Return results of Cmm streams in backends

This generalizes code generators (outputAsm, outputLlvm, outputC, and
the call site codeOutput) so that they'll return the return values of
the passed Cmm streams.

This allows accumulating data during Cmm generation and returning it to
the call site in HscMain.

Previously the Cmm streams were assumed to return (), so the code
generators returned () as well.

This change is required by !1304 and !1530.

Skipping CI as this was tested before and I only updated the commit
message.

[skip ci]

2 files changed, 12 insertions, 8 deletions

diff --git a/compiler/llvmGen/LlvmCodeGen.hs b/compiler/llvmGen/LlvmCodeGen.hs
index 2a568f820f..f649069b97 100644
--- a/compiler/llvmGen/LlvmCodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen.hs
@@ -42,8 +42,8 @@ import System.IO
 -- | Top-level of the LLVM Code generator
 --
 llvmCodeGen :: DynFlags -> Handle -> UniqSupply
-               -> Stream.Stream IO RawCmmGroup ()
-               -> IO ()
+               -> Stream.Stream IO RawCmmGroup a
+               -> IO a
 llvmCodeGen dflags h us cmm_stream
   = withTiming (pure dflags) (text "LLVM CodeGen") (const ()) $ do
        bufh <- newBufHandle h
@@ -66,12 +66,14 @@ llvmCodeGen dflags h us cmm_stream
                             $+$ text "We will try though...")
 
        -- run code generation
-       runLlvm dflags ver bufh us $
+       a <- runLlvm dflags ver bufh us $
          llvmCodeGen' (liftStream cmm_stream)
 
        bFlush bufh
 
-llvmCodeGen' :: Stream.Stream LlvmM RawCmmGroup () -> LlvmM ()
+       return a
+
+llvmCodeGen' :: Stream.Stream LlvmM RawCmmGroup a -> LlvmM a
 llvmCodeGen' cmm_stream
   = do  -- Preamble
         renderLlvm header
@@ -79,13 +81,15 @@ llvmCodeGen' cmm_stream
         cmmMetaLlvmPrelude
 
         -- Procedures
-        () <- Stream.consume cmm_stream llvmGroupLlvmGens
+        a <- Stream.consume cmm_stream llvmGroupLlvmGens
 
         -- Declare aliases for forward references
         renderLlvm . pprLlvmData =<< generateExternDecls
 
         -- Postamble
         cmmUsedLlvmGens
+
+        return a
   where
     header :: SDoc
     header = sdocWithDynFlags $ \dflags ->
diff --git a/compiler/llvmGen/LlvmCodeGen/Base.hs b/compiler/llvmGen/LlvmCodeGen/Base.hs
index 81f3b9f84c..7bed4c7b8d 100644
--- a/compiler/llvmGen/LlvmCodeGen/Base.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Base.hs
@@ -253,10 +253,10 @@ liftIO m = LlvmM $ \env -> do x <- m
                               return (x, env)
 
 -- | Get initial Llvm environment.
-runLlvm :: DynFlags -> LlvmVersion -> BufHandle -> UniqSupply -> LlvmM () -> IO ()
+runLlvm :: DynFlags -> LlvmVersion -> BufHandle -> UniqSupply -> LlvmM a -> IO a
 runLlvm dflags ver out us m = do
-    _ <- runLlvmM m env
-    return ()
+    (a, _) <- runLlvmM m env
+    return a
   where env = LlvmEnv { envFunMap = emptyUFM
                       , envVarMap = emptyUFM
                       , envStackRegs = []