diff --git a/pkgs/development/compilers/llvm/12/llvm/default.nix b/pkgs/development/compilers/llvm/12/llvm/default.nix
index 04321b3598cf..2c035ee67a9e 100644
--- a/pkgs/development/compilers/llvm/12/llvm/default.nix
+++ b/pkgs/development/compilers/llvm/12/llvm/default.nix
@@ -63,6 +63,9 @@ in stdenv.mkDerivation (rec {
     # a native libLLVM.so (which would fail) we force llvm-config to be linked
     # statically against the necessary LLVM components always.
     ../../llvm-config-link-static.patch
+    # Fix llvm being miscompiled by some gccs. See llvm/llvm-project#49955
+    # Fix llvm being miscompiled by some gccs. See https://github.com/llvm/llvm-project/issues/49955
+    ./fix-llvm-issue-49955.patch
 
     ./gnu-install-dirs.patch
     # On older CPUs (e.g. Hydra/wendy) we'd be getting an error in this test.
diff --git a/pkgs/development/compilers/llvm/12/llvm/fix-llvm-issue-49955.patch b/pkgs/development/compilers/llvm/12/llvm/fix-llvm-issue-49955.patch
new file mode 100644
index 000000000000..b515583a0c42
--- /dev/null
+++ b/pkgs/development/compilers/llvm/12/llvm/fix-llvm-issue-49955.patch
@@ -0,0 +1,13 @@
+diff --git a/lib/CodeGen/AsmPrinter/CMakeLists.txt b/lib/CodeGen/AsmPrinter/CMakeLists.txt
+index eb924282..c77c140b 100644
+--- a/lib/CodeGen/AsmPrinter/CMakeLists.txt
++++ b/lib/CodeGen/AsmPrinter/CMakeLists.txt
+@@ -44,3 +44,8 @@ add_llvm_component_library(LLVMAsmPrinter
+   Support
+   Target
+   )
++
++if (CMAKE_COMPILER_IS_GNUCXX)
++  set_source_files_properties(DwarfCompileUnit.cpp PROPERTIES
++                              COMPILE_FLAGS -fno-strict-aliasing)
++endif()