asm-emit-time IND_STATIC elimination

When a new closure identifier is being established to a
local or exported closure already emitted into the same
module, refrain from adding an IND_STATIC closure, and
instead emit an assembly-language alias.

Inter-module IND_STATIC objects still remain, and need to be
addressed by other measures.

Binary-size savings on nofib are around 0.1%.

1 files changed, 31 insertions, 3 deletions

diff --git a/compiler/llvmGen/LlvmCodeGen/Data.hs b/compiler/llvmGen/LlvmCodeGen/Data.hs
index cabfe76762..3651a88cc6 100644
--- a/compiler/llvmGen/LlvmCodeGen/Data.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Data.hs
@@ -32,12 +32,41 @@ import qualified Data.ByteString as BS
 structStr :: LMString
 structStr = fsLit "_struct"
 
+-- | The LLVM visibility of the label
+linkage :: CLabel -> LlvmLinkageType
+linkage lbl = if externallyVisibleCLabel lbl
+              then ExternallyVisible else Internal
+
 -- ----------------------------------------------------------------------------
 -- * Top level
 --
 
 -- | Pass a CmmStatic section to an equivalent Llvm code.
 genLlvmData :: (Section, CmmStatics) -> LlvmM LlvmData
+-- See note [emit-time elimination of static indirections] in CLabel.
+genLlvmData (_, Statics alias [CmmStaticLit (CmmLabel lbl), CmmStaticLit ind, _, _])
+  | lbl == mkIndStaticInfoLabel
+  , let labelInd (CmmLabelOff l _) = Just l
+        labelInd (CmmLabel l) = Just l
+        labelInd _ = Nothing
+  , Just ind' <- labelInd ind
+  , alias `mayRedirectTo` ind' = do
+    label <- strCLabel_llvm alias
+    label' <- strCLabel_llvm ind'
+    let link     = linkage alias
+        link'    = linkage ind'
+        -- the LLVM type we give the alias is an empty struct type
+        -- but it doesn't really matter, as the pointer is only
+        -- used for (bit/int)casting.
+        tyAlias  = LMAlias (label `appendFS` structStr, LMStructU [])
+
+        aliasDef = LMGlobalVar label tyAlias link Nothing Nothing Alias
+        -- we don't know the type of the indirectee here
+        indType  = panic "will be filled by 'aliasify', later"
+        orig     = LMStaticPointer $ LMGlobalVar label' indType link' Nothing Nothing Alias
+
+    pure ([LMGlobal aliasDef $ Just orig], [tyAlias])
+
 genLlvmData (sec, Statics lbl xs) = do
     label <- strCLabel_llvm lbl
     static <- mapM genData xs
@@ -45,11 +74,10 @@ genLlvmData (sec, Statics lbl xs) = do
     let types   = map getStatType static
 
         strucTy = LMStruct types
-        tyAlias = LMAlias ((label `appendFS` structStr), strucTy)
+        tyAlias = LMAlias (label `appendFS` structStr, strucTy)
 
         struct         = Just $ LMStaticStruc static tyAlias
-        link           = if (externallyVisibleCLabel lbl)
-                            then ExternallyVisible else Internal
+        link           = linkage lbl
         align          = case sec of
                             Section CString _ -> Just 1
                             _                 -> Nothing