[MCA] Fix MSVC 19.16 build with libc++

[orivej]

Summary

MSVC (19.16) wants to see the definition of Instruction in std::pair<unsigned, const Instruction &> SourceRef to decide if it is assignable:

libcxx/include/type_traits:2209: error C2139: 'llvm::mca::Instruction': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_assignable'
llvm/include/llvm/mca/sourcemgr.h:31: note: see declaration of 'llvm::mca::Instruction'
libcxx/include/type_traits:2226: note: see reference to class template instantiation 'std::is_assignable<const llvm::mca::Instruction &,const llvm::mca::Instruction &>' being compiled
libcxx/include/utility:524: note: see reference to class template instantiation 'std::is_copy_assignable<const llvm::mca::Instruction &>' being compiled
llvm/include/llvm/mca/sourcemgr.h:51: note: see reference to class template instantiation 'std::pair<unsigned int,const llvm::mca::Instruction &>' being compiled

[andreadb]

A colleague ( @gbedwell ) pointed out that this is likely to be the same MSVC bug originally reported here:
https://developercommunity.visualstudio.com/content/problem/343550/webkit-an-undefined-class-is-not-allowed-as-an-arg.html.

That issue has been fixed by a more recent version of MSVC (presumably 19.20)
https://godbolt.org/resetlayout/vka9Vf

If that's the case, then your change is a good workaround. Maybe add a comment to the new include mentioning the problematic MSVC version, so that we can re-evaluate it in future.

Thanks!
-Andrea

[orivej]

Add a comment that this affects only MSVC >= 19.15, < 19.20.

[orivej]

Could you commit this on my behalf? Please edit the comment if you wish.

[andreadb]

In D65844#1622256, @orivej wrote:

Could you commit this on my behalf? Please edit the comment if you wish.

Sure. I'll commit it on your behalf.
That code comment is perfect. Thanks!

llvm/trunk/include/llvm/MCA/SourceMgr.h

 /// every instruction in the sequence.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_MCA_SOURCEMGR_H
 #define LLVM_MCA_SOURCEMGR_H
 
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/MCA/Instruction.h"
 
 namespace llvm {
 namespace mca {
 
-class Instruction;
-
+// MSVC >= 19.15, < 19.20 need to see the definition of class Instruction to
+// prevent compiler error C2139 about intrinsic type trait '__is_assignable'.
 typedef std::pair<unsigned, const Instruction &> SourceRef;
 
 class SourceMgr {
   using UniqueInst = std::unique_ptr<Instruction>;
   ArrayRef<UniqueInst> Sequence;
   unsigned Current;
   const unsigned Iterations;
   static const unsigned DefaultIterations = 100;