diff --git a/src/librustc_codegen_llvm/llvm/ffi.rs b/src/librustc_codegen_llvm/llvm/ffi.rs
index f1a966d765438..9eefeca70cf7a 100644
--- a/src/librustc_codegen_llvm/llvm/ffi.rs
+++ b/src/librustc_codegen_llvm/llvm/ffi.rs
@@ -1358,6 +1358,7 @@ extern "C" {
     pub fn LLVMRustDebugMetadataVersion() -> u32;
     pub fn LLVMRustVersionMajor() -> u32;
     pub fn LLVMRustVersionMinor() -> u32;
+    pub fn LLVMRustIsRustLLVM() -> bool;
 
     pub fn LLVMRustAddModuleFlag(M: &Module, name: *const c_char, value: u32);
 
diff --git a/src/librustc_codegen_llvm/llvm_util.rs b/src/librustc_codegen_llvm/llvm_util.rs
index 267d7e0d54b63..fdb6373bea1fe 100644
--- a/src/librustc_codegen_llvm/llvm_util.rs
+++ b/src/librustc_codegen_llvm/llvm_util.rs
@@ -70,6 +70,9 @@ unsafe fn configure_llvm(sess: &Session) {
         if sess.opts.debugging_opts.disable_instrumentation_preinliner {
             add("-disable-preinline");
         }
+        if llvm::LLVMRustIsRustLLVM() {
+            add("-mergefunc-use-aliases");
+        }
 
         for arg in &sess.opts.cg.llvm_args {
             add(&(*arg));
diff --git a/src/rustllvm/PassWrapper.cpp b/src/rustllvm/PassWrapper.cpp
index 06de0d6509bab..aa420bf6100c7 100644
--- a/src/rustllvm/PassWrapper.cpp
+++ b/src/rustllvm/PassWrapper.cpp
@@ -284,7 +284,7 @@ static Optional<Reloc::Model> fromRust(LLVMRustRelocMode RustReloc) {
   report_fatal_error("Bad RelocModel.");
 }
 
-#if LLVM_RUSTLLVM
+#ifdef LLVM_RUSTLLVM
 /// getLongestEntryLength - Return the length of the longest entry in the table.
 ///
 static size_t getLongestEntryLength(ArrayRef<SubtargetFeatureKV> Table) {
diff --git a/src/rustllvm/RustWrapper.cpp b/src/rustllvm/RustWrapper.cpp
index b6e07942f8667..6dcd32fe4d719 100644
--- a/src/rustllvm/RustWrapper.cpp
+++ b/src/rustllvm/RustWrapper.cpp
@@ -533,6 +533,14 @@ extern "C" uint32_t LLVMRustVersionMinor() { return LLVM_VERSION_MINOR; }
 
 extern "C" uint32_t LLVMRustVersionMajor() { return LLVM_VERSION_MAJOR; }
 
+extern "C" bool LLVMRustIsRustLLVM() {
+#ifdef LLVM_RUSTLLVM
+  return 1;
+#else
+  return 0;
+#endif
+}
+
 extern "C" void LLVMRustAddModuleFlag(LLVMModuleRef M, const char *Name,
                                       uint32_t Value) {
   unwrap(M)->addModuleFlag(Module::Warning, Name, Value);
diff --git a/src/test/codegen/export-no-mangle.rs b/src/test/codegen/export-no-mangle.rs
index 3546284f9a3c5..95587d9d84cf0 100644
--- a/src/test/codegen/export-no-mangle.rs
+++ b/src/test/codegen/export-no-mangle.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-flags: -C no-prepopulate-passes
+
 #![crate_type = "lib"]
 
 mod private {
diff --git a/src/test/codegen/external-no-mangle-fns.rs b/src/test/codegen/external-no-mangle-fns.rs
index 3f092b802f99a..5823285259602 100644
--- a/src/test/codegen/external-no-mangle-fns.rs
+++ b/src/test/codegen/external-no-mangle-fns.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags: -O
+// compile-flags: -C no-prepopulate-passes
 // `#[no_mangle]`d functions always have external linkage, i.e. no `internal` in their `define`s
 
 #![crate_type = "lib"]
@@ -43,7 +43,7 @@ const HIDDEN: () = {
 };
 
 // The surrounding item should not accidentally become external
-// CHECK: define internal {{.*}} void @_ZN22external_no_mangle_fns1x
+// CHECK: define internal{{.*}} void @_ZN22external_no_mangle_fns1x
 #[inline(never)]
 fn x() {
     // CHECK: define void @g()
diff --git a/src/test/codegen/issue-45222.rs b/src/test/codegen/issue-45222.rs
index 30a03243f0151..3544786e2e60a 100644
--- a/src/test/codegen/issue-45222.rs
+++ b/src/test/codegen/issue-45222.rs
@@ -69,6 +69,6 @@ fn foo3r(n: u64) -> u64 {
 // CHECK-LABEL: @check_foo3r
 #[no_mangle]
 pub fn check_foo3r() -> u64 {
-    // CHECK: ret i64 500005000000000
-    foo3r(100000)
+    // CHECK: ret i64 500050000000
+    foo3r(10000)
 }
diff --git a/src/test/codegen/match-optimizes-away.rs b/src/test/codegen/match-optimizes-away.rs
index d7b779374314d..2136b176eac9b 100644
--- a/src/test/codegen/match-optimizes-away.rs
+++ b/src/test/codegen/match-optimizes-away.rs
@@ -14,6 +14,7 @@
 
 pub enum Three { A, B, C }
 
+#[repr(u16)]
 pub enum Four { A, B, C, D }
 
 #[no_mangle]
@@ -32,7 +33,7 @@ pub fn three_valued(x: Three) -> Three {
 pub fn four_valued(x: Four) -> Four {
     // CHECK-LABEL: @four_valued
     // CHECK-NEXT: {{^.*:$}}
-    // CHECK-NEXT: ret i8 %0
+    // CHECK-NEXT: ret i16 %0
     match x {
         Four::A => Four::A,
         Four::B => Four::B,