—Categories: Compilers, Fortran, LLVM

Failing to Build Flang with Visual C++

Background

Elmer is the first codebase that I have dug into that has a substantial (or really any) amount of Fortran code. I used GFortran to build it but went digging around for a clang based compiler. I found llvm-project/flang and since I had been building LLVM earlier this year, I figured it should be straightforward to build flang and perhaps explore it in a debugger.

My first attempt to build flang (on Windows, my primary OS) resulted in many build errors. Unfortunately, I was using a preview Visual Studio build, so I didn’t want to compare the errors with those from a different machine because it wasn’t the same compiler version in use. I decided to use an RTM Visual Studio compiler (VS 17.2.5) to avoid possible compiler bugs present only in VS preview builds since most people would not be using preview VS builds anyway.

Without giving it much thought, my suspicion was that any build failures probably arose from not using the correct C++ version. The source code I was trying to build (commit c0702ac0) states that it uses C++17. I set this in CMake by defining the CXX_STANDARD property. Here is the full cmake command line I used to set up the build.

cd llvm-project
mkdir build
cd build

cmake \
  -G Ninja \
  ../llvm \
  -DCMAKE_BUILD_TYPE=Release \
  -DFLANG_ENABLE_WERROR=On \
  -DLLVM_ENABLE_ASSERTIONS=ON \
  -DLLVM_TARGETS_TO_BUILD=host \
  -DCMAKE_INSTALL_PREFIX=../install
  -DLLVM_LIT_ARGS=-v \
  -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" \
  -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
  -DCXX_STANDARD=17

# Shown here without \ to be executable in cmd.exe
cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release -DFLANG_ENABLE_WERROR=On -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_TARGETS_TO_BUILD=host -DCMAKE_INSTALL_PREFIX=../install -DLLVM_LIT_ARGS=-v -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" -DLLVM_ENABLE_RUNTIMES="compiler-rt" -DCXX_STANDARD=17

That took about 2 minutes on my machine after which I ran ninja to start the build

ninja

Unfortunately, the build failed! The first error I encountered was in fold-real.cpp. Here is the command line used to invoke the compiler (shown with newlines to simplify interpretation, see Compiler options listed alphabetically | Microsoft Docs for the complete list of compiler options).

 C:\PROGRA~1\MIB055~1\2022\ENTERP~1\VC\Tools\MSVC\1432~1.313\bin\Hostx64\x64\cl.exe
 /nologo
 /TP
 -DFLANG_LITTLE_ENDIAN=1
 -DGTEST_HAS_RTTI=0 -DUNICODE
 -D_CRT_NONSTDC_NO_DEPRECATE
 ...
 -D__STDC_LIMIT_MACROS
 -ID:\dev\repos\llvm-project\build-cpp17\tools\flang\lib\Evaluate
 ...
 -ID:\dev\repos\llvm-project\llvm\include
 -external:I D:\dev\repos\llvm-project\llvm\..\mlir\include
 ...
 -external:I D:\dev\repos\llvm-project\llvm\..\clang\include
 -external:W0
 /DWIN32
 /D_WINDOWS
 /Zc:inline
 /Zc:__cplusplus
 /Oi
 /bigobj
 /permissive-
 /W4
 -wd4141
 ...
 -wd4324
 -w14062
 -we4238
 /Gw
 /WX
 /MD
 /O2
 /Ob2
 /EHs-c-
 /GR-
 -UNDEBUG
 -std:c++17
 /showIncludes
 /Fotools\flang\lib\Evaluate\CMakeFiles\obj.FortranEvaluate.dir\fold-real.cpp.obj
 /Fdtools\flang\lib\Evaluate\CMakeFiles\obj.FortranEvaluate.dir\
 /FS
 -c
 D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-real.cpp

I had to look up the meaning of the C++ syntax on that line to understand why the build could be failing. Turns out to be a lambda, as explained at c++ – What is a lambda expression in C++11? – Stack Overflow.

I tried manually creating a repro for this compiler issue by creating a new Visual C++ project in Visual Studio and recreating the structure of the code failing to build. One of the questions I had was how to set conformance mode in a Visual Studio Cmake project. I still haven’t yet figured this out. However, one of the issues I ran into was that my cmake project was building the code without the /permissive- flag! I ended up switching to a regular Visual C++ project (.vcxproj) since I knew how to change the compiler options reliably for such projects. After struggling with recreating the code, I realized that I would make more progress removing code from flang’s fold-real.cpp instead. Here are some of the other searches and concepts I had to look up to understand the code while trying to create a minimal repro of the build failure.

  1. c++11 – how to remove error : X is not a class template – Stack Overflow
  2. c++ – What does template <unsigned int N> mean? – Stack Overflow
  3. what does this … (three dots) means in c++ – Stack Overflow
  4. create a reference in c++ -> Reference of Reference in C++ – Stack Overflow
  5. c++ using namespace -> Namespaces (C++) | Microsoft Docs
  6. using typedef -> Aliases and typedefs (C++) | Microsoft Docs
  7. move mechanics -> C++ Move Semantics Introduction | hacking C++ (hackingcpp.com)
  8. c++ – What is std::decay and when it should be used? – Stack Overflow

I was eventually able to create a simpler test case showing that the flang code could not build with my RTM compiler.

cl /std:c++17 /permissive- flang-msvc-clang-test.cpp

Microsoft (R) C/C++ Optimizing Compiler Version 19.32.31332 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

flang-msvc-clang-test.cpp
flang-msvc-clang-test.cpp(159): error C2065: 'T': undeclared identifier
flang-msvc-clang-test.cpp(48): note: see reference to function template instantiation 'auto FoldIntrinsicFunction::<lambda_1>::operator ()<_First>(const _T1 &) const' being compiled
        with
        [
            _First=Expr<Type<TypeCategory::Real,1>>,
            _T1=Expr<Type<TypeCategory::Real,1>>
        ]
flang-msvc-clang-test.cpp(171): note: see reference to function template instantiation 'Expr<Type<TypeCategory::Real,2>> FoldIntrinsicFunction<2>(FoldingContext &,FunctionRef<Type<TypeCategory::Real,2>> &&)' being compiled
flang-msvc-clang-test.cpp(159): error C2923: 'Scalar': 'T' is not a valid template type argument for parameter 'T'
flang-msvc-clang-test.cpp(159): note: see declaration of 'T'

So after all that, the RTM LTS Visual C++ compiler turned out to have a bug. Turns out the Visual C++ folks had already fixed this issue so the way to unblock myself was to switch to the preview Visual Studio build :(! The irony…

Suppressing Warnings

Armed with a preview build that correctly compiled the test case, the next obstacle in the build process was a set of warnings that were treated as errors: C4661 and C4101.

FAILED: tools/flang/lib/Evaluate/CMakeFiles/obj.FortranEvaluate.dir/fold.cpp.obj
C:\...\cl.exe ... -c D:\dev\repos\llvm-project\flang\lib\Evaluate\fold.cpp
D:\dev\repos\llvm-project\flang\include\flang\Evaluate\expression.h(101): error C2220: the following warning is treated as an error
D:\dev\repos\llvm-project\flang\include\flang\Evaluate\expression.h(101): warning C4661: 'std::optional<Fortran::evaluate::DynamicType> Fortran::evaluate::ExpressionBase<Fortran::evaluate::SomeDerived>::GetType(void) const': no suitable definition provided for explicit template instantiation request
...
FAILED: tools/flang/lib/Evaluate/CMakeFiles/obj.FortranEvaluate.dir/fold-complex.cpp.obj
C:\...\cl.exe ... -c D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-complex.cpp
D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-implementation.h(1583): error C2220: the following warning is treated as an error
D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-implementation.h(1583): warning C4101: 'buffer': unreferenced local variable

I tried to suppressed them to keep marching forward:

cd \dev\repos\llvm-project
mkdir build-nowarn
cd build-nowarn

cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release -DFLANG_ENABLE_WERROR=On -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_TARGETS_TO_BUILD=host -DCMAKE_INSTALL_PREFIX=../install -DLLVM_LIT_ARGS=-v -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" -DLLVM_ENABLE_RUNTIMES="compiler-rt" -DCXX_STANDARD=17 -DCXX_FLAGS="-wd4661 -wd4101"

ninja

Defining CXX_FLAGS like that did not work so I end up looking around for how to disable warnings in cmake. This was when I discovered that CMAKE_CXX_STANDARD is not necessary on the command line because flang/CMakeLists.txt already requires C++17. Trying to append the warning disable option /wdXXXX to that file didn’t work either. However, the comment on line 329 made me explore HandleLLVMOptions.cmake. There, I discovered support for setting the number of parallel jobs (via /MP for Visual C++). This file also contained the code that sets up most of the compiler options used when building! Closer to the task at hand is the discover of the LLVM_ENABLE_WARNINGS option and the hard-coded list of MSVC warning flags! I therefore made this change (before running cmake and ninja) to get the warning flags to be respected:

diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 56d05f5b5fce..589281b232f1 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -648,6 +648,8 @@ if (MSVC)
           # v15.8.8. Re-evaluate the usefulness of this diagnostic when the bug
           # is fixed.
       -wd4709 # Suppress comma operator within array index expression
+      -wd4101  # Suppress ...
+      -wd4661  # Suppress ...

       # Ideally, we'd like this warning to be enabled, but even MSVC 2019 doesn't
       # support the 'aligned' attribute in the way that clang sources requires (for

cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release -DFLANG_ENABLE_WERROR=On -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_TARGETS_TO_BUILD=host -DCMAKE_INSTALL_PREFIX=../install -DLLVM_LIT_ARGS=-v -DLLVM_ENABLE_PROJECTS="clang;mlir;flang" -DLLVM_ENABLE_RUNTIMES="compiler-rt"

Another Compiler Failure

With the aforementioned change, the build proceeded to a different build failure, this time in fold-integer.cpp.

FAILED: tools/flang/lib/Evaluate/CMakeFiles/obj.FortranEvaluate.dir/fold-integer.cpp.obj
C:\PROGRA~1\MIB055~1\2022\Preview\VC\Tools\MSVC\1433~1.316\bin\Hostx64\x64\cl.exe  /nologo /TP -DFLANG_LITTLE_ENDIAN=1 -DGTEST_HAS_RTTI=0 -DUNICODE -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_NONSTDC_NO_WARNINGS -D_CRT_SECURE_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -D_HAS_EXCEPTIONS=0 -D_SCL_SECURE_NO_DEPRECATE -D_SCL_SECURE_NO_WARNINGS -D_UNICODE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -ID:\dev\repos\llvm-project\build-vsmain\tools\flang\lib\Evaluate -ID:\dev\repos\llvm-project\flang\lib\Evaluate -ID:\dev\repos\llvm-project\flang\include -ID:\dev\repos\llvm-project\build-vsmain\tools\flang\include -ID:\dev\repos\llvm-project\build-vsmain\include -ID:\dev\repos\llvm-project\llvm\include -external:ID:\dev\repos\llvm-project\llvm\..\mlir\include -external:ID:\dev\repos\llvm-project\build-vsmain\tools\mlir\include -external:ID:\dev\repos\llvm-project\build-vsmain\tools\clang\include -external:ID:\dev\repos\llvm-project\llvm\..\clang\include -external:W0 /DWIN32 /D_WINDOWS   /Zc:inline /Zc:__cplusplus /Oi /bigobj /permissive- /W4 -wd4141 -wd4146 -wd4244 -wd4267 -wd4291 -wd4351 -wd4456 -wd4457 -wd4458 -wd4459 -wd4503 -wd4624 -wd4722 -wd4100 -wd4127 -wd4512 -wd4505 -wd4610 -wd4510 -wd4702 -wd4245 -wd4706 -wd4310 -wd4701 -wd4703 -wd4389 -wd4611 -wd4805 -wd4204 -wd4577 -wd4091 -wd4592 -wd4319 -wd4709 -wd4101 -wd4661 -wd4324 -w14062 -we4238 /Gw /WX /MD /O2 /Ob2  /EHs-c- /GR- -UNDEBUG -std:c++17 /showIncludes /Fotools\flang\lib\Evaluate\CMakeFiles\obj.FortranEvaluate.dir\fold-integer.cpp.obj /Fdtools\flang\lib\Evaluate\CMakeFiles\obj.FortranEvaluate.dir\ /FS -c D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp
D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(771): error C2672: 'invoke': no matching overloaded function found
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.33.31627\include\type_traits(1552): note: could be 'unknown-type std::invoke(_Callable &&,_Ty1 &&,_Types2 &&...) noexcept(<expr>)'
D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(771): note: Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Ty1 &&,_Types2 &&...) noexcept(<expr>)'
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.33.31627\include\type_traits(1552): note: see declaration of 'std::invoke'
...

By this point, I knew that simplifying the function containing the error was the fastest path to a repro. One of the little problems I ran into was how to figure out the type of fptr since it is declared using the auto keyword. I ended up assigning it to a new temporary variable of a different type, e.g. char et voila!

D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(505): error C2440: 'initializing': cannot convert from 'int (__cdecl Fortran::evaluate::value::Integer<8,true,8,unsigned char,unsigned short>::* )(void) const' to 'char'

I then removed the temporary assignment and explicitly specified this type as the type of fptr:

using T2 = int (__cdecl Fortran::evaluate::value::Integer<8,true,8,unsigned char,unsigned short>::* )(void) const;

T2 fptr{&Scalar<TI>::LEADZ};

The build then failed because the function pointer types are not the same, which was really confusing given that I had just checked the type of fptr.

D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(504): error C2440: 'initializing': cannot convert from
'int (__cdecl Fortran::evaluate::value::Integer<16,true,16,unsigned short,unsigned int>::* )(void) const' to
'int (__cdecl Fortran::evaluate::value::Integer<8,true,8,unsigned char,unsigned short>::* )(void) const'

I switched the type of fptr and got a different error:

D:\dev\repos\llvm-project\flang\include\flang\Evaluate\integer.h(66): error C2607: static assertion failed
D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(490): note: see reference to class template instantiation 'Fortran::evaluate::value::Integer<16,true,16,unsigned char,unsigned short>' being compiled

Here is a different change I tried:

using T2 = int (__cdecl Fortran::evaluate::value::Integer<8>::* )(void) const;

T2 fptr{&Scalar<TI>::LEADZ};

That still failed with the following error:

D:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(504): error C2440: 'initializing': cannot convert from
'int (__cdecl Fortran::evaluate::value::Integer<8,true,8,unsigned char,unsigned short>::* )(void) const' to
'int (__cdecl Fortran::evaluate::value::Integer<32,true,32,unsigned int,unsigned __int64>::* )(void) const'

It was at this point that I realized that it was time to learn a bit more about decay. What is decay and array-to-pointer conversion? | C++ FAQ (64.github.io) had a good explanation of why the term decay is used. Perhaps a reexamination of std::decay – cppreference.com might lead to some insight. I wasn’t sure what Result referred to in the statement using TI = typename std::decay_t<decltype(n)>::Result; One idea I got was to append a number to the typename and examine the compiler error. Here’s the new line 752 of llvm-project/fold-integer.cpp and the resulting compiler error showing that this name cannot be arbitrary.

using TI = typename std::decay_t<decltype(n)>::Result3;


C:\dev\repos\llvm-project\flang\lib\Evaluate\fold-integer.cpp(502): error C2039: 'Result3': is not a member of 'Fortran::evaluate::Expr<Fortran::evaluate::Type<Fortran::common::TypeCategory::Integer,1>>'

Aha, so what it was referring to is the using statement in llvm-project/expression.h!

template <int KIND>
class Expr<Type<TypeCategory::Integer, KIND>>
    : public ExpressionBase<Type<TypeCategory::Integer, KIND>> {
public:
  using Result = Type<TypeCategory::Integer, KIND>;

...

The problematic lambda is therefore expecting a Scalar<Type<TypeCategory::Integer, KIND>>. Scalar is defined using decay and Type<TypeCategory::Integer, KIND>::Scalar is defined in llvm-project/type.h as the type value::Integer<8 * KIND>. This is when I see the reason for the previous build errors about mismatched Integer sizes no matter which size I picked – the fixed type I was using didn’t allow for the different template instantiations! Note that the problematic lambda is defined as a ScalarFunc.

By this point, I had a self-contained repro of the compiler bug, which ironically, compiled successfully on the RTM C++ compiler so I could use neither the preview nor the RTM to build the flang code.

cl /c /TP /std:c++17 /permissive- flang-msvc-clang-test-02.cpp

This compiler invocation gives the same error seen when compiling the flang code:

Microsoft (R) C/C++ Optimizing Compiler Version 19.33.31627.1 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

flang-msvc-clang-test-02.cpp
flang-msvc-clang-test-02.cpp(193): error C2672: 'invoke': no matching overloaded function found
C:\Program Files\Microsoft Visual Studio\2022\Preview\VC\Tools\MSVC\14.34.31721\include\type_traits(1552): note: could be 'unknown-type std::invoke(_Callable &&,_Ty1 &&,_Types2 &&...) noexcept(<expr>)'
flang-msvc-clang-test-02.cpp(193): note: Failed to specialize function template 'unknown-type std::invoke(_Callable &&,_Ty1 &&,_Types2 &&...) noexcept(<expr>)'

I ended up reporting this compiler bug via the Visual Studio feedback system – see C++17 lambda fails to compile on latest VS preview compiler.

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