Implement Lambda Conversion Operators for All CCs for MSVC.

[erichkeane]

Summary

As described here:
https://devblogs.microsoft.com/oldnewthing/20150220-00/?p=44623

In order to allow Lambdas to be used with traditional Win32 APIs, they
emit a conversion function for (what Raymond Chen claims is all) a
number of the calling conventions. Through experimentation, we
discovered that the list isn't quite 'all'.

This patch implements this by taking the list of conversions that MSVC
emits (across 'all' architectures, I don't see any CCs on ARM), then
emits them if they are supported by the current target.

However, we also add 3 other options (which may be duplicates):
free-function, member-function, and operator() calling conventions. We
do this because we have an extension where we generate both free and
member for these cases so th at people specifying a calling convention
on the lambda will have the expected behavior when specifying one of
those two.

MSVC doesn't seem to permit specifying calling-convention on lambdas,
but we do, so we need to make sure those are emitted as well. We do this
so that clang-only conventions are supported if the user specifies them.

[aaron.ballman]

clang/lib/Sema/SemaLambda.cpp

1281

Should we call out that one of the existing member functions is likely to be thiscall which means we'll generate a version of the operator for that calling convention even though MSVC doesn't, but we want to do this because users can explicitly write the CC on the lambda (unlike in MSVC)? (I'm worried that lack of mention about thiscall may look like a bug to someone a few years down the line.)

[erichkeane]

update comment as @aaron.ballman requested.

[aaron.ballman]

clang/lib/Sema/SemaLambda.cpp

1282

If we're intentionally generating it, should it be listed explicitly in Convs below? (If not, then perhaps change the comment somewhat to "We intentionally generate a 'thiscall' (on Win32) implicitly from the default member call's calling convention..."?)

[erichkeane]

Made another attempt at fixing the comment. Wordsmithing welcomed/encouraged :)

[aaron.ballman]

This LGTM but you should wait a day or so in case @rjmccall has opinions.

[erichkeane]

In D90634#2371436, @aaron.ballman wrote:

This LGTM but you should wait a day or so in case @rjmccall has opinions.

Thanks, will do!

clang/lib/Sema/SemaLambda.cpp

1281

Can do! I put it in the commit message as well,

[rsmith]

clang/lib/Sema/SemaLambda.cpp

1285

clang/test/CodeGenCXX/lambda-conversion-op-cc.cpp

10

Does lambda-to-function-pointer decay still work (eg, +lambda or *lambda)? It'd be good to test that, since it's a fairly common idiom.

[erichkeane]

clang/test/CodeGenCXX/lambda-conversion-op-cc.cpp

10

It does! I tested that in the last patch, but don't seem to have a codegen test for it, so I'll make sure to add it.

I ended up having to do a tiebreaker as you suggested in that patch as well.

[erichkeane]

Added test for +lambda as @rsmith requested.

clang/lib/Sema/SemaLambda.cpp

 static void repeatForLambdaConversionFunctionCallingConvs(
     Sema &S, const FunctionProtoType &CallOpProto, Func F) {
   CallingConv DefaultFree = S.Context.getDefaultCallingConvention(
       CallOpProto.isVariadic(), /*IsCXXMethod=*/false);
   CallingConv DefaultMember = S.Context.getDefaultCallingConvention(
       CallOpProto.isVariadic(), /*IsCXXMethod=*/true);
   CallingConv CallOpCC = CallOpProto.getCallConv();
 
+  /// Implement emitting a version of the operator for many of the calling
+  /// conventions for MSVC, as described here:
+  /// https://devblogs.microsoft.com/oldnewthing/20150220-00/?p=44623.
+  /// Experimentally, we determined that cdecl, stdcall, fastcall, and
+  /// vectorcall are generated by MSVC when it is supported by the target.
+  /// Additionally, we are ensuring that the default-free/default-member and
+  /// call-operator calling convention are generated as well.
+  /// NOTE: We intentionally generate a 'thiscall' on Win32 implicitly from the
+  /// 'member default', despite MSVC not doing so. We do this in order to ensure
+  /// that someone who intentionally places 'thiscall' on the lambda call
+  /// operator will still get that overload, since we don't have the a way of
+  /// detecting the attribute by the time we get here.
+  if (S.getLangOpts().MSVCCompat) {
+    CallingConv Convs[] = {
+        CC_C,        CC_X86StdCall, CC_X86FastCall, CC_X86VectorCall,
+        DefaultFree, DefaultMember, CallOpCC};
+    llvm::sort(Convs);
+    llvm::iterator_range<CallingConv *> Range(
+        std::begin(Convs), std::unique(std::begin(Convs), std::end(Convs)));
+    const TargetInfo &TI = S.getASTContext().getTargetInfo();
+
+    for (CallingConv C : Range) {
+      if (TI.checkCallingConvention(C) == TargetInfo::CCCR_OK)
+        F(C);
+    }
+    return;
+  }
+
   if (CallOpCC == DefaultMember && DefaultMember != DefaultFree) {
     F(DefaultFree);
     F(DefaultMember);
   } else {
     F(CallOpCC);
   }
 }
 
static void addFunctionPointerConversion(Sema &S, SourceRange IntroducerRange,
   } else
     Class->addDecl(Invoke);
 }
 
 /// Add a lambda's conversion to function pointers, as described in
 /// C++11 [expr.prim.lambda]p6. Note that in most cases, this should emit only a
 /// single pointer conversion. In the event that the default calling convention
 /// for free and member functions is different, it will emit both conventions.
-/// FIXME: Implement emitting a version of the operator for EVERY calling
-/// convention for MSVC, as described here:
-/// https://devblogs.microsoft.com/oldnewthing/20150220-00/?p=44623.
 static void addFunctionPointerConversions(Sema &S, SourceRange IntroducerRange,
                                           CXXRecordDecl *Class,
                                           CXXMethodDecl *CallOperator) {
   const FunctionProtoType *CallOpProto =
       CallOperator->getType()->castAs<FunctionProtoType>();
 
   repeatForLambdaConversionFunctionCallingConvs(
       S, *CallOpProto, [&](CallingConv CC) {

clang/test/CodeGenCXX/lambda-conversion-op-cc.cpp

 // RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-linux-gnu | FileCheck %s --check-prefixes=CHECK,LIN64
 // RUN: %clang_cc1 -emit-llvm %s -o - -triple=x86_64-linux-gnu -DCC="__attribute__((vectorcall))" | FileCheck %s --check-prefixes=CHECK,VECCALL
-// RUN: %clang_cc1 -emit-llvm %s -o - -triple=i386-windows-pc -DWIN32 | FileCheck %s --check-prefixes=WIN32
+// RUN: %clang_cc1 -emit-llvm %s -o - -fms-compatibility -triple=i386-windows-pc -DWIN32 | FileCheck %s --check-prefixes=WIN32
 
 #ifndef CC
 #define CC
 #endif
 
 void usage() {
   auto lambda = [](int i, float f, double d) CC { return i + f + d; };
 
   double (*CC fp)(int, float, double) = lambda;
-  fp(0, 1.1, 2.2);
 #ifdef WIN32
   double (*__attribute__((thiscall)) fp2)(int, float, double) = lambda;
+  double (*__attribute__((stdcall)) fp3)(int, float, double) = lambda;
+  double (*__attribute__((fastcall)) fp4)(int, float, double) = lambda;
+  double (*__attribute__((vectorcall)) fp5)(int, float, double) = lambda;
+#endif // WIN32
+  fp(0, 1.1, 2.2);
+#ifdef WIN32
   fp2(0, 1.1, 2.2);
+  fp3(0, 1.1, 2.2);
+  fp4(0, 1.1, 2.2);
+  fp5(0, 1.1, 2.2);
 #endif // WIN32
+
+  auto x = +lambda;
 }
 
-// void usage function, calls convrsion operator.
+// void usage function, calls conversion operator.
 // LIN64: define void @_Z5usagev()
 // VECCALL: define void @_Z5usagev()
 // WIN32: define dso_local void @"?usage@@YAXXZ"()
 // CHECK: call double (i32, float, double)* @"_ZZ5usagevENK3$_0cvPFdifdEEv"
 // WIN32: call x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6A?A?<auto>@@HMN@ZXZ"
 // WIN32: call x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6E?A?<auto>@@HMN@ZXZ"
+// WIN32: call x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6G?A?<auto>@@HMN@ZXZ"
+// WIN32: call x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6I?A?<auto>@@HMN@ZXZ"
+// WIN32: call x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6Q?A?<auto>@@HMN@ZXZ"
+// Operator+ calls 'default' calling convention.
+// CHECK: call double (i32, float, double)* @"_ZZ5usagevENK3$_0cvPFdifdEEv"
+// WIN32: call x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6A?A?<auto>@@HMN@ZXZ"
 //
 // Conversion operator, returns __invoke.
 // CHECK: define internal double (i32, float, double)* @"_ZZ5usagevENK3$_0cvPFdifdEEv"
 // CHECK: ret double (i32, float, double)* @"_ZZ5usagevEN3$_08__invokeEifd"
 // WIN32: define internal x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6A?A?<auto>@@HMN@ZXZ"
 // WIN32: ret double (i32, float, double)* @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CA?A?<auto>@@HMN@Z"
 // WIN32: define internal x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6E?A?<auto>@@HMN@ZXZ"
 // WIN32: ret double (i32, float, double)* @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CE?A?<auto>@@HMN@Z"
+// WIN32: define internal x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6G?A?<auto>@@HMN@ZXZ"
+// WIN32: ret double (i32, float, double)* @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CG?A?<auto>@@HMN@Z"
+// WIN32: define internal x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6I?A?<auto>@@HMN@ZXZ"
+// WIN32: ret double (i32, float, double)* @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CI?A?<auto>@@HMN@Z"
+// WIN32: define internal x86_thiscallcc double (i32, float, double)* @"??B<lambda_0>@?0??usage@@YAXXZ@QBEP6Q?A?<auto>@@HMN@ZXZ"
+// WIN32: ret double (i32, float, double)* @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CQ?A?<auto>@@HMN@Z"
 //
 // __invoke function, calls operator(). Win32 should call both.
 // LIN64: define internal double @"_ZZ5usagevEN3$_08__invokeEifd"
 // LIN64: call double @"_ZZ5usagevENK3$_0clEifd"
 // VECCALL: define internal x86_vectorcallcc double @"_ZZ5usagevEN3$_08__invokeEifd"
 // VECCALL: call x86_vectorcallcc double @"_ZZ5usagevENK3$_0clEifd"
 // WIN32: define internal double @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CA?A?<auto>@@HMN@Z"
 // WIN32: call x86_thiscallcc double @"??R<lambda_0>@?0??usage@@YAXXZ@QBE?A?<auto>@@HMN@Z"
 // WIN32: define internal x86_thiscallcc double @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CE?A?<auto>@@HMN@Z"
 // WIN32: call x86_thiscallcc double @"??R<lambda_0>@?0??usage@@YAXXZ@QBE?A?<auto>@@HMN@Z"
+// WIN32: define internal x86_stdcallcc double @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CG?A?<auto>@@HMN@Z"
+// WIN32: call x86_thiscallcc double @"??R<lambda_0>@?0??usage@@YAXXZ@QBE?A?<auto>@@HMN@Z"
+// WIN32: define internal x86_fastcallcc double @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CI?A?<auto>@@HMN@Z"
+// WIN32: call x86_thiscallcc double @"??R<lambda_0>@?0??usage@@YAXXZ@QBE?A?<auto>@@HMN@Z"
+// WIN32: define internal x86_vectorcallcc double @"?__invoke@<lambda_0>@?0??usage@@YAXXZ@CQ?A?<auto>@@HMN@Z"
+// WIN32: call x86_thiscallcc double @"??R<lambda_0>@?0??usage@@YAXXZ@QBE?A?<auto>@@HMN@Z"