[llvm-dev] Exploding compile times for long functions

[Florian Hahn via llvm-dev]

> On Dec 8, 2021, at 16:59, Jonas Hahnfeld via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Dear list,
> 
> we recently found very long compile times for generated functions with
> many, many lines. I've extracted a tiny reproducer to mimic what is
> going on, and the core problem seems to be calling a function that
> takes a std::string_view and a newly heap-allocated pointer (see
> generate.string_view.sh in the attached archive).
> 
> Generating 10k calls with
> $ ./generate_string_view.sh 10000 > generated.string_view.10000.cpp
> and then compiling with
> $ clang++ -std=c++17 -O1 generated.string_view.10000.cpp -c
> takes 27s (with Clang 12.0.1), while -O0 is only 1.5s. With more recent
> versions of LLVM, it gets much worse: Clang 13.0.0 takes around 4m30s,
> current main is in the same area.
> 
> Is this expected? Is this a case of "don't do it" (such long functions)
> or a problem that should be fixed in the optimization passes?

Unfortunately this is kind of expected. Large auto-generated can be very good at highlighting inefficiencies in optimizations.

It seems there might be an issue in instcombine. Building a generated file with 1k calls spends ~0.25s in instcombine, for a file with 10k calls we spend 53s in instcombine (as per -ftime-report).

Cheers,
Florian