Re: [RFC/PATCH 3/3] btf_encoder: Func generation fix

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On Thu, Nov 12, 2020 at 5:01 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 11/12/20 4:30 PM, Andrii Nakryiko wrote:
> > On Thu, Nov 12, 2020 at 4:19 PM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> >>
> >> On Thu, Nov 12, 2020 at 4:08 PM Andrii Nakryiko
> >> <andrii.nakryiko@xxxxxxxxx> wrote:
> >>>
> >>> So I looked at your vmlinux image. I think we should just keep
> >>> everything mostly as it it right now (without changes in this patch),
> >>> but add just two simple checks:
> >>>
> >>> 1. Skip if fn->declaration (ignore correctly marked func declarations)
> >>> 2. Skip if DW_AT_inline: 1 (ignore inlined functions).
> >>>
> >>> I'd keep the named arguments check as is, I think it's helpful. 1)
> >>> will skip stuff that's explicitly marked as declaration. 2) inline
> >>> check will partially mitigate dropping of fn->external check (and we
> >>> can't really attach to inlined functions).
> >>
> >> I thought DW_AT_inline is an indication that the function was marked "inline"
> >> in C code. That doesn't mean that the function was actually inlined.
> >> So I don't think pahole should check that bit.
> >
> > According to DWARF spec, there are 4 possible values:
> >
> > DW_INL_not_inlined = 0            Not declared inline nor inlined by
> > the compiler
> > DW_INL_inlined = 1                Not declared inline but inlined by
> > the compiler
> > DW_INL_declared_not_inlined = 2   Declared inline but not inlined by
> > the compiler
> > DW_INL_declared_inlined = 3       Declared inline and inlined by the compiler
> >
> > So DW_INL_inlined is supposed to be added to functions that are not
> > marked inline, but were nevertheless inlined. I saw this for one of
> > vfs_getattr entries in DWARF, which clearly is not marked inline.
>
> I looked at llvm source code, llvm only tries to assign DW_INL_inlined
> and also only at certain conditions. Not sure about gcc. Probably
> similar. So this field is not reliable, esp. without it does not mean it
> is not inlined.

Can't say I'm surprised...

>
> >
> > But also that DWARF entry had proper args with names, so it would work
> > fine as well. I don't know, with DWARF it's always some guessing game.
> > Let's leave DW_AT_inline alone for now.
> >
> > Important part is skipping declarations (when they are marked as
> > such), though I'm not claiming it will solve the problem completely...
> > :)
> >



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