[LLVMdev] bit code file incompatibility due to debug info changes

[Eric Christopher]

On Mon, Nov 18, 2013 at 10:37 AM, Chandler Carruth <chandlerc at google.com> wrote:
>
> On Mon, Nov 18, 2013 at 10:35 AM, Eric Christopher <echristo at gmail.com>
> wrote:
>>
>> >> At a minimum, it seems like we need a version number in the debug info
>> >> metadata so we can detect this situation and avoid crashing.
>> >
>> >
>> > Or to put it in the terms of the IR: we need to autoupgrade the debug
>> > info
>> > metadata just like we do intrinsics. With debug info this might (at the
>> > worst) involve dropping old metadata.
>> >
>>
>> The verifier is probably the best route here. I think if we fail debug
>> info verification we can just strip it and continue.
>
>
> I'm slightly worried about relying on verification based "autoupgrade"
> (where by upgrade I mean discard old cruft since it doesn't impact
> correctness) due to compile time costs, but if the verification is fast
> enough to be "always on", then cool.

Agreed, it's not really that fast either. The problem with versions is
that it makes small changes that only require a bit of modification to
the testcase to be pretty heinous upgrade paths and a lot of work -
you can see that from the work any of us have had to do when changing
the format. We'd like to keep the testcases and upgrade those as well
because otherwise we're just dropping testcases like mad constantly.
It's a lot of work, but the format has been changing frequently enough
that we'd be on version 1000 or something by now and it wouldn't
really accomplish much. The verifier seems to be the best route for
that. It isn't too fast, but then reading in bitcode files from disk
isn't particularly speedy either.

-eric