[llvm-dev] RFC: Representing unions in TBAA

[Hal Finkel via llvm-dev]

On 05/14/2017 12:49 PM, Daniel Berlin wrote:
>
>
> On Sun, May 14, 2017 at 10:20 AM, Hal Finkel <hfinkel at anl.gov 
> <mailto: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
>>     <mailto: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
>>>         <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.


Ah, okay. I don't think that's desirable in general. There are frontends 
emitting perfectly self-consistent TBAA metadata, and there's no reason 
they should change. Clang's metadata is broken because the mapping 
between language rules and TBAA is broken. If we'd like, we can 
blacklist TBAA metadata coming from root nodes named "Simple C++ TBAA" 
and "Simple C/C++ TBAA" (or provide an option to do that because I'm not 
convinced it is worth trying to retroactively "fix" old IR by default 
given the associated performance penalties). After the fix, Clang can 
emit root nodes with different names (they're arbitrary). Changing the 
root-node names will also give the conservatively-correct behavior when 
linking old IR with new IR.

Thanks again,
Hal

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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