[LLVMdev] Tablegen binary literals

[Sean Silva]

On Thu, Jul 31, 2014 at 12:28 AM, Adam Nemet <anemet at apple.com> wrote:

>
> On Jul 30, 2014, at 10:56 PM, Pete Cooper <peter_cooper at apple.com> wrote:
>
> Hi Adam
>
> On Jul 30, 2014, at 10:28 PM, Adam Nemet <anemet at apple.com> wrote:
>
> Hi Pete,
>
> Just to clarify, are you proposing two things here?  First, 0b… literals
> to have type bits<n> and second to allow bits<n> initializer to  contain
> other bits<m> elements which would initialize the next m elements.
>
> Yeah, exactly those 2 things.  I have them in separate patches, but I
> think we only get the benefit from sized binary literals if we also allow
> them to initialize multiple bits in another bits<n> type.
>
>
> Looks like bits<n> is already valid in a bits initializer context; it
> yields the bottom bit.
>
> def a {
>   bits<2> opc = { 0, 1 };
>   bits<2> opc2 = { 1, 0 };
>   bits<2> oo = { opc, opc2 };
> }
>
> is valid and produces:
>
> ..
>   bits<2> oo = { 1, 0 };
> ..
>
> Are you aware of this?  This may lead to some ambiguity with your proposed
> extension.  (I have no idea whether this behavior is relied on anywhere
> though.)
>

I think it would make sense to change this behavior to follow the behavior
that Pete describes. I don't have the code handy but my guess is that what
the code is doing here is requesting each element in the braces of a
bits<n> initializer to be casted to `bit`; in that case, we would want to
change that code to first try casting to bits and give that preference (and
issues relevant diagnostics). Pete, is that what your patch does? Can you
attach your patches?

Of course, before changing the behavior, we would probably want to place an
assert on that codepath to catch all existing cases where the behavior
would change.
Also it is important to scan all the docs in docs/TableGen/ and make any
necessary updates (if there is nowhere that needs to be updated, then
documentation needs to be *added* covering this behavior).
Finally, we should post a notice to LLVMdev for out-of-tree maintainers
including information about how to add the necessary assert to check for
places where this behavior would change their .td files.

-- Sean Silva


>
> Adam
>
>
> Thanks,
> Pete
>
> I.e. I don’t think we currently accept:
>
> bits<4> x = { opc, opc }
>
> Thanks,
> Adam
>
>
>
>
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