Implement function-sections for Haskell code, #8405

This adds a flag -split-sections that does similar things to
-split-objs, but using sections in single object files instead of
relying on the Satanic Splitter and other abominations. This is very
similar to the GCC flags -ffunction-sections and -fdata-sections.

The --gc-sections linker flag, which allows unused sections to actually
be removed, is added to all link commands (if the linker supports it) so
that space savings from having base compiled with sections can be
realized.

Supported both in LLVM and the native code-gen, in theory for all
architectures, but really tested on x86 only.

In the GHC build, a new SplitSections variable enables -split-sections
for relevant parts of the build.

Test Plan: validate with both settings of SplitSections

Reviewers: dterei, Phyx, austin, simonmar, thomie, bgamari

Reviewed By: simonmar, thomie, bgamari

Subscribers: hsyl20, erikd, kgardas, thomie

Differential Revision: https://phabricator.haskell.org/D1242

GHC Trac Issues: #8405

4 files changed, 46 insertions, 12 deletions

diff --git a/compiler/llvmGen/LlvmCodeGen/Base.hs b/compiler/llvmGen/LlvmCodeGen/Base.hs
index 7a673b8ec3..3367cdaf45 100644
--- a/compiler/llvmGen/LlvmCodeGen/Base.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Base.hs
@@ -26,7 +26,7 @@ module LlvmCodeGen.Base (
 
         cmmToLlvmType, widthToLlvmFloat, widthToLlvmInt, llvmFunTy,
         llvmFunSig, llvmFunArgs, llvmStdFunAttrs, llvmFunAlign, llvmInfAlign,
-        llvmPtrBits, tysToParams,
+        llvmPtrBits, tysToParams, llvmFunSection,
 
         strCLabel_llvm, strDisplayName_llvm, strProcedureName_llvm,
         getGlobalPtr, generateExternDecls,
@@ -140,6 +140,12 @@ llvmFunAlign dflags = Just (wORD_SIZE dflags)
 llvmInfAlign :: DynFlags -> LMAlign
 llvmInfAlign dflags = Just (wORD_SIZE dflags)
 
+-- | Section to use for a function
+llvmFunSection :: DynFlags -> LMString -> LMSection
+llvmFunSection dflags lbl
+    | gopt Opt_SplitSections dflags = Just (concatFS [fsLit ".text.", lbl])
+    | otherwise                     = Nothing
+
 -- | A Function's arguments
 llvmFunArgs :: DynFlags -> LiveGlobalRegs -> [LlvmVar]
 llvmFunArgs dflags live =
diff --git a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
index aa3a0c3f1e..fb79a9d973 100644
--- a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
@@ -144,7 +144,9 @@ getInstrinct2 fname fty@(LMFunction funSig) = do
         return []
       Nothing -> do
         funInsert fname fty
-        return [CmmData Data [([],[fty])]]
+        un <- runUs getUniqueM
+        let lbl = mkAsmTempLabel un
+        return [CmmData (Section Data lbl) [([],[fty])]]
 
     return (fv, nilOL, tops)
 
diff --git a/compiler/llvmGen/LlvmCodeGen/Data.hs b/compiler/llvmGen/LlvmCodeGen/Data.hs
index b306748d23..3c1af4f587 100644
--- a/compiler/llvmGen/LlvmCodeGen/Data.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Data.hs
@@ -15,6 +15,7 @@ import LlvmCodeGen.Base
 import BlockId
 import CLabel
 import Cmm
+import DynFlags
 
 import FastString
 import Outputable
@@ -36,6 +37,7 @@ genLlvmData :: (Section, CmmStatics) -> LlvmM LlvmData
 genLlvmData (sec, Statics lbl xs) = do
     label <- strCLabel_llvm lbl
     static <- mapM genData xs
+    lmsec <- llvmSection sec
     let types   = map getStatType static
 
         strucTy = LMStruct types
@@ -45,21 +47,43 @@ genLlvmData (sec, Statics lbl xs) = do
         link           = if (externallyVisibleCLabel lbl)
                             then ExternallyVisible else Internal
         const          = if isSecConstant sec then Constant else Global
-        varDef         = LMGlobalVar label tyAlias link Nothing Nothing const
+        varDef         = LMGlobalVar label tyAlias link lmsec Nothing const
         globDef        = LMGlobal varDef struct
 
     return ([globDef], [tyAlias])
 
 -- | Should a data in this section be considered constant
 isSecConstant :: Section -> Bool
-isSecConstant Text                    = True
-isSecConstant ReadOnlyData            = True
-isSecConstant RelocatableReadOnlyData = True
-isSecConstant ReadOnlyData16          = True
-isSecConstant Data                    = False
-isSecConstant UninitialisedData       = False
-isSecConstant (OtherSection _)        = False
-
+isSecConstant (Section t _) = case t of
+    Text                    -> True
+    ReadOnlyData            -> True
+    RelocatableReadOnlyData -> True
+    ReadOnlyData16          -> True
+    Data                    -> False
+    UninitialisedData       -> False
+    (OtherSection _)        -> False
+
+-- | Format the section type part of a Cmm Section
+llvmSectionType :: SectionType -> FastString
+llvmSectionType t = case t of
+    Text                    -> fsLit ".text"
+    ReadOnlyData            -> fsLit ".rodata"
+    RelocatableReadOnlyData -> fsLit ".data.rel.ro"
+    ReadOnlyData16          -> fsLit ".rodata.cst16"
+    Data                    -> fsLit ".data"
+    UninitialisedData       -> fsLit ".bss"
+    (OtherSection _)        -> panic "llvmSectionType: unknown section type"
+
+-- | Format a Cmm Section into a LLVM section name
+llvmSection :: Section -> LlvmM LMSection
+llvmSection (Section t suffix) = do
+  dflags <- getDynFlags
+  let splitSect = gopt Opt_SplitSections dflags
+  if not splitSect
+  then return Nothing
+  else do
+    lmsuffix <- strCLabel_llvm suffix
+    return (Just (concatFS [llvmSectionType t, fsLit ".", lmsuffix]))
 
 -- ----------------------------------------------------------------------------
 -- * Generate static data
diff --git a/compiler/llvmGen/LlvmCodeGen/Ppr.hs b/compiler/llvmGen/LlvmCodeGen/Ppr.hs
index d7ddf804f2..1de630ef10 100644
--- a/compiler/llvmGen/LlvmCodeGen/Ppr.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Ppr.hs
@@ -114,6 +114,7 @@ pprLlvmCmmDecl (CmmProc mb_info entry_lbl live (ListGraph blks))
        dflags <- getDynFlags
        let buildArg = fsLit . showSDoc dflags . ppPlainName
            funArgs = map buildArg (llvmFunArgs dflags live)
+           funSect = llvmFunSection dflags (decName funDec)
 
        -- generate the info table
        prefix <- case mb_info of
@@ -123,7 +124,8 @@ pprLlvmCmmDecl (CmmProc mb_info entry_lbl live (ListGraph blks))
                        let infoTy = LMStruct $ map getStatType infoStatics
                        return $ Just $ LMStaticStruc infoStatics infoTy
 
-       let fun = LlvmFunction funDec funArgs llvmStdFunAttrs Nothing
+
+       let fun = LlvmFunction funDec funArgs llvmStdFunAttrs funSect
                               prefix lmblocks
            name = decName $ funcDecl fun
            defName = name `appendFS` fsLit "$def"