[llvm-dev] Exceptions on Windows & MSVC

[Alexandre Ganea via llvm-dev]

Hello everyone,

I was wondering what is the status regarding exceptions for the windows-msvc target? The corresponding part of the documentation (llvm/decs/ExceptionHandling.rst) hasn't been updated since 2015, was there any progress since?

This is most likely known, but there's significant divergence between the behavior of MSVC cl.exe and clang-cl.exe:
Consider:

void crash() {
  struct A {
    ~A() {}
  } Obj;
  *(volatile int *)0x11 = 0;
}

#ifdef SEH
  #define TRY __try
  #define CATCH_ALL __except (1)
#else
  #define TRY try
  #define CATCH_ALL catch (...)
#endif

int main() {
  TRY { crash(); }
  CATCH_ALL {}
  return 0;
}

using try/catch (SEH not defined):

         |               (default)               |          /EHa            |          /EHs            |
--------------------------------------------------------------------------------------------------------
MSVC cl  | warning [1], no unwind, crash catched |    unwind, crash catched |          crash unhandled |
clang-cl | compilation error [2]                 |          crash unhandled |          crash unhandled |


using __try/__except (SEH defined):

         |               (default)               |          /EHa            |           /EHs           |
--------------------------------------------------------------------------------------------------------
MSVC cl  |              no unwind, crash catched |    unwind, crash catched |          crash unhandled |
clang-cl |              no unwind, crash catched | no unwind, crash catched | no unwind, crash catched |


According to Microsoft's documentation [3],
                (default) means:              async exceptions + C++ exceptions, but no unwinding
                /EHa means:                      async exceptions + C++ exceptions, with unwinding in both cases
                /EHs means:                      C++ exceptions only, with unwinding

I'm using MSVC VS2017 15.9.16 and clang-cl from github at HEAD. In both cases I'm targeting x64.
Using _set_se_translator [4] and /EHa does not help with clang-cl.

Are there any plans to improve  the async exceptions unwinding in the short term?

Thanks!
Alex.


[1] warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc
[2] error: cannot use 'try' with exceptions disabled
[3] https://docs.microsoft.com/en-us/cpp/build/reference/eh-exception-handling-model?view=vs-2019
[4] https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/set-se-translator?view=vs-2019
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