Revert "Multiple fixes / improvements for LLVM backend"

This reverts commit 73273be476a8cc6c13368660b042b3b0614fd928.

Unfortunately we were unable to come to a fix that didn't sacrifice the
ability to bootstrap GHC using the LLVM backend. Reverting for 8.6.2.

5 files changed, 61 insertions, 67 deletions

diff --git a/compiler/llvmGen/Llvm/Types.hs b/compiler/llvmGen/Llvm/Types.hs
index a45004d3a8..bc7bbaab1b 100644
--- a/compiler/llvmGen/Llvm/Types.hs
+++ b/compiler/llvmGen/Llvm/Types.hs
@@ -564,7 +564,7 @@ instance Outputable LlvmFuncAttr where
   ppr OptSize            = text "optsize"
   ppr NoReturn           = text "noreturn"
   ppr NoUnwind           = text "nounwind"
-  ppr ReadNone           = text "readnone"
+  ppr ReadNone           = text "readnon"
   ppr ReadOnly           = text "readonly"
   ppr Ssp                = text "ssp"
   ppr SspReq             = text "ssqreq"
diff --git a/compiler/llvmGen/LlvmCodeGen/Base.hs b/compiler/llvmGen/LlvmCodeGen/Base.hs
index ec91bacc4c..6e20da48c1 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, llvmFunSection, padLiveArgs, isSSE,
+        llvmPtrBits, tysToParams, llvmFunSection,
 
         strCLabel_llvm, strDisplayName_llvm, strProcedureName_llvm,
         getGlobalPtr, generateExternDecls,
@@ -58,8 +58,6 @@ import ErrUtils
 import qualified Stream
 
 import Control.Monad (ap)
-import Data.List (sort)
-import Data.Maybe (mapMaybe)
 
 -- ----------------------------------------------------------------------------
 -- * Some Data Types
@@ -149,58 +147,16 @@ llvmFunSection dflags lbl
 -- | A Function's arguments
 llvmFunArgs :: DynFlags -> LiveGlobalRegs -> [LlvmVar]
 llvmFunArgs dflags live =
-    map (lmGlobalRegArg dflags) (filter isPassed allRegs)
+    map (lmGlobalRegArg dflags) (filter isPassed (activeStgRegs platform))
     where platform = targetPlatform dflags
-          allRegs = activeStgRegs platform
-          paddedLive = map (\(_,r) -> r) $ padLiveArgs live
-          isLive r = r `elem` alwaysLive || r `elem` paddedLive
+          isLive r = not (isSSE r) || r `elem` alwaysLive || r `elem` live
           isPassed r = not (isSSE r) || isLive r
-
-
-isSSE :: GlobalReg -> Bool
-isSSE (FloatReg _)  = True
-isSSE (DoubleReg _) = True
-isSSE (XmmReg _)    = True
-isSSE (YmmReg _)    = True
-isSSE (ZmmReg _)    = True
-isSSE _             = False
-
-sseRegNum :: GlobalReg -> Maybe Int
-sseRegNum (FloatReg i)  = Just i
-sseRegNum (DoubleReg i) = Just i
-sseRegNum (XmmReg i)    = Just i
-sseRegNum (YmmReg i)    = Just i
-sseRegNum (ZmmReg i)    = Just i
-sseRegNum _             = Nothing
-
--- the bool indicates whether the global reg was added as padding.
--- the returned list is not sorted in any particular order,
--- but does indicate the set of live registers needed, with SSE padding.
-padLiveArgs :: LiveGlobalRegs -> [(Bool, GlobalReg)]
-padLiveArgs live = allRegs
-    where
-        sseRegNums = sort $ mapMaybe sseRegNum live
-        (_, padding) = foldl assignSlots (1, []) $ sseRegNums
-        allRegs = padding ++ map (\r -> (False, r)) live
-
-        assignSlots (i, acc) regNum
-            | i == regNum = -- don't need padding here
-                  (i+1, acc)
-            | i < regNum = let -- add padding for slots i .. regNum-1
-                  numNeeded = regNum-i
-                  acc' = genPad i numNeeded ++ acc
-                in
-                  (regNum+1, acc')
-            | otherwise = error "padLiveArgs -- i > regNum ??"
-
-        genPad start n =
-            take n $ flip map (iterate (+1) start) (\i ->
-                (True, FloatReg i))
-                -- NOTE: Picking float should be fine for the following reasons:
-                -- (1) Float aliases with all the other SSE register types on
-                -- the given platform.
-                -- (2) The argument is not live anyways.
-
+          isSSE (FloatReg _)  = True
+          isSSE (DoubleReg _) = True
+          isSSE (XmmReg _)    = True
+          isSSE (YmmReg _)    = True
+          isSSE (ZmmReg _)    = True
+          isSSE _             = False
 
 -- | Llvm standard fun attributes
 llvmStdFunAttrs :: [LlvmFuncAttr]
diff --git a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
index 041329ef8d..f5d06e54b6 100644
--- a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
@@ -14,7 +14,7 @@ import LlvmCodeGen.Base
 import LlvmCodeGen.Regs
 
 import BlockId
-import CodeGen.Platform ( activeStgRegs )
+import CodeGen.Platform ( activeStgRegs, callerSaves )
 import CLabel
 import Cmm
 import PprCmm
@@ -211,6 +211,7 @@ genCall t@(PrimTarget (MO_Prefetch_Data localityInt)) [] args
     fptr    <- liftExprData $ getFunPtr funTy t
     argVars' <- castVarsW Signed $ zip argVars argTy
 
+    doTrashStmts
     let argSuffix = [mkIntLit i32 0, mkIntLit i32 localityInt, mkIntLit i32 1]
     statement $ Expr $ Call StdCall fptr (argVars' ++ argSuffix) []
   | otherwise = panic $ "prefetch locality level integer must be between 0 and 3, given: " ++ (show localityInt)
@@ -293,6 +294,7 @@ genCall t@(PrimTarget op) [] args
     fptr          <- getFunPtrW funTy t
     argVars' <- castVarsW Signed $ zip argVars argTy
 
+    doTrashStmts
     let alignVal = mkIntLit i32 align
         arguments = argVars' ++ (alignVal:isVolVal)
     statement $ Expr $ Call StdCall fptr arguments []
@@ -447,6 +449,7 @@ genCall target res args = runStmtsDecls $ do
                  | never_returns     = statement $ Unreachable
                  | otherwise         = return ()
 
+    doTrashStmts
 
     -- make the actual call
     case retTy of
@@ -1775,9 +1778,12 @@ genLit _ CmmHighStackMark
 funPrologue :: LiveGlobalRegs -> [CmmBlock] -> LlvmM StmtData
 funPrologue live cmmBlocks = do
 
+  trash <- getTrashRegs
   let getAssignedRegs :: CmmNode O O -> [CmmReg]
       getAssignedRegs (CmmAssign reg _)  = [reg]
-      getAssignedRegs (CmmUnsafeForeignCall _ rs _) = map CmmLocal rs
+      -- Calls will trash all registers. Unfortunately, this needs them to
+      -- be stack-allocated in the first place.
+      getAssignedRegs (CmmUnsafeForeignCall _ rs _) = map CmmGlobal trash ++ map CmmLocal rs
       getAssignedRegs _                  = []
       getRegsBlock (_, body, _)          = concatMap getAssignedRegs $ blockToList body
       assignedRegs = nub $ concatMap (getRegsBlock . blockSplit) cmmBlocks
@@ -1807,9 +1813,14 @@ funPrologue live cmmBlocks = do
 funEpilogue :: LiveGlobalRegs -> LlvmM ([LlvmVar], LlvmStatements)
 funEpilogue live = do
 
-    -- the bool indicates whether the register is padding.
-    let alwaysNeeded = map (\r -> (False, r)) alwaysLive
-        livePadded = alwaysNeeded ++ padLiveArgs live
+    -- Have information and liveness optimisation is enabled?
+    let liveRegs = alwaysLive ++ live
+        isSSE (FloatReg _)  = True
+        isSSE (DoubleReg _) = True
+        isSSE (XmmReg _)    = True
+        isSSE (YmmReg _)    = True
+        isSSE (ZmmReg _)    = True
+        isSSE _             = False
 
     -- Set to value or "undef" depending on whether the register is
     -- actually live
@@ -1821,17 +1832,39 @@ funEpilogue live = do
           let ty = (pLower . getVarType $ lmGlobalRegVar dflags r)
           return (Just $ LMLitVar $ LMUndefLit ty, nilOL)
     platform <- getDynFlag targetPlatform
-    let allRegs = activeStgRegs platform
-    loads <- flip mapM allRegs $ \r -> case () of
-      _ | (False, r) `elem` livePadded
-                             -> loadExpr r   -- if r is not padding, load it
-        | not (isSSE r) || (True, r) `elem` livePadded
-                             -> loadUndef r
+    loads <- flip mapM (activeStgRegs platform) $ \r -> case () of
+      _ | r `elem` liveRegs  -> loadExpr r
+        | not (isSSE r)      -> loadUndef r
         | otherwise          -> return (Nothing, nilOL)
 
     let (vars, stmts) = unzip loads
     return (catMaybes vars, concatOL stmts)
 
+
+-- | A series of statements to trash all the STG registers.
+--
+-- In LLVM we pass the STG registers around everywhere in function calls.
+-- So this means LLVM considers them live across the entire function, when
+-- in reality they usually aren't. For Caller save registers across C calls
+-- the saving and restoring of them is done by the Cmm code generator,
+-- using Cmm local vars. So to stop LLVM saving them as well (and saving
+-- all of them since it thinks they're always live, we trash them just
+-- before the call by assigning the 'undef' value to them. The ones we
+-- need are restored from the Cmm local var and the ones we don't need
+-- are fine to be trashed.
+getTrashStmts :: LlvmM LlvmStatements
+getTrashStmts = do
+  regs <- getTrashRegs
+  stmts <- flip mapM regs $ \ r -> do
+    reg <- getCmmReg (CmmGlobal r)
+    let ty = (pLower . getVarType) reg
+    return $ Store (LMLitVar $ LMUndefLit ty) reg
+  return $ toOL stmts
+
+getTrashRegs :: LlvmM [GlobalReg]
+getTrashRegs = do plat <- getLlvmPlatform
+                  return $ filter (callerSaves plat) (activeStgRegs plat)
+
 -- | Get a function pointer to the CLabel specified.
 --
 -- This is for Haskell functions, function type is assumed, so doesn't work
@@ -1953,6 +1986,11 @@ getCmmRegW = lift . getCmmReg
 genLoadW :: Atomic -> CmmExpr -> CmmType -> WriterT LlvmAccum LlvmM LlvmVar
 genLoadW atomic e ty = liftExprData $ genLoad atomic e ty
 
+doTrashStmts :: WriterT LlvmAccum LlvmM ()
+doTrashStmts = do
+    stmts <- lift getTrashStmts
+    tell $ LlvmAccum stmts mempty
+
 -- | Return element of single-element list; 'panic' if list is not a single-element list
 singletonPanic :: String -> [a] -> a
 singletonPanic _ [x] = x
diff --git a/llvm-passes b/llvm-passes
index 14eb62d87c..5183c9f2ab 100644
--- a/llvm-passes
+++ b/llvm-passes
@@ -1,5 +1,5 @@
 [
-(0, "-mem2reg -globalopt -lower-expect"),
+(0, "-mem2reg -globalopt"),
 (1, "-O1 -globalopt"),
 (2, "-O2")
 ]
diff --git a/testsuite/tests/codeGen/should_run/all.T b/testsuite/tests/codeGen/should_run/all.T
index eaf0e77b97..3935574549 100644
--- a/testsuite/tests/codeGen/should_run/all.T
+++ b/testsuite/tests/codeGen/should_run/all.T
@@ -172,7 +172,7 @@ test('T13825-unit',
 test('T14619', normal, compile_and_run, [''])
 test('T14754', normal, compile_and_run, [''])
 test('T14346', only_ways(['threaded1','threaded2']), compile_and_run, ['-O -threaded'])
-test('T14251', [expect_broken_for(14251, [''])],
+test('T14251', [expect_broken_for(14251, ['optllvm'])],
      compile_and_run, [''])
 
 # These actually used to fail with all optimisation settings, but adding -O just