Re: stdc++ issue: extremely long compile time with large number of string literals

[Marc Glisse <marc.glisse@inria.fr>]

On Thu, 9 Jul 2020, Mandeep Sandhu via Gcc-help wrote:

> I have an strange (to me) issue, where trying to compile a header
> which has a single "std::unordered_set<std::string>" initialized with
> around 50K short strings is taking forever.
>
> The set is declared as:
> const std::unordered_set<std::string> my_set ({"item1", "item2", ....});
>
> (The header is auto-generated using a script which takes a JSON array
> and puts its elements in an unordered_set)
>
> I understand that creation of many strings has an overhead, but this
> issue seems to affect compilation time, not runtime.
>
> Can someone explain to me why it takes such a long time to compile?
> Keeping the strings to under 5K, makes the program compile in about 8
> secs.
>
> I'm using the following compiler on Linux:
> $ g++ --version
> g++ (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008
> Copyright (C) 2019 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

You didn't say what flags you are using. For anything autogenerated like 
that, going above -O1 is always risky.

One first thing to try is -ftime-report

At -O0
  phase opt and generate             :   3.46 ( 84%)   0.15 ( 45%)   3.62 ( 81%)  188385 kB ( 70%)

without a specific pass that stands out.

At -O1
  phase opt and generate             :  87.96 ( 99%)   6.57 ( 97%)  94.56 ( 99%)24503776 kB (100%)
  callgraph ipa passes               :  85.93 ( 97%)   6.52 ( 96%)  92.47 ( 97%)24413253 kB ( 99%)
  tree eh                            :  84.32 ( 95%)   6.43 ( 95%)  90.78 ( 95%)24340697 kB ( 99%)

so, it looks related to the optimization of exceptions...

-- 
Marc Glisse