[llvm-dev] RFC: Representing unions in TBAA

[Daniel Berlin via llvm-dev]

On Sun, May 14, 2017 at 10:20 AM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 05/14/2017 11:06 AM, Daniel Berlin wrote:
>
>
>
> On Sun, May 14, 2017 at 8:37 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>>
>> On 03/01/2017 05:30 PM, Daniel Berlin via llvm-dev wrote:
>>
>> So, https://bugs.llvm.org/show_bug.cgi?id=32056 is an example showing
>> our current TBAA tree for union generation is definitely irretrievably
>> broken.
>> I'll be honest here. I'm pretty sure your proposal doesn't go far enough.
>> But truthfully,  I would rather see us come closer to a representation we
>> know works, which is GCC's.
>> Let me try to simplify what you are suggesting, and what we have.
>> Our current representation is basically inverted from GCC, but we don't
>> allow things that would enable it to work.
>>
>> Given
>> union {int a, short b};
>>
>> GCC's will be:
>>
>>  union
>>   /   \
>> short int
>>
>>
>> Everything is implicitly a subset of alias set 0 (which for C/C++ is
>> char) just to avoid representing it.
>>
>> Our metadata has no child links, and only a single parent link.
>>
>> You can't represent the above form because you can't make a single short
>> node a child  of every union/struct it needs to be (lack of multiple
>> parents means you can't just frob them all to offset zero).
>>
>> Your suggestion is to invert this as a struct
>>
>> short  int
>>    |    /
>> union
>>
>> We don't allow multiple parents, however.
>> Because of that, you suggest we follow all nodes for unions, special
>> casing union-type nodes somehow
>>
>>
>> Now that I've spent a bunch of time looking at this, I'd like to voice
>> support for Steven's original proposal. In the context of what we have, it
>> makes sense, seems sound, and should fix the representational mapping
>> problem we currently have.
>>
>
> Except you can't easily differentiate it from the current one, and if we
> are going to have to upgrade/break compatibility, why not just do it once
> right, a way we know works, instead of risk screwing it up again, and
> playing with a representation we aren't actually sure we can make efficient
> for this case?
>
>
> I don't see why need to break backward compatibility. Do you have
> something specific in mind? Once we extend the TBAA implementation to treat
> repeated offsets as disjunction, then we'll extend Clang to emit metadata
> for unions in this form. Old IR will work exactly as it does now.
>

Except the Old IR is irretrievably broken. and in this method, you can't
tell whether you are dealing with correct TBAA or not.

Earlier, the discussion was basically "detect old IR, disable TBAA since
it's broken, make new IR work".

If we do that, we need a way to distinguish new vs old IR.
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