hadrian: Add a viaLlvmBackend modifier

Note that this also slightly changes the semantics of these flavours as
we only use LLVM for >= stage1 builds.

3 files changed, 12 insertions, 6 deletions

diff --git a/hadrian/doc/flavours.md b/hadrian/doc/flavours.md
index c5441f05d6..254f2e0666 100644
--- a/hadrian/doc/flavours.md
+++ b/hadrian/doc/flavours.md
@@ -216,6 +216,10 @@ The supported transformers are listed below:
         <td><code>thread_sanitizer</code></td>
         <td>Build the runtime system with ThreadSanitizer support</td>
     </tr>
+    <tr>
+        <td><code>llvm</code></td>
+        <td>Use GHC's LLVM backend (`-fllvm`) for all stage1+ compilation.</td>
+    </tr>
 </table>
 
 ## Ways
diff --git a/hadrian/src/Flavour.hs b/hadrian/src/Flavour.hs
index 06f72a06eb..6c68df21ff 100644
--- a/hadrian/src/Flavour.hs
+++ b/hadrian/src/Flavour.hs
@@ -8,6 +8,7 @@ module Flavour
   , splitSections, splitSectionsIf
   , enableThreadSanitizer
   , enableDebugInfo, enableTickyGhc
+  , viaLlvmBackend
   ) where
 
 import Expression
@@ -84,6 +85,7 @@ flavourTransformers = M.fromList
     , "ticky_ghc" =: enableTickyGhc
     , "split_sections" =: splitSections
     , "thread_sanitizer" =: enableThreadSanitizer
+    , "llvm" =: viaLlvmBackend
     ]
   where (=:) = (,)
 
@@ -191,3 +193,7 @@ enableThreadSanitizer = addArgs $ mconcat
     , builder (Cabal Flags) ? arg "thread-sanitizer"
     , builder  RunTest ? arg "--config=have_thread_sanitizer=True"
     ]
+
+-- | Use the LLVM backend in stages 1 and later.
+viaLlvmBackend :: Flavour -> Flavour
+viaLlvmBackend = addArgs $ notStage0 ? builder Ghc ? arg "-fllvm"
diff --git a/hadrian/src/Settings/Flavours/Llvm.hs b/hadrian/src/Settings/Flavours/Llvm.hs
index ddf07a248d..3882e73f45 100644
--- a/hadrian/src/Settings/Flavours/Llvm.hs
+++ b/hadrian/src/Settings/Flavours/Llvm.hs
@@ -5,7 +5,6 @@ module Settings.Flavours.Llvm (
   quickLlvmFlavour,
 ) where
 
-import Expression
 import Flavour
 
 import Settings.Flavours.Benchmark
@@ -22,8 +21,5 @@ quickLlvmFlavour       = mkLlvmFlavour quickFlavour
 
 -- | Turn a flavour into an LLVM flavour
 mkLlvmFlavour :: Flavour -> Flavour
-mkLlvmFlavour flav = flav
-    { name = name flav ++ "-llvm"
-    , args = mconcat [ args flav
-                     , builder Ghc ? arg "-fllvm" ]
-    }
+mkLlvmFlavour flav = viaLlvmBackend $ flav
+    { name = name flav ++ "-llvm" }