Columns UI can now be compiled using Clang

After a fair bit of work, release builds of Columns UI can now be compiled with Clang1 (and the builds produced seem to work πŸ™Œ).

Clang attempts to be compatible with Visual C++ – in terms of ABI, some Microsoft language extensions and supporting the same compiler options (not all are implemented, though). It also integrates with Visual Studio and can be used from both the IDE and using MSBuild (by changing the project platform toolset to e.g. LLVM-vs2014).

The Columns UI code base would not compile as is, because Clang is far stricter on C++ compliance. I encountered problems both in the foobar2000 SDK and in Columns UI code.

The bulk of errors were due to:

Visual C++ allowing rvalues to bind to non-const lvalues

That often presented itself when working with the foobar2000 in code following patterns like pfc::string_formatter() << ... << ... and console::formatter() << ... << ....

It also presented itself when temporary objects were used as function arguments when the parameter type was an lvalue, e.g. run_v2(..., abort_callback_dummy()); (Here, the type of the last parameter is abort_callback&.)

The fix I took in instances of this was just to use a non-temporary object:

pfc::string_formatter formatter;  
formatter << ... << ...;  

This class of problem is actually logged by Visual C++ under warning C4239, but you have to use at least warning level 4 (/W4) or explicitly turn the warning on. (Apparently, the default warning level is 1...)

Taking the address of rvalues

Not dissimilar to the previous problem, this often cropped up in function arguments with temporary objects: run(&titleformat_hook_impl_file_info(...), ...);

This one appears to be warning C4238 in Visual C++.

These are examples of some of the changes I made to the foobar2000 SDK to get it compiling with Clang.

What was the point of doing this?

I don’t intend to switch from the Visual C++ compiler for releases. However, the Clang compiler revealed various code-related errors that were previously unknown to me. This is partly due to it being better at picking up problematic code, and partly due to better C++ compliance.

In general terms, I have more confidence in the code base, and reassurance that it won't take too much effort to get it to work with new tools in the future.

It only compiles in release configurations and using Clang 3.9.0 pre-release builds

Trying to compile with Clang 3.8.0 64-bit in release mode results in the following:

         Unwind edges out of a funclet pad must have the same unwind dest
           %94 = cleanuppad within none [], !dbg !787
           cleanupret from %100 unwind to caller, !dbg !839
           cleanupret from %94 unwind label %ehcleanup101, !dbg !787
    10>CL : fatal error : error in backend: Broken function found, compilation aborted! [F:\foobar2000\columns_ui\foo_ui_columns\foo_ui_columns.vcxproj]

A quick search on their bug tracker throws up Clang/LLVM bug 26698. That was marked as fixed only in February, and, indeed, the current snapshot build of Clang 3.9.0 (r268958) works fine for release builds.

Debug builds with both 3.8.0 and snapshot build r268958 yield the following, though:

CL : fatal error : error in backend: assembler label 'L?query_cpu_feature_set@pfc@@YA_NI@Z$parent_frame_offset' can not be undefined [F:\foobar2000\columns_ui\pfc\pfc.vcxproj]  

I haven't worked out as yet why that only happens for debug builds.

Compiling using Clang

Rather than faffing about with my project files, I am just overriding the platform toolset using MSBuild in a VS2015 x86 Native Tools command prompt.

For example, running:

msbuild /m /p:PlatformToolset=LLVM-vs2014 /p:Platform=Win32 /p:Configuration=Release /t:Rebuild vc14\columns_ui-public.sln  

One other problem I ran into is that if Inherit from project or project defaults isn't turned on in Visual Studio for Additional Options in C/C++/Command Line in your project properties, you will encounter various problems such as compiler errors in Windows SDK headers (about e.g. char16_t and char32_t not being defined). (This happens because having that option off stops certain options like -fmsc-version=1900 and -m32/-m64 from being passed to clang-cl.)

It's worth noting that building Columns UI using Clang is far slower than the Visual C++ compiler build (~9 minutes vs ~33 seconds on my PC). The Clang build is not using precompiled headers (it only supports them if using /FI at present).


Another benefit of the code being compilable with clang is that it opens up the possibility of using clang-tidy.

Apart from checking code for possible errors, clang-tidy can modernise code by, for example, replacing null constants (e.g. NULL and 0) with nullptr and adding the override specifier to member functions that override virtual functions of a base class.

A full list of checks available in clang-tidy can be found here.

Here's an example command for running it (from a VS2015 x86 Native Tools command prompt, under Clang 3.9.0 build r268958):

clang-tidy foo_ui_columns/main_window.cpp -checks=-*,modernize-* -fix -- -fmsc-version=1900 -DUNICODE -D_UNICODE -DNDEBUG -DWIN32_WINNT=0x0600 -DWIN32 -D_WINDOWS -D_WINDLL -D_USRDLL -m32  

-checks=-*,modernize-* enables code-modernisation checks (and disables the default checks), and -fix applies any available fixes.

It almost works:

C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\um\winnt.h:935:5: error: MS-style inline assembly is not available: Unable to find target for this triple (no targets are registered) [clang-diagnostic-error]  
    __asm    {
C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\um\winnt.h:951:5: error: MS-style inline assembly is not available: Unable to find target for this triple (no targets are registered) [clang-diagnostic-error]  
    __asm {
C:\Program Files (x86)\Windows Kits\10\include\10.0.10586.0\um\winnt.h:967:5: error: MS-style inline assembly is not available: Unable to find target for this triple (no targets are registered) [clang-diagnostic-error]  
    __asm    {
F:\foobar2000\columns_ui\foo_ui_columns/../foobar2000/SDK/../../pfc/audio_sample.h:48:4: error: MS-style inline assembly is not available: Unable to find target for this triple (no targets are registered) [clang-diagnostic-error]  
                        _asm {
F:\foobar2000\columns_ui\foo_ui_columns/../foobar2000/SDK/../../pfc/audio_sample.h:56:4: error: MS-style inline assembly is not available: Unable to find target for this triple (no targets are registered) [clang-diagnostic-error]  
                        _asm {

I've tried specifying a target triple using --target, but that either breaks things completely, or it doesn't help. Perhaps I ought to post about this on their mailing list.

However, despite those errors, clang-tidy does carry on and perform the requested checks on the input file specified, but stops short of applying the fixes. You can additionally supply -fix-errors to make it apply fixes in spite of errors, though (so I am still able to use it).

Update 17 September 2016

As it turns out, someone has already submitted a patch for the clang-tidy MS-style inline assembly problem. However, it hasn't been accepted as yet, unfortunately.

What's next

It wasn't easy, but getting Columns UI compiling with Clang was a worthwhile effort. When I get some time, I'll have a look at the checks available in clang-tidy in more detail.

Another thing worth mentioning is that ReSharper C++ has been invaluable throughout all this, but perhaps I'll write about that another time.

  1. There are a large number of warnings, which are still to be looked at in detail. ↩