diff --git a/pkgs/development/compilers/llvm/4/fix-gcc9.patch b/pkgs/development/compilers/llvm/4/fix-gcc9.patch
new file mode 100644
index 000000000000..8ea5ca970855
--- /dev/null
+++ b/pkgs/development/compilers/llvm/4/fix-gcc9.patch
@@ -0,0 +1,33 @@
+diff --git a/lib/Target/Mips/MipsFastISel.cpp b/lib/Target/Mips/MipsFastISel.cpp
+index f79cb0e6..c6279046 100644
+--- a/lib/Target/Mips/MipsFastISel.cpp
++++ b/lib/Target/Mips/MipsFastISel.cpp
+@@ -32,6 +32,7 @@
+ #include "llvm/MC/MCSymbol.h"
+ #include "llvm/Target/TargetInstrInfo.h"
+ #include "llvm/Support/Debug.h"
++#include <array>
+ 
+ #define DEBUG_TYPE "mips-fastisel"
+ 
+@@ -1268,13 +1269,13 @@ bool MipsFastISel::fastLowerArguments() {
+     return false;
+   }
+ 
+-  const ArrayRef<MCPhysReg> GPR32ArgRegs = {Mips::A0, Mips::A1, Mips::A2,
+-                                            Mips::A3};
+-  const ArrayRef<MCPhysReg> FGR32ArgRegs = {Mips::F12, Mips::F14};
+-  const ArrayRef<MCPhysReg> AFGR64ArgRegs = {Mips::D6, Mips::D7};
+-  ArrayRef<MCPhysReg>::iterator NextGPR32 = GPR32ArgRegs.begin();
+-  ArrayRef<MCPhysReg>::iterator NextFGR32 = FGR32ArgRegs.begin();
+-  ArrayRef<MCPhysReg>::iterator NextAFGR64 = AFGR64ArgRegs.begin();
++   std::array<MCPhysReg, 4> GPR32ArgRegs = {{Mips::A0, Mips::A1, Mips::A2,
++                                            Mips::A3}};
++   std::array<MCPhysReg, 2> FGR32ArgRegs = {{Mips::F12, Mips::F14}};
++   std::array<MCPhysReg, 2> AFGR64ArgRegs = {{Mips::D6, Mips::D7}};
++   auto NextGPR32 = GPR32ArgRegs.begin();
++   auto NextFGR32 = FGR32ArgRegs.begin();
++   auto NextAFGR64 = AFGR64ArgRegs.begin();
+ 
+   struct AllocatedReg {
+     const TargetRegisterClass *RC;
diff --git a/pkgs/development/compilers/llvm/4/llvm.nix b/pkgs/development/compilers/llvm/4/llvm.nix
index ac5dcbe6b942..c443303f3739 100644
--- a/pkgs/development/compilers/llvm/4/llvm.nix
+++ b/pkgs/development/compilers/llvm/4/llvm.nix
@@ -52,6 +52,13 @@ stdenv.mkDerivation ({
       url = "https://bugzilla.redhat.com/attachment.cgi?id=1389687";
       sha256 = "0ga2123aclq3x9w72d0rm0az12m8c1i4r1106vh701hf4cghgbch";
     })
+    ./fix-gcc9.patch
+    (fetchpatch {
+      name = "llvm4-avoid-undefined-behavior-in-unittest.patch";
+      url = "https://aur.archlinux.org/cgit/aur.git/plain/D32089-Avoid-undefined-behavior-in-unittest.patch?h=llvm40&id=f459b0bad8aa3b94bc2733d79d176071a32846a6";
+      sha256 = "0x5q6a8lk6xg4ns4qh75fxvvmfnifwvyrq17ck85q8c0753i1irf";
+      extraPrefix = "";
+    })
   ];
 
   # TSAN requires XPC on Darwin, which we have no public/free source files for. We can depend on the Apple frameworks