diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index 5d648e60f995d8b4cd9379d364aea913d1ae8ac9..6e61d20dc8638ddc797d4f806ef765d8c656ad56 100644
--- a/compiler/main/DriverPipeline.hs
+++ b/compiler/main/DriverPipeline.hs
@@ -2099,8 +2099,12 @@ getBackendDefs :: DynFlags -> IO [String]
 getBackendDefs dflags | hscTarget dflags == HscLlvm = do
     llvmVer <- figureLlvmVersion dflags
     return $ case llvmVer of
-               Just n -> [ "-D__GLASGOW_HASKELL_LLVM__="++show n ]
+               Just n -> [ "-D__GLASGOW_HASKELL_LLVM__=" ++ format n ]
                _      -> []
+  where
+    format (major, minor)
+      | minor >= 100 = error "getBackendDefs: Unsupported minor version"
+      | otherwise = show $ (100 * major + minor :: Int) -- Contract is Int
 
 getBackendDefs _ =
     return []
diff --git a/docs/users_guide/8.0.2-notes.rst b/docs/users_guide/8.0.2-notes.rst
index 43c956209a99de98b29cd9bfff628014672cedd9..82c214e534411d77e940912964a3a634bdf7cff5 100644
--- a/docs/users_guide/8.0.2-notes.rst
+++ b/docs/users_guide/8.0.2-notes.rst
@@ -40,6 +40,12 @@ Compiler
    defaulting to decimal, hexadecimal if the address starts with `0x`, and
    octal if the address starts with `0`.
 
+-  Due to an oversight in GHC 8.0.1, the value of the preprocessor macro
+   ``__GLASGOW_HASKELL_LLVM__``, which exposes the LLVM version used by GHC, was
+   no longer an integer. This value is now turned into an integer again, but the
+   formatting is changed to be in line with ``__GLASGOW_HASKELL__``
+   (:ghc-ticket:`12628`).
+
 Runtime system
 ~~~~~~~~~~~~~~
 
diff --git a/docs/users_guide/phases.rst b/docs/users_guide/phases.rst
index 0c3b59f54c45a0a7c2886bf9914f6798a0ca34d9..01c2e1f35fba76c35158beca9678ec3508516629 100644
--- a/docs/users_guide/phases.rst
+++ b/docs/users_guide/phases.rst
@@ -291,7 +291,9 @@ defined by your local GHC installation, the following trick is useful:
 
     Only defined when ``-fllvm`` is specified. When GHC is using version
     ``x.y.z`` of LLVM, the value of ``__GLASGOW_HASKELL_LLVM__`` is the
-    integer ⟨xy⟩.
+    integer ⟨xyy⟩ (if ⟨y⟩ is a single digit, then a leading zero
+    is added, so for example when using version 3.7 of LLVM,
+    ``__GLASGOW_HASKELL_LLVM__==307``).
 
 ``__PARALLEL_HASKELL__``
     .. index::