在 2020/10/8 22:56, Jason Merrill 写道: > > Hmm, why isn't the mingw implementation used for all programs? That would > avoid the bug. >
There was a little further discussion about this [1]. TL;DR: The mingw-w64 function is linked statically and subject to issues about order of destruction. Recently mingw-w64 has got its own `__cxa_thread_atexit()` so libstdc++ no longer exposes it. This patch for libstdc++ fixes calling conventions for destructors on i686 so they match mingw-w64 ones. [1] https://github.com/msys2/MINGW-packages/issues/7096 [2] Below is a direct quote from #mingw-w64 on OFTC: (lh_ideapad is me and wbs is Martin Storsjö.) ``` [14:29:32] <lh_ideapad> wbs, what was the rationale for the `__thiscall` convention for `__cxa_thread_atexit()` and `__cxa_atexit()` in our CRT? I suspect it is correct, but it is not specified anywhere in Itanium ABI. [14:30:41] <lh_ideapad> In case of evidence for that, the GCC prototype (with default __cdecl) should be wrong. [14:31:10] <lh_ideapad> See this: https://github.com/msys2/MINGW-packages/issues/7096 [14:52:05] <wbs> lh_ideapad: itanium ABI doesn't really talk about windows things, but, the function that is passed to __cxa_thread_atexit is the object's destructor function, and when calling the destructor, which is technically a member function, it's done with the thiscall calling convention [14:52:31] <wbs> lh_ideapad: example: https://godbolt.org/z/qbfWT1 (only clang as there's no gcc-mingw there, but if you try the same there you'll see the same thing) [14:52:35] <minifox> Title: Compiler Explorer (at godbolt.org) [14:52:58] <wbs> lh_ideapad: the destruct function shows that when calling __ZN7MyClassD1Ev, the destructor, it passes the object pointer in ecx, i.e. thiscall [14:53:42] <wbs> lh_ideapad: and when adding the object to the cleanup list, the __ZN7MyClassD1Ev function is passed directly to ___cxa_thread_atexit, which then will need to call the function using the thiscall convention [14:59:54] <wbs> lh_ideapad: so yes, I would agree with your patch changing libsupc++ to use thiscall [15:13:01] <lh_ideapad> gcc is doing the same thing with a wrong calling convention , leaving a garbage value on i686-w64-mingw32. [15:13:38] <wbs> yup, so definite +1 on your libsupc++ patch for that [15:14:00] <lh_ideapad> then how about `__cxa_atexit`? [15:14:26] <wbs> I would say it should work the same, but gcc doesn't normally use that one, right? [15:14:29] <lh_ideapad> it's not used by GCC (the standard `atexit()` is used). [15:15:26] <wbs> clang has a flag -fuse-cxa-atexit, which makes it use cxa_atexit instead of atexit [15:15:40] <lh_ideapad> I was a bit dubious on it. [15:18:59] <lh_ideapad> GCC has `-fuse-cxa-atexit` too . Let me check. [15:18:59] <wbs> (I tested it), clang does use __cxa_atexit if the -fuse-cxa-atexit flag is used, and then the dtor (thiscall) is passed directly to __cxa_atexit, so that's +1 datapoint to that it should have thiscall. gcc doesn't use __cxa_atexit for i686 windows despite -fuse-cxa-atexit, so that's no points in either direction [15:19:28] <wbs> both clang and gcc use a wrapper function that fixes the calling convention, when using atexit at least [15:20:22] <lh_ideapad> `-fuse-cxa-atexit` seems to have no effect on `i686-w64-mingw32-g++`. [15:20:46] <wbs> exactly. so in practice it doesn't matter for gcc, but I think libsupc++ should handle it the same [15:23:11] <lh_ideapad> ok I will compose a new patch for both functions later today. [15:23:13] <lh_ideapad> :) [15:23:25] <wbs> \o/ [15:24:40] <wbs> then for the other issue that the user was posting about; I remember testing and noticing that the mingw-w64-crt implementation of __cxa_thread_atexit doesn't work with emutls, but in all of my tests, it has been a non-issue as it has ended up using the libsupc++ code instead [15:50:50] <lh_ideapad> probably static linking is broken, so one must link against shared libstdc++. [15:52:20] <lh_ideapad> it doesn't matter whether it is the CRT or libsupc++ implementation that is linked statically. [15:53:13] <wbs> it seems like current msys builds of libstdc++ doesn't include __cxa_thread_atexit in libstdc++ at all. I'm pretty sure I tested this back when I made the mingw version, but I'll investigate and try to pinpoint what changed (did gcc at some point start noticing that mingw-w64-crt contains it and stop providing their own?) or whether I just missed something when I tested it back when I wrote it... [15:59:47] <wbs> I'll follow up on that other issue and mingw bugtracker issue, but I'll do a couple hours of comple testing/studying things first to be able to give an informed comment [16:15:34] * pamaury (~pama...@ip-41.net-89-3-53.rev.numericable.fr) has joined [16:27:34] <lh_ideapad> wbs, there is a check in `libstdc++-v3/configure.ac` around line 275. [16:29:22] <wbs> lh_ideapad: yeah, but in my tests it doesn't pick up on it. I test by cross-bootstrapping a toolchain, so maybe this check runs before the mingw-w64-crt actually is built [16:29:50] <wbs> so it doesn't detect if just bootstrapping once, but if building a new gcc in an already complete environment, it behaves differently [16:33:21] * Pali (~p...@0001caa5.user.oftc.net) has joined [16:34:52] <wbs> ah, here it is: [16:34:52] <wbs> # Only do link tests if native. Else, hardcode. [16:34:52] <wbs> if $GLIBCXX_IS_NATIVE; then ``` -- Best regards, LH_Mouse
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