mattjgalloway
commented
2 years ago I've done a bit more digging on this and seem to have come up with an even more minimal test case, this time not using std::future/std::promise at all. And this time it crashes when compiled with plain clang.exe and also clang-cl.exe. It still crashes when using a PCH and doesn't crash when not using a PCH. Hopefully this is a little easier to digest therefore.
thing.hpp:
#pragma once
#include <cstdlib>
struct Thing {
Thing() { mem_ = malloc(1); }
~Thing() { free(mem_); }
Thing& operator=(const Thing& other) {
if (this == &other) return *this;
mem_ = malloc(1);
return *this;
}
Thing(const Thing& other) {
mem_ = malloc(1);
}
void* mem_;
};pch.hpp:
#include "thing.hpp"
void foo() {
try {
Thing t;
throw t;
} catch (const Thing& t) {}
}crash.cpp:
#include <cstdio>
#include "thing.hpp"
int main(int argc, char* argv[]) {
printf("started\n");
try {
Thing t;
throw t;
} catch (Thing t) {}
printf("didn't crash\n");
return 0;
}Then compiling with a PCH:
PS1> & 'C:\Program Files\LLVM13.0.0\bin\clang++.exe' -Xclang -emit-pch -o pch.gch pch.hpp
PS1> & 'C:\Program Files\LLVM13.0.0\bin\clang++.exe' -include-pch pch.gch -O0 -o crash.exe crash.cpp
PS1> .\crash.exe
startedWithout a PCH:
PS1> & 'C:\Program Files\LLVM13.0.0\bin\clang++.exe' -O0 -o crash.exe crash.cpp
PS1> .\crash.exe
started
didn't crashAgain it’s crashing because the copy ctor of Thing was not called when it needed to be. That results in a double free of the mem_ member of Thing.
Note that it will also not crash, even with the PCH, if in crash.cpp you catch the exception by reference. But that's because there's no exception copying going on. The catch by value is there to force the copy ctor to need to be called.
smeenai
WenleiHe
tentzen
rnk
glebov-andrey
[UPDATE: I found a more minimal example and I've put that as a separate comment below. Preserving the original here as it shows the direction we went in and may help diagnosis.]
We've come across a strange issue when using
clang-cl.exeon Windows. It seems to have started somewhere between 10.0.0 and 11.0.0. And I can confirm I am seeing it with the latest, LLVM13.0.0, as well.Important to note is that the issue only happens when using precompiled headers too.
(I am using
clangwith the standard library from Visual Studio 2019 with Windows SDK version 10.0.18362.0 and MSVC version 14.28.29333.)First let me show the minimal test case I've come up with.
crash.cpp:
pch.hpp:
Then we compile, link and run with
clang++.exelike this:However if we compile, link and run with
clang-cl.exethen it crashes:If you omit the use of PCH but still use
clang-cl.exethen it works again:When tracing through why the crash is happening, it seems to be coming from the fact that at runtime, the
CatchableTypefor thefuture_errorexception has nocopyFunction. That means that inside_CopyExceptionObjectin ucrt, we justmemcpythe exception data, which is wrong because thefuture_errorneeds the proper copy constructor to run. It seems that the compiler emittingnullptrforcopyFunctionin theclang-cl.exe+PCH case is wrong. That's my understanding at least thus far!