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

[Dwight Guth via llvm-dev]

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. 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. 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).

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.

Or am I going to have to resign myself to waiting until lld is well
supported at linking mach-o files?

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>
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