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

[David Blaikie via llvm-dev]

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.

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.


> 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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