Multiple fixes / improvements for LLVM backend

- Fix for #13904 -- stop "trashing" callee-saved registers, since it is
  not actually doing anything useful.

- Fix for #14251 -- fixes the calling convention for functions passing
  raw SSE-register values by adding padding as needed to get the values
  in the right registers. This problem cropped up when some args were
  unused an dropped from the live list.

- Fixed a typo in 'readnone' attribute

- Added 'lower-expect' pass to level 0 LLVM optimization passes to
  improve block layout in LLVM for stack checks, etc.

Test Plan: `make test WAYS=optllvm` and `make test WAYS=llvm`

Reviewers: bgamari, simonmar, angerman

Reviewed By: angerman

Subscribers: rwbarton, carter

GHC Trac Issues: #13904, #14251

Differential Revision: https://phabricator.haskell.org/D5190

(cherry picked from commit adcb5fb4)

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 "readnon"
+  ppr ReadNone           = text "readnone"
   ppr ReadOnly           = text "readonly"
   ppr Ssp                = text "ssp"
   ppr SspReq             = text "ssqreq"

compiler/llvmGen/LlvmCodeGen/Base.hs

@@ -26,7 +26,7 @@ module LlvmCodeGen.Base (
 
         cmmToLlvmType, widthToLlvmFloat, widthToLlvmInt, llvmFunTy,
         llvmFunSig, llvmFunArgs, llvmStdFunAttrs, llvmFunAlign, llvmInfAlign,
-        llvmPtrBits, tysToParams, llvmFunSection,
+        llvmPtrBits, tysToParams, llvmFunSection, padLiveArgs, isSSE,
 
         strCLabel_llvm, strDisplayName_llvm, strProcedureName_llvm,
         getGlobalPtr, generateExternDecls,
@@ -58,6 +58,8 @@ import ErrUtils
 import qualified Stream
 
 import Control.Monad (ap)
+import Data.List (sort)
+import Data.Maybe (mapMaybe)
 
 -- ----------------------------------------------------------------------------
 -- * Some Data Types
@@ -147,16 +149,58 @@ llvmFunSection dflags lbl
 -- | A Function's arguments
 llvmFunArgs :: DynFlags -> LiveGlobalRegs -> [LlvmVar]
 llvmFunArgs dflags live =
-    map (lmGlobalRegArg dflags) (filter isPassed (activeStgRegs platform))
+    map (lmGlobalRegArg dflags) (filter isPassed allRegs)
     where platform = targetPlatform dflags
-          isLive r = not (isSSE r) || r `elem` alwaysLive || r `elem` live
+          allRegs = activeStgRegs platform
+          paddedLive = map (\(_,r) -> r) $ padLiveArgs live
+          isLive r = r `elem` alwaysLive || r `elem` paddedLive
           isPassed r = not (isSSE r) || isLive r
-          isSSE (FloatReg _)  = True
-          isSSE (DoubleReg _) = True
-          isSSE (XmmReg _)    = True
-          isSSE (YmmReg _)    = True
-          isSSE (ZmmReg _)    = True
-          isSSE _             = False
+
+
+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.
+
 
 -- | Llvm standard fun attributes
 llvmStdFunAttrs :: [LlvmFuncAttr]

compiler/llvmGen/LlvmCodeGen/CodeGen.hs

@@ -14,7 +14,7 @@ import LlvmCodeGen.Base
 import LlvmCodeGen.Regs
 
 import BlockId
-import CodeGen.Platform ( activeStgRegs, callerSaves )
+import CodeGen.Platform ( activeStgRegs )
 import CLabel
 import Cmm
 import PprCmm
@@ -211,7 +211,6 @@ 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)
@@ -294,7 +293,6 @@ 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 []
@@ -449,7 +447,6 @@ genCall target res args = runStmtsDecls $ do
                  | never_returns     = statement $ Unreachable
                  | otherwise         = return ()
 
-    doTrashStmts
 
     -- make the actual call
     case retTy of
@@ -1778,12 +1775,9 @@ genLit _ CmmHighStackMark
 funPrologue :: LiveGlobalRegs -> [CmmBlock] -> LlvmM StmtData
 funPrologue live cmmBlocks = do
 
-  trash <- getTrashRegs
   let getAssignedRegs :: CmmNode O O -> [CmmReg]
       getAssignedRegs (CmmAssign reg _)  = [reg]
-      -- 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 (CmmUnsafeForeignCall _ rs _) = map CmmLocal rs
       getAssignedRegs _                  = []
       getRegsBlock (_, body, _)          = concatMap getAssignedRegs $ blockToList body
       assignedRegs = nub $ concatMap (getRegsBlock . blockSplit) cmmBlocks
@@ -1813,14 +1807,9 @@ funPrologue live cmmBlocks = do
 funEpilogue :: LiveGlobalRegs -> LlvmM ([LlvmVar], LlvmStatements)
 funEpilogue live = do
 
-    -- 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
+    -- the bool indicates whether the register is padding.
+    let alwaysNeeded = map (\r -> (False, r)) alwaysLive
+        livePadded = alwaysNeeded ++ padLiveArgs live
 
     -- Set to value or "undef" depending on whether the register is
     -- actually live
@@ -1832,39 +1821,17 @@ funEpilogue live = do
           let ty = (pLower . getVarType $ lmGlobalRegVar dflags r)
           return (Just $ LMLitVar $ LMUndefLit ty, nilOL)
     platform <- getDynFlag targetPlatform
-    loads <- flip mapM (activeStgRegs platform) $ \r -> case () of
-      _ | r `elem` liveRegs  -> loadExpr r
-        | not (isSSE r)      -> loadUndef r
+    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
         | 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
@@ -1986,11 +1953,6 @@ 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

llvm-passes

 [
-(0, "-mem2reg -globalopt"),
+(0, "-mem2reg -globalopt -lower-expect"),
 (1, "-O1 -globalopt"),
 (2, "-O2")
 ]

testsuite/tests/codeGen/should_run/all.T

@@ -172,5 +172,4 @@ 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, ['optllvm'])],
-     compile_and_run, [''])
+test('T14251', normal, compile_and_run, [''])