From b0d53a839da0149e0142da036b6ebf5a01b3216f Mon Sep 17 00:00:00 2001
From: Nicolas Trangez <ikke@nicolast.be>
Date: Sat, 1 Oct 2016 17:58:11 -0400
Subject: [PATCH] Turn `__GLASGOW_HASKELL_LLVM__` into an integer again

In GHC < 8.0.1, the value of `__GLASGOW_HASKELL_LLVM__`, exposed
through the preprocessor when compiled with `-fllvm`, was an integer
value, encoded according to some rules specified in the user guide.

Due to an oversight, in GHC 8.0.1 the value of this define became a
tuple, exposed as e.g. `(3, 7)`. This was an unintended regression.

This patch turns the value of the `__GLASGOW_HASKELL_LLVM__` definition
into a single integer again, but changes the formatting of said number
slightly. Before, any LLVM version where the major or minor component >=
10 would cause ambiguous values for `__GLASGOW_HASKELL_LLVM__`. With
this patch, the value is in line with `__GLASGOW_HASKELL__`, adding a
padding `0` in-between major and minor component if applicable (we
assume no minors >= 100 will ever exist).

The documentation in the user guide is updated accordingly, and a
reference is made in the 8.0.2 release notes.

Test Plan: validate

Reviewers: bgamari, erikd

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2552

GHC Trac Issues: #12628
---
 compiler/main/DriverPipeline.hs  | 6 +++++-
 docs/users_guide/8.0.2-notes.rst | 6 ++++++
 docs/users_guide/phases.rst      | 4 +++-
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/compiler/main/DriverPipeline.hs b/compiler/main/DriverPipeline.hs
index 5d648e60f9..6e61d20dc8 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 43c956209a..82c214e534 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 0c3b59f54c..01c2e1f35f 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::
-- 
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