Fix function alias support (#178)

Author: dpaoliello

lib/Target/CBackend/CBackend.cpp

@@ -2460,6 +2460,13 @@ void CWriter::generateHeader(Module &M) {
       continue;
     printTypeName(NullOut, I->getValueType(), false);
   }
+  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
+       ++I) {
+    if (!I->hasLocalLinkage() && I->getValueType()->isFunctionTy()) {
+      // Make sure that the aliasee function type name exists.
+      getFunctionName(cast<Function>(I->getAliaseeObject()));
+    }
+  }
   printModuleTypes(Out);
 
   // Global variable declarations...
@@ -2643,9 +2650,18 @@ void CWriter::generateHeader(Module &M) {
         headerUseAligns();
         Out << "__PREFIXALIGN__(" << Alignment << ") ";
       }
+      std::optional<std::string> CastAs{};
+      if (ElTy->isFunctionTy()) {
+        std::string FunctionName =
+            getFunctionName(cast<Function>(I->getAliaseeObject()));
+        Out << FunctionName;
+        CastAs = FunctionName;
+      } else {
+        printTypeName(Out, ElTy, false);
+      }
       // GetValueName would resolve the alias, which is not what we want,
       // so use getName directly instead (assuming that the Alias has a name...)
-      printTypeName(Out, ElTy, false) << " *" << I->getName();
+      Out << " *" << I->getName();
       if (IsOveraligned) {
         headerUseAligns();
         Out << " __POSTFIXALIGN__(" << Alignment << ")";
@@ -2656,6 +2672,9 @@ void CWriter::generateHeader(Module &M) {
         Out << " __EXTERNAL_WEAK__";
       }
       Out << " = ";
+      if (CastAs.has_value()) {
+        Out << '(' << CastAs.value() << "*)";
+      }
       writeOperand(I->getAliasee(), ContextStatic);
       Out << ";\n";
     }

test/ll_tests/test_function_alias.ll

@@ -0,0 +1,36 @@
+; Test function aliases
+; It is inspired by LLVM IR produced by rustc 1.69 for code like
+;
+; #[no_mangle]
+; unsafe extern "C" fn drop_vec_of_a(
+;     data: *mut A,
+;     len: usize,
+;     capacity: usize,
+; ) {
+;     drop(Vec::from_raw_parts(data, len, capacity))
+; }
+; #[no_mangle]
+; unsafe extern "C" fn drop_vec_of_b(
+;     data: *mut B,
+;     len: usize,
+;     capacity: usize,
+; ) {
+;     drop(Vec::from_raw_parts(data, len, capacity))
+; }
+;
+; but with some simplifications.
+
+; Function Attrs: noinline nounwind optnone uwtable
+define i32 @drop_vec_of_a(i32 %retVal) #0 {
+  ret i32 %retVal
+}
+
+@drop_vec_of_b = unnamed_addr alias i32 (i32), ptr @drop_vec_of_a
+
+; Function Attrs: noinline nounwind optnone uwtable
+define dso_local i32 @main() #1 {
+  %a = call i32 @drop_vec_of_a(i32 2)
+  %b = call i32 @drop_vec_of_b(i32 4)
+  %sum = add i32 %a, %b
+  ret i32 %sum
+}