diff --git a/pkgs/development/compilers/llvm/10/compiler-rt.nix b/pkgs/development/compilers/llvm/10/compiler-rt.nix
index 68f64ef42560..00c4f0677ebc 100644
--- a/pkgs/development/compilers/llvm/10/compiler-rt.nix
+++ b/pkgs/development/compilers/llvm/10/compiler-rt.nix
@@ -57,7 +57,7 @@ stdenv.mkDerivation rec {
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd
   # get no binary cache for the entire platform. If you really find yourself wanting the TSAN, make this controllable by
   # a flag and turn the flag off during the stdenv build.
-  postPatch = ''
+  postPatch = stdenv.lib.optionalString (!stdenv.isDarwin) ''
     substituteInPlace cmake/builtin-config-ix.cmake \
       --replace 'set(X86 i386)' 'set(X86 i386 i486 i586 i686)'
   '' + stdenv.lib.optionalString stdenv.isDarwin ''
diff --git a/pkgs/development/compilers/llvm/6/compiler-rt.nix b/pkgs/development/compilers/llvm/6/compiler-rt.nix
index 5ae8bb01f050..13abf6d95611 100644
--- a/pkgs/development/compilers/llvm/6/compiler-rt.nix
+++ b/pkgs/development/compilers/llvm/6/compiler-rt.nix
@@ -55,7 +55,7 @@ stdenv.mkDerivation {
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd
   # get no binary cache for the entire platform. If you really find yourself wanting the TSAN, make this controllable by
   # a flag and turn the flag off during the stdenv build.
-  postPatch = ''
+  postPatch = stdenv.lib.optionalString (!stdenv.isDarwin) ''
     substituteInPlace cmake/builtin-config-ix.cmake \
       --replace 'set(X86 i386)' 'set(X86 i386 i486 i586 i686)'
   '' + stdenv.lib.optionalString stdenv.isDarwin ''
diff --git a/pkgs/development/compilers/llvm/7/compiler-rt.nix b/pkgs/development/compilers/llvm/7/compiler-rt.nix
index e9853c58ca57..84ca6af3b36d 100644
--- a/pkgs/development/compilers/llvm/7/compiler-rt.nix
+++ b/pkgs/development/compilers/llvm/7/compiler-rt.nix
@@ -56,7 +56,7 @@ stdenv.mkDerivation {
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd
   # get no binary cache for the entire platform. If you really find yourself wanting the TSAN, make this controllable by
   # a flag and turn the flag off during the stdenv build.
-  postPatch = ''
+  postPatch = stdenv.lib.optionalString (!stdenv.isDarwin) ''
     substituteInPlace cmake/builtin-config-ix.cmake \
       --replace 'set(X86 i386)' 'set(X86 i386 i486 i586 i686)'
   '' + stdenv.lib.optionalString stdenv.isDarwin ''
diff --git a/pkgs/development/compilers/llvm/8/compiler-rt.nix b/pkgs/development/compilers/llvm/8/compiler-rt.nix
index 1f58cb98e3d3..15e55800dc8e 100644
--- a/pkgs/development/compilers/llvm/8/compiler-rt.nix
+++ b/pkgs/development/compilers/llvm/8/compiler-rt.nix
@@ -56,7 +56,7 @@ stdenv.mkDerivation {
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd
   # get no binary cache for the entire platform. If you really find yourself wanting the TSAN, make this controllable by
   # a flag and turn the flag off during the stdenv build.
-  postPatch = ''
+  postPatch = stdenv.lib.optionalString (!stdenv.isDarwin) ''
     substituteInPlace cmake/builtin-config-ix.cmake \
       --replace 'set(X86 i386)' 'set(X86 i386 i486 i586 i686)'
   '' + stdenv.lib.optionalString stdenv.isDarwin ''
diff --git a/pkgs/development/compilers/llvm/9/compiler-rt.nix b/pkgs/development/compilers/llvm/9/compiler-rt.nix
index 3b92264ad692..0183754a2fd4 100644
--- a/pkgs/development/compilers/llvm/9/compiler-rt.nix
+++ b/pkgs/development/compilers/llvm/9/compiler-rt.nix
@@ -55,7 +55,7 @@ stdenv.mkDerivation rec {
   # can build this. If we didn't do it, basically the entire nixpkgs on Darwin would have an unfree dependency and we'd
   # get no binary cache for the entire platform. If you really find yourself wanting the TSAN, make this controllable by
   # a flag and turn the flag off during the stdenv build.
-  postPatch = ''
+  postPatch = stdenv.lib.optionalString (!stdenv.isDarwin) ''
     substituteInPlace cmake/builtin-config-ix.cmake \
       --replace 'set(X86 i386)' 'set(X86 i386 i486 i586 i686)'
   '' + stdenv.lib.optionalString stdenv.isDarwin ''