From d01397347887162a6d5a9d82e750bef91bdba131 Mon Sep 17 00:00:00 2001
From: Jasmine Tang <jjasmine@igalia.com>
Date: Mon, 10 Nov 2025 11:38:25 -0800
Subject: [PATCH] [WebAssembly] Enable musttail only when tail-call is enabled
 (#163618)

Fixes https://github.com/llvm/llvm-project/issues/163256
---
 clang/docs/ReleaseNotes.rst               |  2 ++
 clang/include/clang/Basic/Attr.td         |  6 +++++-
 clang/include/clang/Basic/TargetInfo.h    |  3 +++
 clang/lib/Basic/TargetInfo.cpp            |  1 +
 clang/lib/Basic/Targets/WebAssembly.cpp   |  3 +++
 clang/test/CodeGen/WebAssembly/musttail.c | 20 ++++++++++++++++++++
 6 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/CodeGen/WebAssembly/musttail.c

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 98034cfcd8413..72197380cf5af 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1197,6 +1197,8 @@ NetBSD Support
 WebAssembly Support
 ^^^^^^^^^^^^^^^^^^^
 
+- Fix a bug so that ``__has_attribute(musttail)`` is no longer true when WebAssembly's tail-call is not enabled. (#GH163256)
+
 AVR Support
 ^^^^^^^^^^^
 
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index 2953c1b63b3dc..26fc0c33eb20e 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -527,6 +527,10 @@ def TargetMicrosoftRecordLayout : TargetArch<["x86", "x86_64", "arm", "thumb",
   let CustomCode = [{ Target.hasMicrosoftRecordLayout() }];
 }
 
+def TargetMustTailAvaiable:  TargetSpec {
+  let CustomCode = [{ Target.hasMustTail() }];
+}
+
 def TargetELF : TargetSpec {
   let ObjectFormats = ["ELF"];
 }
@@ -1914,7 +1918,7 @@ 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">;
diff --git a/clang/include/clang/Basic/TargetInfo.h b/clang/include/clang/Basic/TargetInfo.h
index 76fdb5a2c860d..0ae46f355e7fd 100644
--- a/clang/include/clang/Basic/TargetInfo.h
+++ b/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;
@@ -674,6 +675,8 @@ 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) ||
diff --git a/clang/lib/Basic/TargetInfo.cpp b/clang/lib/Basic/TargetInfo.cpp
index 75000dbb804a2..7575eb3fc7f79 100644
--- a/clang/lib/Basic/TargetInfo.cpp
+++ b/clang/lib/Basic/TargetInfo.cpp
@@ -58,6 +58,7 @@ 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;
diff --git a/clang/lib/Basic/Targets/WebAssembly.cpp b/clang/lib/Basic/Targets/WebAssembly.cpp
index af25d25a3af3b..910bb28cf1807 100644
--- a/clang/lib/Basic/Targets/WebAssembly.cpp
+++ b/clang/lib/Basic/Targets/WebAssembly.cpp
@@ -209,6 +209,7 @@ 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;
@@ -333,10 +334,12 @@ 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") {
diff --git a/clang/test/CodeGen/WebAssembly/musttail.c b/clang/test/CodeGen/WebAssembly/musttail.c
new file mode 100644
index 0000000000000..37fed70028bbc
--- /dev/null
+++ b/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-warning@+1 {{unknown attribute 'clang::musttail' ignored}}
+ [[clang::musttail]] return foo(1);
+}