Add -fno-builtin command line flag

driver/cl_options.cpp

@@ -20,6 +20,12 @@ namespace opts {
 // This vector is filled by parseCommandLine in main.cpp.
 llvm::SmallVector<const char *, 32> allArguments;
 
+cl::opt<bool> noBuiltIn(
+    "fno-builtin",
+    cl::desc("Disable implicit builtin knowledge of functions"),
+    cl::ZeroOrMore
+);
+
 cl::OptionCategory linkingCategory("Linking options");
 
 // Positional options first, in order:

driver/cl_options.h

@@ -41,6 +41,7 @@ extern cl::OptionCategory linkingCategory;
    egrep -e '^(cl::|#if|#e)' gen/cl_options.cpp \
     | sed -re 's/^(cl::.*)\(.*$/    extern \1;/'
  */
+extern cl::opt<bool> noBuiltIn;
 extern cl::list<std::string> fileList;
 extern cl::list<std::string> runargs;
 extern cl::opt<bool> invokedByLDMD;

gen/functions.cpp

@@ -1190,7 +1190,9 @@ void DtoDefineFunction(FuncDeclaration *fd, bool linkageAvailableExternally) {
   if (opts::isUsingSampleBasedPGOProfile()) {
     func->addFnAttr("use-sample-profile");
   }
-
+  if (opts::noBuiltIn) {
+    func->addFnAttr("no-built-in");
+  }
   if (fd->hasInlineAsm()) {
     // disable frame-pointer-elimination for functions with DMD-style inline asm
     func->addFnAttr("frame-pointer", "all");

tests/codegen/attr_fno_builtin.d

@@ -0,0 +1,6 @@
+// RUN: %ldc -c -output-ll -fno-builtin -of=%t.ll %s && FileCheck %s < %t.ll
+
+// CHECK: define {{.*}} @builtin{{.*}} [[ATTR:#[0-9]+]]
+extern(C) void builtin() {}
+
+// CHECK: attributes [[ATTR]] = { {{.*}}"no-built-in"{{.*}} }