'this' is unavailable when entering constructor

[SLTozer]

Bugzilla Link	43195
Version	trunk
OS	Windows NT
Attachments	File containing the reproducible case
CC	@dwblaikie,@pogo59,@rnk

Extended Description

When building the attached file with clang-cl using the flags /Od /Zi and running the output with the Visual Studio 15 2017 debugger, evaluating this at line 8 (before initialization) gives the result "Variable is optimized away and not available.". This behaviour is different to both clang+lldb and pure MSVC, both of which give this the address of the uninitialized object. This occurs whether the MSVC linker or lld is used.

clang version 10.0.0
Target: x86_64-pc-windows-msvc
Thread model: posix

[SLTozer]

Correction to the above: the line at which this is not available is line 3, not line 8.

[rnk]

I haven't opened the attached source file yet, but I believe this is known behavior. Currently the user has to step into a function body once in order to see parameters. There may be a duplicate issue in the tracker already, I don't have time to search.

This is difficult to fix because clang copies all parameters into fresh allocas in the newly allocated stack frame instead of using the memory locations provided by the caller, if they exist.

[SLTozer]

That makes sense to me, but it sounds like that would mean that this is always unavailable before stepping into the body of a member function. When using clang -O0 -g and debugging with lldb (all on linux), breaking in the function header and entering p *this gives you the uninitialized value of the object. Is this due to a known difference clang-cl+MSVC on windows and clang+lldb on linux?

[rnk]

IIRC they have some mechanism for tracking where the end of the prologue is, so when you break on a function, the current PC is actually somewhere later in the function body. I think there may be analogous functionality in VS (I was hoping it was DbgStart in S_FRAMEPROC...), but I don't know where it is.

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Stephen Tozer (SLTozer)

| | | | --- | --- | | Bugzilla Link | [43195](https://llvm.org/bz43195) | | Version | trunk | | OS | Windows NT | | Attachments | [File containing the reproducible case](https://user-images.githubusercontent.com/5277830/143759707-c1db6faf-4e74-4ce7-b91b-867b77a0531b.gz) | | CC | @dwblaikie,@pogo59,@rnk |

Extended Description

When building the attached file with clang-cl using the flags /Od /Zi and running the output with the Visual Studio 15 2017 debugger, evaluating this at line 8 (before initialization) gives the result "Variable is optimized away and not available.". This behaviour is different to both clang+lldb and pure MSVC, both of which give this the address of the uninitialized object. This occurs whether the MSVC linker or lld is used.

clang version 10.0.0
Target: x86_64-pc-windows-msvc
Thread model: posix

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Stephen Tozer (SLTozer)

| | | | --- | --- | | Bugzilla Link | [43195](https://llvm.org/bz43195) | | Version | trunk | | OS | Windows NT | | Attachments | [File containing the reproducible case](https://user-images.githubusercontent.com/5277830/143759707-c1db6faf-4e74-4ce7-b91b-867b77a0531b.gz) | | CC | @dwblaikie,@pogo59,@rnk |

Extended Description

When building the attached file with clang-cl using the flags /Od /Zi and running the output with the Visual Studio 15 2017 debugger, evaluating this at line 8 (before initialization) gives the result "Variable is optimized away and not available.". This behaviour is different to both clang+lldb and pure MSVC, both of which give this the address of the uninitialized object. This occurs whether the MSVC linker or lld is used.

clang version 10.0.0
Target: x86_64-pc-windows-msvc
Thread model: posix