[Modules] EvaluateAsInitializer executed on used variable templates in every importer

[davidstone]

Given the following two translation units:

export module a;

#define REPEAT8(x) \
	(x), \
	(x), \
	(x), \
	(x), \
	(x), \
	(x), \
	(x), \
	(x)


unsigned a() {
	unsigned x = 0;
	REPEAT8(REPEAT8(REPEAT8(REPEAT8(REPEAT8(REPEAT8(++x))))));
	return x;
}

template<typename>
unsigned aa = a();

void aaa() {
	aa<int>;
}

export module b;

import a;

And compiled with

clang++ -std=c++20 -x c++-module --precompile -c a.cpp -o a.pcm
clang++ -std=c++20 -fmodule-file=a=a.pcm -x c++-module -c b.cpp

Causes the compilation of b.cpp to take longer than expected, especially if optimizations are turned on. Passing -ftime-trace shows that most of the non-optimizer time is spent in EvaluateAsInitializer on aa. If optimizations are turned on, it then spends a bunch of extra time compiling module b for optimizations, primarily SROAPass, but that is possibly just a duplicate of #60996

[davidstone]

In this example, if I declare unsigned a() as constexpr unsigned a() and thereby cause aa to have constant initialization, then in this example module b no longer spends time in EvaluateAsInitializer. However, the real program I reduced this from has a module that spends time on EvaluateAsInitializer even when the module contains nothing but imports, and the variable templates it is evaluating are all declared constexpr.

[llvmbot]

@llvm/issue-subscribers-clang-modules

[ChuanqiXu9]

Fixed in af86957.