https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122614
Bug ID: 122614
Summary: Dependency file created when using import std causes
full rebuilds even if nothing has changed
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jussi.pakkanen at mailbox dot org
Target Milestone: ---
Take this source file:
import std;
int main() {
std::println("Hello");
return 0;
}
compile it with the following two commands:
g++ -std=c++23 -c -fmodules -fmodule-only -fsearch-include-path bits/std.cc
g++ -std=c++23 -fmodules standalone.cpp -o standalone -MD -MF standalone.deps
The output of the deps file looks like this:
standalone: standalone.cpp /usr/include/stdc-predef.h gcm.cache/std.gcm
standalone: std.c++-module
CXX_IMPORTS += std.c++-module
This dependency file specifies that `standalone` depends on a file called
`std.c++-module`. As such a file does not exist, the dependency is always out
of date and thus running `ninja` will always rebuild the std module and by
extension every source file that uses `import std`. Effectively this causes a
full rebuild every time, even when nothing has changed.
To fix this the output file should not contain the line `standalone:
std.c++-module`.
