[llvm-dev] Aggregate load/stores
David Majnemer via llvm-dev
llvm-dev at lists.llvm.org
Sun Aug 16 23:21:05 PDT 2015
On Sun, Aug 16, 2015 at 10:27 PM, deadal nix <deadalnix at gmail.com> wrote:
> 2015-08-16 22:10 GMT-07:00 David Majnemer <david.majnemer at gmail.com>:
>> I would argue that a fix in the wrong direction is worse than the status
> How is proposed change worse than status quo ?
Because a solution which doesn't generalize is not a very powerful
solution. What happens when somebody says that they want to use atomics +
large aggregate loads and stores? Give them yet another, different answer?
That would mean our earlier, less general answer, approach was either a
bandaid (bad) or the new answer requires a parallel code path in their
>>> The argument that target are relying on InstCombine to mitigate IR
>>> requiring legalization seems dubious to me. First, because both aggregate
>>> and large scalar require legalization, so, if not ideal, the proposed
>>> change does not makes things any worse than they already are. In fact, as
>>> far as legalization is concerned, theses are pretty much the same. It
>>> should also be noted that InstCombine is not guaranteed to run before the
>>> target, so it seems like a bad idea to me to rely on it in the backend.
>> InstCombine is not guaranteed to run before IR hits the backend but the
>> result of legalizing the machinations of InstCombine's output during
>> SelectionDAG is worse than generating illegal IR in the first place.
> That does not follow. InstCombine is not creating new things that require
> legalisation, it changes one thing that require legalization into another
> that a larger part of LLVM can understand.
I'm afraid I don't understand what you are getting at here. InstCombine
carefully avoids ptrtoint to weird types, truncs to weird types, etc. when
creating new IR.
>>> As for the big integral thing, I really don't care. I can change it to
>>> create multiple loads/stores respecting data layout, I have the code for
>>> that and could adapt it for this PR without too much trouble. If this is
>>> the only thing that is blocking this PR, then we can proceed. But I'd like
>>> some notion that we are making progress. Would you be willing to accept a
>>> solution based on creating a serie of load/store respecting the datalayout ?
>> Splitting the memory operation into smaller operations is not semantics
>> preserving from an IR-theoretic perspective. For example, splitting a
>> volatile memory operation into several volatile memory operations is not
>> OK. Same goes with atomics. Some targets provide atomic memory operations
>> at the granularity of a cache line and splitting at legal integer
>> granularity would be observably different.
> That is off topic. Proposed patch explicitly gate for this.
Then I guess we agree to disagree about what is "on topic". I think that
our advice to frontend authors regarding larger-than-legal loads/stores
should be uniform and not dependent on whether or not the operation was or
was not volatile.
>> With the above in mind, I don't see it as unreasonable for frontends to
>> generate IR that LLVM is comfortable with. We seem fine telling frontend
>> authors that they should strive to avoid large aggregate memory operations
>> in our performance tips guide <
>> Implementation experience with Clang hasn't shown this to be particularly
>> odious to follow and none of the LLVM-side solutions seem satisfactory.
> Most front end do not have clang resources. Additionally, this tip is not
> quite accurate. I'm not interested in large aggregate load/store at this
> stage. I'm interested in ANY aggregate load/store. LLVM is just unable to
> handle any of it in a way that make sense. It could certainly do better for
> small aggregate, without too much trouble.
I'm confused what you mean about "clang resources" here, you haven't made
it clear what the burden it is to your frontend. I'm not saying that there
isn't such a burden, I just haven't seen it been articulated and I have
heard nothing similar from other folks using LLVM. What prevents you from
performing field-at-a-time loads and stores or calls to the memcpy
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the llvm-dev