[clang] unused-function diagnostic regression?

[kornman00]

I recently downloaded 2.9.2, after using 2.8.2. I was using -Werror=unused-function to remove some dead code but noticed I was not hitting any errors after performing a rebuild on a library. After switching back to 2.8.2 in my msbuild .props files I once again get errors about unused functions as expected.

For 2.8.2, I'm setting LLVMInstallDir to my local llvm-msvc 2.8.2 copy and LLVMToolsVersion to 17. For 2.9.2, I'm setting LLVMInstallDir to my local llvm-msvc 2.9.2 copy and LLVMToolsVersion to 18.

I'm using the latest VS2022 Preview (17.8 Preview 1) in the event that it matters.

Note, I have not tried stock LLVM v18 - may be possible it was regressed there?

[kornman00]

Isn't the compatible solution to default to or implicitly set -Wno-unused-function :)? Then people that desire the warning can say -Wunused-function?

For context, I'm building code that was previously only ever compiled with MSVC. I'm building with warnings-as-errors but have toggled off or no-error'd some warnings which I want to address later, like unused-function. Also going through the exercise of figuring out how to get clang or the linker warn about which public symbols are unused (or if its even possible) since unused-function only seems to apply to static linkage or LLVM's nature of omitting unused symbols short circuits any additional reporting.

[kornman00]

Sorry, haven't had time to setup a test until today. I tested with v3.0.8 and it still is silent about unused functions and variables, even though I have them explicitly set as warnings.

I realize that in my original post that I mentioned I had these set as errors, but for iteration of catching the list of everything that appears as unused (at least in 2.8.2, before the change you mentioned on August 8th) I set these to show up as warnings still, while not changing the overall default of warnings-as-errors. Here's a subset of what I have in a .props file:

    <ClangClAdditionalOptions>
      -Wno-unused-value
      -Wno-error=unused-variable
      -Wno-error=unused-function
      -Wno-error=unused-member-function
      -Wno-error=unused-const-variable
      -Wno-unused-but-set-variable
      -Wno-unused-local-typedef
      -Wno-unused-but-set-parameter
      $(ClangClAdditionalOptions)
    </ClangClAdditionalOptions>

Here's the current output, for what it is worth (names have been changed to protect the innocent):

1>------ Rebuild All started: Project: TheProject, Configuration: TheProjectConfiguation_clang x64 ------
1>llvm-msvc(v3.0.8) spent 4.193s in TheProjectPCH.cpp
1>llvm-msvc(v3.0.8) spent 0.100s in TheProjectPCH.cpp
1>TheProject.vcxproj -> TheProject.lib
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========
========== Rebuild started at 10:53 AM and took 01:33.943 minutes ==========

But when I compile with 2.8.2 there is of course much more output (and takes ~12s less time to rebuild, whatever that anecdote may mean to you).

[kornman00]

#include <cstdio>

static void this_function_is_unused();

int main()
{
    puts("Hello, World");
}

static void this_function_is_unused()
{
    // This function should generate a -Wunused-function warning under ClangCL
}

1>------ Build started: Project: llvm-msvc-issue-109, Configuration: Debug x64 ------
1>llvm-msvc-issue-109.cpp(10,13): warning : unused function 'this_function_is_unused' [-Wunused-function]
1>llvm-msvc-issue-109.vcxproj -> D:\SourceCode\KS\llvm-msvc-issue-109\x64\Debug\llvm-msvc-issue-109.exe

In the attached minimal repro vcxproj file, if you change <PlatformToolsetUseLlvmMsvcFork> on line 25 to true and ensure <LLVMInstallDir> on line 31 is updated to your llvm-msvc install (I tested today with 3.2.6) you will hit the issue where -Wunused-function is not surfaced.

1>------ Rebuild All started: Project: llvm-msvc-issue-109, Configuration: Debug x64 ------
1>llvm-msvc(v3.2.6) spent 0.040s in llvm-msvc-issue-109.cpp
1>llvm-msvc-issue-109.vcxproj -> D:\SourceCode\KS\llvm-msvc-issue-109\x64\Debug\llvm-msvc-issue-109.exe

Even though I have the warning level set to 4, with warnings as error, and -Wunused-function set to be non-error warning:

  <PropertyGroup Label="LLVM" Condition="$(PlatformToolset)=='ClangCL'">
    <ClangClAdditionalOptions>
      -Wno-error=unused-variable
      -Wno-error=unused-function
      -Wno-error=unused-member-function
      -Wno-error=unused-const-variable
      -Wno-unused-but-set-variable
      -Wno-unused-local-typedef
      -Wno-unused-but-set-parameter
      $(ClangClAdditionalOptions)
    </ClangClAdditionalOptions>
  </PropertyGroup>

llvm-msvc-issue-109.min_repro.zip

[kornman00]

Ah, okay, if I have the following:

  <PropertyGroup Label="LLVM" Condition="$(PlatformToolset)=='ClangCL'">
    <ClangClAdditionalOptions>
      -Wunused-function
      -Wno-error=unused-function
      $(ClangClAdditionalOptions)
    </ClangClAdditionalOptions>
  </PropertyGroup>

Where -Wunused-function is explicitly stated, then it reports the unused function. So, this issue can be closed as fixed.

For reference, where in llvm-msvc's code do you configure these normally-on-by-default warnings to be off by default for the sake of MSVC users that don't expect them? Is it in llvm-msvc specific code, or do you modify the llvm code where the warning is defined directly?

Thanks!