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

[Steven Wu via llvm-dev]

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.

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.

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 <mailto: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 <mailto: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 <mailto: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 <mailto:ruiu at google.com> and 
> +Eric Christopher <mailto:echristo at gmail.com> to comment on lld Mach-O support.
> 
> Teresa
>  
> 
> Thanks,
> 
> -- 
> Dwight Guth
> Director of Engineering
> 
> Email: dwight.guth at runtimeverification.com <mailto:dwight.guth at runtimeverification.com>
>  <https://www.runtimeverification.com/>
>  <https://github.com/dwightguth>  <https://www.linkedin.com/company/3142238/>   <https://twitter.com/rv_inc>
> 
> 
> 
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> 
> -- 
> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com <mailto:tejohnson at google.com> |

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