[WebAssembly] Enable musttail only when tail-call is enabled

clang/include/clang/Basic/Attr.td

@@ -483,6 +483,7 @@ def TargetAnyX86 : TargetArch<["x86", "x86_64"]>;
 def TargetSPIRV : TargetArch<["spirv", "spirv32", "spirv64"]>;
 def TargetWebAssembly : TargetArch<["wasm32", "wasm64"]>;
 def TargetNVPTX : TargetArch<["nvptx", "nvptx64"]>;
+def TargetPowerPC : TargetArch<["ppc", "ppcle", "ppc64", "ppc64le"]>;
 def TargetWindows : TargetSpec {
   let OSes = ["Win32"];
 }
@@ -508,6 +509,10 @@ def TargetMicrosoftRecordLayout : TargetArch<["x86", "x86_64", "arm", "thumb",
   let CustomCode = [{ Target.hasMicrosoftRecordLayout() }];
 }
 
+def TargetMustTailAvaiable: TargetArch<!listconcat(TargetARM.Arches, TargetAArch64.Arches, TargetAnyX86.Arches, TargetWebAssembly.Arches, TargetPowerPC.Arches)> {
+  let CustomCode = [{ Target.hasMustTail() }];
+}
+
 def TargetELF : TargetSpec {
   let ObjectFormats = ["ELF"];
 }
@@ -1896,7 +1901,7 @@ def NoMerge : DeclOrStmtAttr {
                              "functions, statements and variables">;
 }
 
-def MustTail : StmtAttr {
+def MustTail : StmtAttr, TargetSpecificAttr<TargetMustTailAvaiable> {
   let Spellings = [Clang<"musttail">];
   let Documentation = [MustTailDocs];
   let Subjects = SubjectList<[ReturnStmt], ErrorDiag, "return statements">;

clang/include/clang/Basic/DiagnosticCommonKinds.td

@@ -374,6 +374,8 @@ def err_ppc_impossible_musttail: Error<
   >;
 def err_aix_musttail_unsupported: Error<
   "'musttail' attribute is not supported on AIX">;
+def err_wasm_musttail_unsupported: Error<
+  "'musttail' attribute is not supported on this target without tail-call feature">;
 
 // Source manager
 def err_cannot_open_file : Error<"cannot open file '%0': %1">, DefaultFatal;

clang/include/clang/Basic/TargetInfo.h

@@ -229,6 +229,7 @@ class TargetInfo : public TransferrableTargetInfo,
 protected:
   // Target values set by the ctor of the actual target implementation.  Default
   // values are specified by the TargetInfo constructor.
+  bool HasMustTail;
   bool BigEndian;
   bool TLSSupported;
   bool VLASupported;
@@ -669,6 +670,8 @@ class TargetInfo : public TransferrableTargetInfo,
                                        : getLongFractScale() + 1;
   }
 
+  virtual bool hasMustTail() const { return HasMustTail; }
+
   /// Determine whether the __int128 type is supported on this target.
   virtual bool hasInt128Type() const {
     return (getPointerWidth(LangAS::Default) >= 64) ||

clang/lib/Basic/TargetInfo.cpp

@@ -59,6 +59,7 @@ static const LangASMap FakeAddrSpaceMap = {
 TargetInfo::TargetInfo(const llvm::Triple &T) : Triple(T) {
   // Set defaults.  Defaults are set for a 32-bit RISC platform, like PPC or
   // SPARC.  These should be overridden by concrete targets as needed.
+  HasMustTail = true;
   BigEndian = !T.isLittleEndian();
   TLSSupported = true;
   VLASupported = true;

clang/lib/Basic/Targets/WebAssembly.cpp

@@ -213,6 +213,7 @@ bool WebAssemblyTargetInfo::initFeatureMap(
 
 bool WebAssemblyTargetInfo::handleTargetFeatures(
     std::vector<std::string> &Features, DiagnosticsEngine &Diags) {
+  HasMustTail = false;
   for (const auto &Feature : Features) {
     if (Feature == "+atomics") {
       HasAtomics = true;
@@ -345,10 +346,12 @@ bool WebAssemblyTargetInfo::handleTargetFeatures(
     }
     if (Feature == "+tail-call") {
       HasTailCall = true;
+      HasMustTail = true;
       continue;
     }
     if (Feature == "-tail-call") {
       HasTailCall = false;
+      HasMustTail = false;
       continue;
     }
     if (Feature == "+wide-arithmetic") {

clang/lib/Sema/SemaStmt.cpp

@@ -695,7 +695,6 @@ bool Sema::checkMustTailAttr(const Stmt *St, const Attr &MTA) {
 
   const Expr *E = cast<ReturnStmt>(St)->getRetValue();
   const auto *CE = dyn_cast_or_null<CallExpr>(IgnoreParenImplicitAsWritten(E));
-
   if (!CE) {
     Diag(St->getBeginLoc(), diag::err_musttail_needs_call) << &MTA;
     return false;

clang/lib/Sema/SemaStmtAttr.cpp

@@ -672,6 +672,12 @@ static Attr *ProcessStmtAttribute(Sema &S, Stmt *St, const ParsedAttr &A,
       !(A.existsInTarget(S.Context.getTargetInfo()) ||
         (S.Context.getLangOpts().SYCLIsDevice && Aux &&
          A.existsInTarget(*Aux)))) {
+    // Special case: musttail on WebAssembly without tail-call feature
+    if (A.getKind() == ParsedAttr::AT_MustTail &&
+        !S.Context.getTargetInfo().hasMustTail()) {
+      S.Diag(A.getLoc(), diag::err_wasm_musttail_unsupported);
+      return nullptr;
+    }
     if (A.isRegularKeywordAttribute()) {
       S.Diag(A.getLoc(), diag::err_keyword_not_supported_on_target)
           << A << A.getRange();

clang/test/CodeGen/WebAssembly/musttail.c

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -target-feature +tail-call -o /dev/null -emit-llvm -verify=tail
+// RUN: %clang_cc1 %s -triple wasm32-unknown-unknown -o /dev/null -emit-llvm -verify=notail
+
+int foo(int x) {
+  return x;
+}
+
+#if __has_attribute(musttail)
+// tail-warning@+1 {{HAS IT}}
+#warning HAS IT
+#else
+// notail-warning@+1 {{DOES NOT HAVE}}
+#warning DOES NOT HAVE
+#endif
+
+int bar(int x)
+{
+  // notail-error@+1 {{'musttail' attribute is not supported on this target without tail-call feature}}
+ [[clang::musttail]] return foo(1);
+}