basic SEH test fails

[jaykrell]

C:\s>clang --version
swift.org clang version 16.0.0
Target: x86_64-unknown-windows-msvc
Thread model: posix
InstalledDir: C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\bin

C:\s>type 1.c
int main()
{
 __try { *(volatile char*) 0; } __except(1) { return 2; }
}

C:\s>clang-cl  1.c

C:\s>1.exe

C:\s>echo %errorlevel%
-1073741819

That is 0xc0000005 access violation.
This program returns 2 with Visual C++.

Hm not sure about this toolchain, I'll try another later.

[EugeneZelenko]

Could you please try 19 or main branch? https://godbolt.org should be helpful.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Jay Krell (jaykrell)

``` C:\s>clang --version swift.org clang version 16.0.0 Target: x86_64-unknown-windows-msvc Thread model: posix InstalledDir: C:\Library\Developer\Toolchains\unknown-Asserts-development.xctoolchain\usr\bin

C:\s>type 1.c
int main()
{
__try { (volatile char) 0; } __except(1) { return 2; }
}

C:\s>clang-cl 1.c

C:\s>1.exe

C:\s>echo %errorlevel%
-1073741819

That is 0xc0000005 access violation.
This program returns 2 with Visual C++.

Hm not sure about this toolchain, I'll try another later.
</details>

[malkia]

It's known limitation of MSVC compatibility:

This works

void test()
{
  *(volatile char*) 0;
}

int main()
{
 __try { test(); } __except(1) { return 2; }
}

From - https://clang.llvm.org/docs/MSVCCompatibility.html

    Asynchronous Exceptions (SEH): Partial. Structured exceptions (__try / __except / __finally) mostly work on x86 and x64. LLVM does not model asynchronous exceptions, so it is currently impossible to catch an asynchronous exception generated in the `same frame` as the catching __try.

So as long as it's not in the same frame - e.g. move this behind a non-inlinable function.

I was actually wondering about this, as we are heavy memory mapped file users, and have to guard parts of the code with this for when STATUS_IN_PAGE_ERROR may happen. Knowing that this workaround is going to help us, we can write a helper utility function that rewraps this (granted, possibly slower)