when required): for types with bitfields and when
alignas() was specified in code.
What do you mean by "include" in this context? Are you arguing for
making an alignment of 8 the implicit default in textual IR? Or do you want to
make DICompositeType variable length?

-- adrian
> 
> Please advise.
> 
> -- 
> Best Regards,
> Victor
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Hello Adrian, sorry for the delay with the response. Somehow I missed 
your message..

On 09/13/2016 12:43 AM, Adrian Prantl wrote:
>> On Sep 10, 2016, at 12:50 PM, Victor Leschuk via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
>>
>> Hello all,
>>
>> I am currently implementing support for DWARFv5 DW_AT_alignment attr
and I got a question about align field in debug info section of IR/Bitcode.
> Thanks for looking into this!
No problem =)
>
>> Currently it is being dumped almost in any case, however according to
code we use align from DI* objects only when dealing with class/structure
bitfields: DwarfUnit::constructMemberDIE.
>>
>> Dumping align information everywhere only for 1 case looks like
overhead to me.
> One your patch is finished it will be used in more cases, right? :-)
My point is that we dump align information everywhere and it is 
currently used only in 1 case. I am trying to make it so that it is 
dumped only when it is necessary.
>> Consider the following code:
>>
>> struct S {
>>   char c;
>> } s;
>>
>> When compiled with debug enabled in IR we get smth like that:
>>
>> !6 = distinct !DICompositeType(tag: DW_TAG_structure_type, name:
"S", file: !5, line: 1, size: 8, align: 8, elements: !7)
>>
>> Am I missing something? What is the purpose of "align: 8"
here? Maybe we could include alignment information into DI* objects only when
required (and thus dump it only when required): for types with bitfields and
when alignas() was specified in code.
> What do you mean by "include" in this context? Are you arguing
for making an alignment of 8 the implicit default in textual IR? Or do you want
to make DICompositeType variable length?
I mean that in this case alignment isn't used anywhere in code further. 
I think we don't need alignment information in textual DI IR in case it 
is not default. Dumping default alignment in every case doesn't make 
sense for me: if we dump align info each time we need to mark
"special"
cases with additional flag in order to be able to know which case to 
"report" in DWARF. If we dump only "forced" alignment, we
can just say:
"Ok, alignment is specified, we need to put this into
DWARF".
>
> -- adrian
>
>> Please advise.
>>
>> -- 
>> Best Regards,
>> Victor
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Best Regards,
Victor