hadrian: Add `collect_stats` flavour transformer

This is useful for later consumption with
https://gitlab.haskell.org/bgamari/ghc-utils/-/blob/master/ghc_timings.py

2 files changed, 20 insertions, 0 deletions

diff --git a/hadrian/doc/flavours.md b/hadrian/doc/flavours.md
index b088b8ab92..b49ef5ddeb 100644
--- a/hadrian/doc/flavours.md
+++ b/hadrian/doc/flavours.md
@@ -240,6 +240,11 @@ The supported transformers are listed below:
         <td><code>ipe</code></td>
         <td>Build the stage2 libraries with IPE debugging information for use with -hi profiling.</td>
     </tr>
+    <tr>
+        <td><code>collect_timings</code></td>
+        <td>Collects timings while building the stage2+ compiler by adding the
+        flags <code>-ddump-to-file -ddump-timings</code>.</td>
+    </tr>
 </table>
 
 ### Static
diff --git a/hadrian/src/Flavour.hs b/hadrian/src/Flavour.hs
index 40cce02e51..f31e7667e1 100644
--- a/hadrian/src/Flavour.hs
+++ b/hadrian/src/Flavour.hs
@@ -46,6 +46,7 @@ flavourTransformers = M.fromList
     , "omit_pragmas" =: omitPragmas
     , "ipe" =: enableIPE
     , "fully_static" =: fullyStatic
+    , "collect_timings" =: collectTimings
     ]
   where (=:) = (,)
 
@@ -246,6 +247,20 @@ fullyStatic flavour =
         , builder (Ghc LinkHs) ? pure [ "-optl", "-static" ]
         ]
 
+-- | Build stage2 dependencies with options to enable collection of compiler
+-- stats.
+collectTimings :: Flavour -> Flavour
+collectTimings =
+  -- Why both -ddump-timings *and* -v?
+  -- In contrast to -ddump-timings, -v will seq the whole CoreProgram and
+  -- produce less missleading information; otherwise, due to laziness some
+  -- allocations might be attributed to a subsequent pass instead of the pass
+  -- that has been causing the allocation. So we want -v.
+  -- On the other hand, -v doesn't work with -ddump-to-file, so we need
+  -- -ddump-timings.
+  addArgs $ notStage0 ? builder (Ghc CompileHs) ?
+    pure ["-ddump-to-file", "-ddump-timings", "-v"]
+
 
 -- * CLI and <root>/hadrian.settings options