C++ modules appear to be exceedignly strict with intrinsic headers #98021
Comments
EugeneZelenko
added
clang:modules
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@llvm/issue-subscribers-clang-modules Author: Chris Elrod (chriselrod)
That is, code using `immintrin.h` tends to fail to compile when using modules while working fine with headers.
I'll try to produce a minimal example in the next few hours.
For now, I have an example using boost_unordered.
When problems showed up in my own code using intrinsics, I could generally fix it by declaring all arguments as variables, and then passing the lvalues to the intriinsic function.
Hello.cxxm: #ifndef USE_HEADERS
module;
#endif
#include <boost/unordered/unordered_flat_map.hpp>
#include <iostream>
#ifndef USE_HEADERS
export module Hello;
export {
#endif
void hello() { std::cout << "Hello World!\n"; }
template <typename K, typename V> using map = boost::unordered_flat_map<K, V>;
#ifndef USE_HEADERS
}
#endifuser.cpp: #ifndef USE_HEADERS
import Hello;
#else
#include "hello.cxxm"
#endif
int main() {
hello();
int x = 0;
long y = 0;
map<int*,long*> m;
m[&x] = &y;
[[maybe_unused]] auto f = m.find(&x);
return 0;
}Compiling with headers: $ clang++ -std=c++23 use.cpp -DUSE_HEADERS -o Hello.out
$ ./Hello.out
Hello World!With modules: $ clang++ -std=c++23 --precompile hello.cxxm -o M-hello.pcm
$ clang++ -std=c++23 use.cpp -fmodule-file=Hello=M-hello.pcm M-hello.pcm -o Hello_mod.outresults in I could file this as a |
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Here is the minimal example using
#ifndef USE_HEADERS
module;
#endif
#include <concepts>
#include <immintrin.h>
#include <iostream>
#ifndef USE_HEADERS
export module Hello;
export {
#endif
void hello() { std::cout << "Hello World!\n"; }
template <typename T> auto vload128(const T *p) {
if constexpr (std::same_as<T, float>) {
return _mm_loadu_ps(p);
} else if constexpr (std::same_as<T, double>) {
return _mm_loadu_pd(p);
} else {
return _mm_loadu_si128(reinterpret_cast<const __m128i *>(p));
}
}
#ifndef USE_HEADERS
}
#endif
#ifndef USE_HEADERS
import Hello;
#else
#include "hello.cxxm"
#endif
int main() {
hello();
float x[4];
[[maybe_unused]] auto v = vload128(x);
return 0;
}Compiling with headers: $ clang++ -std=c++23 use.cpp -DUSE_HEADERS -o Hello.
out
$ ./Hello.out
Hello World!Compiling with modules: $ clang++ -std=c++23 --precompile hello.cxxm -o M-he
llo.pcm
$ clang++ -std=c++23 use.cpp -fmodule-file=Hello=M-hell
o.pcm M-hello.pcm -o Hello_mod.out
In file included from use.cpp:2:
/home/chriselrod/Documents/progwork/cxx/experiments/modules/hello.cxxm:16:14: error: no matching function for call to '_mm_loadu_ps'
16 | return _mm_loadu_ps(p);
| ^~~~~~~~~~~~
use.cpp:10:29: note: in instantiation of function template specialization 'vload128<float>' requested here
10 | [[maybe_unused]] auto v = vload128(x);
| ^
1 error generated. |
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using something like template <typename T> auto vload128(const T *p) {
if constexpr (std::same_as<T, float>) {
const float *fp = p;
return _mm_loadu_ps(fp);
} else if constexpr (std::same_as<T, double>) {
const double *dp = p;
return _mm_loadu_pd(dp);
} else {
const __m128i *ip = reinterpret_cast<const __m128i *>(p);
return _mm_loadu_si128(ip);
}
}instead allows it to compile, even though |
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Another workaround is to declare explicit instantiations of the template within the module that defines it. |
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I can't reproduce this in my local environment with trunk (0182f51). My standard library is libstdc++ 10.2 (this may not be relevent) in linux. Can you try again with trunk? |
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I just stumbled across this bug as well, upgrading from a clang trunk from around end of June to this commit: ChuanqiXu9/clangd-for-modules@e583176. I'm also using I managed to bisect it to 91d40ef . Maybe something in that commit inadvertently touched some code related to name lookup or module visibility? It is a bit weird though that the date of that commit is after the report date of this bug... |
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I managed to work around it for now by re-applying 91d40ef on top of ChuanqiXu9/clangd-for-modules@e583176 . I guess reapplying it on |
That is, code using
immintrin.htends to fail to compile when using modules while working fine with headers.I'll try to produce a minimal example in the next few hours.
For now, I have an example using boost_unordered.
When problems showed up in my own code using intrinsics, I could generally fix it by declaring all arguments as variables, and then passing the lvalues to the intriinsic function.
Hello.cxxm:
user.cpp:
Compiling with headers:
$ clang++ -std=c++23 use.cpp -DUSE_HEADERS -o Hello.out $ ./Hello.out Hello World!With modules:
results in
I could file this as a
boost_unorderedissue or make a PR there, as I've generally found I can work around the problem.But I'll see about creating a minimal reproducer using
#include <immintrin.h>directlry that works with headers but fails with modules.The text was updated successfully, but these errors were encountered: