[llvm-dev] Setting llvm::TargetOptions::GuaranteedTailCallOpt in LTO Code Generation

Wed Sep 18 13:01:31 PDT 2019

On Wed, Sep 18, 2019, 2:39 PM Steven Wu <stevenwu at apple.com> wrote:

>
>
> On Sep 18, 2019, at 10:24 AM, Steven Wu <stevenwu at apple.com> wrote:
>
> Hi Dwight
>
> Thanks for the feedback. For the issue you reported, there has been few
> reviews trying to tweak the -mllvm option when using legacy LTO interfaces
> (myself included) but it never got enough traction to moving forward. Note
> how -tailcallopt is implemented as a -mllvm flag means that it is a debug
> option and probably not well tested. The option is also not stable which
> means it can be renamed without notification.
>
>
> Digging out the patch Teresa once pointed out to me:
> https://reviews.llvm.org/D19015
> It is really a two line change. If you build your own libLTO, it is a very
> patch to maintain downstream.
>

Thanks for linking this to me! I will try and see if I can get this to work
because it might be the simplest short term solution so we can have
something working. It is not ideal as a long term solution because I really
don't want us to have to maintain a build of any of the components of llvm
ourselves though. So I will respond below to your question and we can
figure out the best long term solution.

>
> I also feel like passing -tailcallopt in the linker stage is kind of
> fragile. It is better to create an attribute (on function or callInst) to
> force tailcallopt and some compiler flag to generated that during IRGen.
>
>
> I think I missed you comments about `musttail`. Do you have any example to
> show why `musttail` doesn’t work for you? Is there anything we can do to
> make it work so we don’t need to rely on `-mllvm` options?
>
> Steven
>

The problem with musttail is that the behavior of the feature is that IR
does not verify if it includes a musttail call where the caller and callee
have different numbers of arguments or otherwise differ in certain
respects. However, we need guaranteed tail calls for mutually recursive
functions, which obviously may not have the same signature. I would love to
be able to use musttail though. Maybe you could make musttail functions
with a compatible calling convention use the same codepath as -tailcallopt,
and then loosen the restrictions? I'm not really sure. I can foresee there
might be problems with calling convention if the function was externally
visible, but for our use case it should be fine if the looser musttail
attribute only worked for functions local to a module, I think...

>
> Steven
>
> On Sep 18, 2019, at 10:09 AM, Dwight Guth <
> dwight.guth at runtimeverification.com> wrote:
>
> On Wed, Sep 18, 2019, 11:57 AM Teresa Johnson <tejohnson at google.com>
> wrote:
>
>> Hi Dwight,
>>
>> Welcome to LLVM-dev! A few comments below. Cc'ing a few people who
>> hopefully can add info on some of the specific issues here.
>>
>> Teresa
>>
>> On Wed, Sep 18, 2019 at 9:04 AM Dwight Guth via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>>
>>> I am lead developer of a project that is using LLVM to implement an
>>> ahead-of-time compiled functional language. We use llc -tailcallopt to
>>> ensure that functions that end in a tail call are compiled to a tail call
>>> at the machine level, because we have a number of cases in our interpreter
>>> where functions with different function signatures call one another in
>>> deeply nested recursive calls.
>>>
>>
>> Maybe a naive question - would that be fixable?
>>
>
> I doubt that we can get around this easily. It's a programming language
> compiler, so the guarantee that if the user writes a tail call in their
> code, they will get a tail call at the machine level is pretty important.
> Restricting that guarantee to only functions that call themselves would
> probably cause a lot of problems, including stack overflows, for code
> written in our programming language. Recursion is basically the only way to
> loop in most functional languages; that's why the tailcallopt flag was
> created.
>
>
>>
>>> We can't use `musttail` because the callee and caller often have
>>> different signatures.
>>>
>>> We would like to support link time optimization in our programming
>>> language, because performance is important to us. However, there is no
>>> clang flag to enable the GuaranteedTailCallOpt flag, and the only way to
>>> pass target options to the lto plugin currently is via an unsupported API
>>> that parses those flags to static variables.
>>>
>>
>> I assume you mean passing internal options via -mllvm through the linker?
>>
>
> Yes that's correct, we are passing -mllvm -tailcallopt to lld on Linux.
>
>
>>
>>> This works on Linux, but the Mac OS linker does not actually initialize
>>> the TargetOptions that it passes as an llvm::lto::Config based on the
>>> parsed static variables, and Apple is uninterested in spending time
>>> supporting an unsupported LLVM API like -mllvm (understandably).
>>>
>>
>> lto::Config is part of the new LTO API. For the most part ld64 uses the
>> old legacy LTO API, and therefore does not even use llvm::lto::Config (the
>> one exception is to share the code for computing a cache key). But it
>> doesn't use this when invoking the code generation passes. I'm surprised
>> that ld64 would not have a way to pass through internal llvm options -
>> presumably that is necessary for debugging and tuning. +Steven Wu
>> <stevenwu at apple.com> to give more info here (I work on Linux code and
>> therefore have only directly used gold and lld, which both use the new LTO
>> API).
>>
>
> ld64 does have an -mllvm flag but when you pass -mllvm -tailcallopt, it
> will happily parse this flag, but it ignores the resulting value when
> initializing the code generator, and when I reported the issue to Apple,
> they said they would not fix it because -mllvm is not an officially
> supported API.
>
>
>>
>>> Is there a change to the LLVM project that you might be willing to
>>> accept that we might be able to create a patch for that would allow us to
>>> get link time optimization enabled in our programming language on Mac OS,
>>> at least in the future? And if so, is it possible that someone could give
>>> me pointers on how to proceed? I'm a relative novice studying this code and
>>> I'm not really sure how all the components fit together at a high level and
>>> thus what the correct design for something like this would be.
>>>
>>
>> I guess the question is what interface would work for you. Would passing
>> an internal option like what works on lld or what you are doing with llc be
>> acceptable?
>>
>
> Yes, this would be fine with us, if it's possible. How would I go about
> making this happen?
>
>
>> If you need a more officially supported mechanism, IMO the best way is
>> probably to create a new function attribute (e.g. 'forcetailcall' or
>> something equivalent to what GuaranteedTailCallOpt implies). That would be
>> completely linker agnostic and also not rely on internal options.
>>
>>
>>> Or am I going to have to resign myself to waiting until lld is well
>>> supported at linking mach-o files?
>>>
>>
>> +Rui Ueyama <ruiu at google.com> and
>> +Eric Christopher <echristo at gmail.com> to comment on lld Mach-O support.
>>
>> Teresa
>>
>>
>>>
>>> Thanks,
>>>
>>> --
>>> Dwight Guth
>>> Director of Engineering
>>>
>>> Email: dwight.guth at runtimeverification.com
>>>
>>> <https://www.runtimeverification.com/>
>>>
>>> <https://github.com/dwightguth>
>>> <https://www.linkedin.com/company/3142238/>
>>> <https://twitter.com/rv_inc>
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>>
>> --
>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>>
>
>
>
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