[llvm-dev] DebugInfo: purpose of align field

[David Majnemer via llvm-dev]

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
>
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