[llvm-dev] RFC: DI: Stop preserving types from dead functions

[David Blaikie via llvm-dev]

On Tue, Sep 1, 2015 at 5:56 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2015-Sep-01, at 16:07, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Tue, Sep 1, 2015 at 3:59 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> > Way back in r107027, we started preserving type information of local
> > variables of functions that are optimized away.  This seemed strange to
> > me so I dug into the history: apparently, this is so that ctfconvert can
> > find these types (so they can be exposed in dtrace).
> >
> > E.g., this commit made it so that for this C code:
> >
> >     static void foo(void) { struct X { int b; } v; }
> >
> > we always get the type information for foo.v, even if foo() is optimized
> > away.
> >
> > I came across this when trying to reverse the direction of the IR's
> > DICompileUnit/DISubprogram links.  r107027 effectively forces us to
> > hold onto subprogram definitions that describe deleted functions.
> >
> > Aside: there's a CR currently underway on llvm-dev about whether we
> should (or should not) emit declarations for functions that have been
> optimized away (neglecting local variables/types/etc, we currently emit a
> function declaration for a function even if it gets inlined and optimized
> away - it's unclear if that's the right call (given that we don't emit
> declarations for functions that are never called, I'm not sure there's a
> strong argument to be made to keep these, but I'm undecided)). So if you're
> interested in removing optimized-away subprogrcams, you might want to weigh
> in on that thread.
>
> I can't find the thread :(.  Can you give me the subject line?
>

D12426 (should find you the thread, etc)


>
> I'm guessing it's something along the lines of deleting this block:
> --
>     if (!D && !includeMinimalInlineScopes())
>       // Lazily construct the subprogram if we didn't see either concrete
> or
>       // inlined versions during codegen. (except in -gmlt ^ where we want
>       // to omit these entirely)
>       D = getOrCreateSubprogramDIE(SP);
> --
> in DwarfCompileUnit::finishSubprogramDefinition()?


Yep, more or less.


> That was something
> I was planning on bringing up next...
>
> > For my money I'd apply a similar logic to those types: we don't emit any
> number of types we know during IRGen are unused, so I'm not sure there's a
> good reason to keep them if we discover they're unused a bit later.
>
> Yeah, it seems like the logic should be pretty similar.
>
> >
> > This
> > seems quite weird to me :/.
> >
> > I'm talking to people internally and hoping to find that we just don't
> > care anymore.  Even if we do, perhaps shoving these types into
> > 'retainedTypes:' in `DICompileUnit` (only if -gkeep-all-types or some
> > such) will solve the problem for the ctfconvert use case (without
> > burdening others).
> >
> > While I sort that out... does anyone else rely on this?  How?  Why?
> >
> > (The attach patch effectively reverts r107027.)
>
>
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