[llvm] r221167 - IR: MDNode => Value: Instruction::getAllMetadataOtherThanDebugLoc()

[David Blaikie]

On Mon, Nov 3, 2014 at 11:06 AM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

>
> > On 2014-Nov-03, at 10:40, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Mon, Nov 3, 2014 at 10:13 AM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> > Author: dexonsmith
> > Date: Mon Nov  3 12:13:57 2014
> > New Revision: 221167
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=221167&view=rev
> > Log:
> > IR: MDNode => Value: Instruction::getAllMetadataOtherThanDebugLoc()
> >
> > Change `Instruction::getAllMetadataOtherThanDebugLoc()` from a vector of
> > `MDNode` to one of `Value`.  Part of PR21433.
> >
> > This does seem like a rather unfortunate loss in type safety - is there
> no common type here other than Value*? Should we introduce one.
> >
> > (more broadly, I do wonder if there's some common API we could expose
> via MDNode so as not to end up with two kinds of metadata... but I've not
> really been able to wrap my head around this enough to have a well informed
> perspective)
> >
>
> Because of the overlap in API between `User` and `MDNode` -- they both
> have operands, but deal with them completely differently -- the only
> sane common base class between `MDNode` and `MDUser`/`CustomMD` is
> `Value`.  Otherwise, `MDNode` would have to inherit from `User`
> somehow which is (IMO) significantly worse.
>

What's the cost in having MDNode inherit from User (and thus, indirectly,
from Value)? You mention that we already pretty regularly have Values that
aren't Users - would this instance be particularly worse? Is any instance
so bad that it's worth doing these other contortions instead?


>
> I threw some thoughts on a longer term fix into PR21438 [1] last week
> though, since this is bothering me too.
>
> [1]: http://llvm.org/bugs/show_bug.cgi?id=21438


Right, I vaguely recall reading that - thanks for the (re)pointer.
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