[llvm-dev] RFC: We should stop merging allocas in the inliner

[Chandler Carruth via llvm-dev]

Sorry I missed these comments in my first read through David.

On Mon, Aug 1, 2016 at 1:06 AM Xinliang David Li via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Sun, Jul 31, 2016 at 9:47 PM, Chandler Carruth via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Thoughts? The code changes are easy and mechanical. My plan would be:
>>
>
> There is one caveat: stop doing stack merging in inliner will change the
> inliner cost analysis which may have affect inlining decisions and
> performance in an unexpected way.  For instance, the allocatedSize estimate
> won't be as accurate due to double counting.
>

There is the possibility of this, but I think it is relatively unlikely for
a few reasons.

The first is that I don't think the alloca merging is helping that much
here. Specifically, we only merge certain kinds of allocas and we only
merge them in fairly limited cases. So if the thresholds were very
sensitive to this, I would expect us to have seen these thresholds blocking
things more often than we have.

Also, as you mention, there are two places we might hit this. One is due to
the increased number of alloca instructions causing skew in the inline
cost. I think we should "fix" this by stopping counting *static* alloca
instructions for the purpose of inline cost estimation. I think this will
be a net win. The alloca instructions are completely synthetic when static.
All of them will be folded into a single stack adjustment that is even
shared with register spills etc. I think we should model them as free.
(Clearly, *dynamic* alloca instructions are different here!) But I don't
think this is likely enough to be urgent to fix. I think we could do both
changes in parallel.

The other limit is the allocated size limit, and I think that is the more
likely issue (it sounds like it was the one you were worried about as
well). This one might be an issue, but I also think we can adjust it pretty
freely. The goal of this limit, from my recollection of when it went in, is
more to avoid run-away stack bloating, and the value of the limit is
somewhat arbitrary. And it only impacts inlining into recursive functions.
So I'm less worried about this in general. If it proves to be a problem in
practice, we can do something about it, and if that happens it would
probably be good to know because we shouldn't be relying on merging to
model this anyways.



>
>> 1) Add a flag to turn this off.
>> 2) Run benchmarks with flag to make sure things are actually working.
>> 3) Change default for the flag, make sure chaos doesn't ensue.
>> 4) Delete the (quite substantial) complexity associated with this and the
>> flag.
>>
>
> The code handling the merging seems to be in an isolated place with <100
> lines of code. While I agree this change may be something good to do in
> principle -- are there any practical reason for doing this?
>

Yes, its complexity which is better addressed by another layer. Every piece
of complexity, especially in something like the inliner, makes it harder to
change and reason about. I'm trying to minimize and isolate the complex
parts of the iniliner in preparation to porting it to the new pass manager.
This seemed like low-hanging fruit.


> you mentioned information loss (aliasing, sanitizer etc) -- are there any
> real issues caused by the merging (with tracking bugs)?
>

I don't have any, but I also don't think we should wait for bugs to come up
to make principled changes to the compiler, especially those that are
simplifying!
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