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
