[SIL] Use mangled names + asmname for @c functions

Instead of using the C name for `@c` functions in SIL, retain mangled
names and apply the `asmname` attribute, so we retain more type
information until later in the pipeline and avoid collisions.

Another part of rdar://137014448.

Author: DougGregor

include/swift/SIL/SILDeclRef.h

@@ -333,6 +333,10 @@ struct SILDeclRef {
   /// Produce a mangled form of this constant.
   std::string mangle(ManglingKind MKind = ManglingKind::Default) const;
 
+  /// If the symbol has a specific name for use at the LLVM IR level,
+  /// produce that name. This may be different than the mangled name in SIL.
+  std::optional<StringRef> getAsmName() const;
+
   /// True if the SILDeclRef references a function.
   bool isFunc() const {
     return kind == Kind::Func;

lib/IRGen/TBDGen.cpp

@@ -450,6 +450,15 @@ void TBDGenVisitor::didVisitDecl(Decl *D) {
 }
 
 void TBDGenVisitor::addFunction(SILDeclRef declRef) {
+  // If there is a specific symbol name this should have at the IR level,
+  // use it instead.
+  if (auto asmName = declRef.getAsmName()) {
+    addSymbol(*asmName, SymbolSource::forSILDeclRef(declRef),
+              SymbolFlags::Text);
+    return;
+  }
+
+
   addSymbol(declRef.mangle(), SymbolSource::forSILDeclRef(declRef),
             SymbolFlags::Text);
 }

lib/SIL/IR/SILDeclRef.cpp

@@ -1443,8 +1443,11 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
         return NameA->Name.str();
       }
 
-    // Use a given cdecl name for native-to-foreign thunks.
-    if (getDecl()->getAttrs().hasAttribute<CDeclAttr>())
+    // Use a given cdecl name for native-to-foreign thunks. Don't do this
+    // for functions that only have a C entrypoint.
+    if (getDecl()->getAttrs().hasAttribute<CDeclAttr>() &&
+        !(getDecl()->hasOnlyCEntryPoint() &&
+          !getDecl()->getImplementedObjCDecl())) {
       if (isNativeToForeignThunk() || isForeign) {
         // If this is an @implementation @_cdecl, mangle it like the clang
         // function it implements.
@@ -1455,6 +1458,7 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
         }
         return getDecl()->getCDeclName().str();
       }
+    }
 
     if (SKind == ASTMangler::SymbolKind::DistributedThunk) {
       return mangler.mangleDistributedThunk(cast<FuncDecl>(getDecl()));
@@ -1543,6 +1547,19 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
   llvm_unreachable("bad entity kind!");
 }
 
+std::optional<StringRef> SILDeclRef::getAsmName() const {
+  if (isForeign && isFunc()) {
+    auto func = getFuncDecl();
+    if (auto *EA = ExternAttr::find(func->getAttrs(), ExternKind::C))
+      return EA->getCName(func);
+    if (func->hasOnlyCEntryPoint() && !func->getImplementedObjCDecl())
+      return func->getCDeclName();
+  }
+
+  return std::nullopt;
+}
+
+
 // Returns true if the given JVP/VJP SILDeclRef requires a new vtable entry.
 // FIXME(https://github.com/apple/swift/issues/54833): Also consider derived declaration `@derivative` attributes.
 static bool derivativeFunctionRequiresNewVTableEntry(SILDeclRef declRef) {

lib/SIL/IR/SILFunctionBuilder.cpp

@@ -127,10 +127,8 @@ void SILFunctionBuilder::addFunctionAttributes(
       }
     }
 
-    if (constant.isFunc() && constant.hasFuncDecl()) {
-      auto func = constant.getFuncDecl();
-      if (auto *EA = ExternAttr::find(Attrs, ExternKind::C))
-        F->setAsmName(EA->getCName(func));
+    if (auto asmName = constant.getAsmName()) {
+      F->setAsmName(*asmName);
     }
   }
 

test/SILGen/cdecl-official.swift

@@ -6,21 +6,21 @@
 
 // REQUIRES: swift_feature_CDecl
 
-// CHECK-LABEL: sil hidden [ossa] @pear : $@convention(c) (@convention(c) (Int) -> Int) -> () {
+// CHECK-LABEL: sil hidden [asmname "pear"] [ossa] @$s5cdecl5appleyyS2iXCFTo : $@convention(c) (@convention(c) (Int) -> Int) -> () {
 @c(pear)
 func apple(_ f: @convention(c) (Int) -> Int) { }
 
 func acceptSwiftFunc(_ f: (Int) -> Int) { }
 
 // CHECK-LABEL: sil hidden [ossa] @$s5cdecl16forceCEntryPoint{{[_0-9a-zA-Z]*}}F
-// CHECK: [[GRAPEFRUIT:%[0-9]+]] = function_ref @grapefruit : $@convention(c) (Int) -> Int
-// CHECK: [[PEAR:%[0-9]+]] = function_ref @pear : $@convention(c) (@convention(c) (Int) -> Int) -> ()
+// CHECK: [[GRAPEFRUIT:%[0-9]+]] = function_ref @$s5cdecl6orangeyS2iFTo : $@convention(c) (Int) -> Int
+// CHECK: [[PEAR:%[0-9]+]] = function_ref @$s5cdecl5appleyyS2iXCFTo : $@convention(c) (@convention(c) (Int) -> Int) -> ()
 // CHECK: apply [[PEAR]]([[GRAPEFRUIT]])
 func forceCEntryPoint() {
   apple(orange)
 }
 
-// CHECK-LABEL: sil hidden [ossa] @grapefruit : $@convention(c) (Int) -> Int {
+// CHECK-LABEL: sil hidden [asmname "grapefruit"] [ossa] @$s5cdecl6orangeyS2iFTo : $@convention(c) (Int) -> Int {
 @c(grapefruit)
 func orange(_ x: Int) -> Int {
   return x
@@ -36,23 +36,23 @@ func requiresThunk() {
   acceptSwiftFunc(orange)
 }
 
-// CHECK-LABEL: sil [serialized] [ossa] @cauliflower : $@convention(c) (Int) -> Int {
+// CHECK-LABEL: sil [serialized] [asmname "cauliflower"] [ossa] @$s5cdecl8broccoliyS2iFTo : $@convention(c) (Int) -> Int {
 // CHECK-NOT: apply
 // CHECK: return
 @c(cauliflower)
 public func broccoli(_ x: Int) -> Int {
   return x
 }
 
-// CHECK-LABEL: sil private [ossa] @collard_greens : $@convention(c) (Int) -> Int {
+// CHECK-LABEL: sil private [asmname "collard_greens"] [ossa] @$s5cdecl4kale33_{{.*}}FTo : $@convention(c) (Int) -> Int {
 // CHECK-NOT: apply
 // CHECK: return
 @c(collard_greens)
 private func kale(_ x: Int) -> Int {
   return x
 }
 
-// CHECK-LABEL: sil private [ossa] @defaultName : $@convention(c) (Int) -> Int {
+// CHECK-LABEL: sil private [asmname "defaultName"] [ossa] @$s5cdecl11defaultName33{{.*}}iFTo : $@convention(c) (Int) -> Int {
 // CHECK-NOT: apply
 // CHECK: return
 @c