Re: [GHC] #6084: Add stg_ap_pnnv and related call patterns

16 Sep 2017

      #6084: Add stg_ap_pnnv and related call patterns
-------------------------------------+-------------------------------------
        Reporter:  SimonMeier        |                Owner:  simonmar
            Type:  feature request   |               Status:  closed
        Priority:  high              |            Milestone:  7.8.1
       Component:  Runtime System    |              Version:  7.7
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  8313
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by angerman):

 cmm, as per `-ddump-cmm`.

 I believe the relevant part is
 {{{
 ==================== Output Cmm ====================
 [section ""relreadonly" . S4QA_srt" {
      S4QA_srt:
          const Main.q1_closure;
  },
  section ""data" . Main.p_closure" {
      Main.p_closure:
          const Main.p_info;
          const 0;
  },
  Main.p_slow() //  [R1]
          { info_tbl: []
            stack_info: arg_space: 0 updfr_space: Nothing
          }
      {offset
        c4Qt: // global
            D2 = F64[Sp + 16];
            F1 = F32[Sp + 8];
            R2 = I64[Sp];
            R1 = R1;
            Sp = Sp + 24;
            call Main.p_info(D2, F1, R2, R1) args: 8, res: 0, upd: 8;
      }
  },
  Main.p_entry() //  []
          { info_tbl: [(c4Qx,
                        label: Main.p_info
                        rep:HeapRep static {
                              Fun {arity: 3 fun_type: ArgGen [True, True,
 True]} })]
            stack_info: arg_space: 0 updfr_space: Nothing
          }
      {offset
        c4Qx: // global
            R1 = Main.q1_closure;
            call (I64[R1])(R1) args: 8, res: 0, upd: 8;
      }
  }]

 }}}

 However, my Cmm knowledge is rather limited.

 The llvm backend then does:
 {{{
 pprLlvmCmmDecl (CmmProc mb_info entry_lbl live (ListGraph blks))
   = do let lbl = case mb_info of
                      Nothing                   -> entry_lbl
                      Just (Statics info_lbl _) -> info_lbl
            link = if externallyVisibleCLabel lbl
                       then ExternallyVisible
                       else Internal
            lmblocks = map (\(BasicBlock id stmts) ->
                                 LlvmBlock (getUnique id) stmts) blks

        funDec <- llvmFunSig live lbl link
        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
                      Nothing -> return Nothing
                      Just (Statics _ statics) -> do
                        infoStatics <- mapM genData statics
                        let infoTy = LMStruct $ map getStatType infoStatics
                        return $ Just $ LMStaticStruc infoStatics infoTy

        let fun = LlvmFunction funDec funArgs llvmStdFunAttrs funSect
                               prefix lmblocks
            name = decName $ funcDecl fun
            defName = name `appendFS` fsLit "$def"
            funcDecl' = (funcDecl fun) { decName = defName }
            fun' = fun { funcDecl = funcDecl' }
            funTy = LMFunction funcDecl'
            funVar = LMGlobalVar name
                                 (LMPointer funTy)
                                 link
                                 Nothing
                                 Nothing
                                 Alias
            defVar = LMGlobalVar defName
                                 (LMPointer funTy)
                                 (funcLinkage funcDecl')
                                 (funcSect fun)
                                 (funcAlign funcDecl')
                                 Alias
            alias = LMGlobal funVar
                             (Just $ LMBitc (LMStaticPointer defVar)
                                            (LMPointer $ LMInt 8))

        return (ppLlvmGlobal alias $+$ ppLlvmFunction fun', [])

 -- ...

 llvmFunSig :: LiveGlobalRegs ->  CLabel -> LlvmLinkageType -> LlvmM
 LlvmFunctionDecl
 llvmFunSig live lbl link = do
   lbl' <- strCLabel_llvm lbl
   llvmFunSig' live lbl' link

 llvmFunSig' :: LiveGlobalRegs -> LMString -> LlvmLinkageType -> LlvmM
 LlvmFunctionDecl
 llvmFunSig' live lbl link
   = do let toParams x | isPointer x = (x, [NoAlias, NoCapture])
                       | otherwise   = (x, [])
        dflags <- getDynFlags
        return $ LlvmFunctionDecl lbl link (llvmGhcCC dflags) LMVoid
 FixedArgs
                                  (map (toParams . getVarType) (llvmFunArgs
 dflags live))
                                  (llvmFunAlign dflags)

 -- ...

 -- | A Function's arguments
 llvmFunArgs :: DynFlags -> LiveGlobalRegs -> [LlvmVar]
 llvmFunArgs dflags live =
     map (lmGlobalRegArg dflags) (filter isPassed (activeStgRegs platform))
     where platform = targetPlatform dflags
           isLive r = not (isSSE r) || r `elem` alwaysLive || r `elem` live
           isPassed r = not (isSSE r) || isLive r
           isSSE (FloatReg _)  = True
           isSSE (DoubleReg _) = True
           isSSE (XmmReg _)    = True
           isSSE (YmmReg _)    = True
           isSSE (ZmmReg _)    = True
           isSSE _             = False

 -- ...

 -- | A list of STG Registers that should always be considered alive
 alwaysLive :: [GlobalReg]
 alwaysLive = [BaseReg, Sp, Hp, SpLim, HpLim, node]

 }}}

 As the set of live registers for `Main.p_info` (`Main.p_entry()`) is
 empty, we end up generating the default signature for
 {{{
 ["BaseReg","Sp","Hp","R1","R2","R3","R4","R5","R6","SpLim"]
 }}}
 only, when we supposedly would want to have included `F1` and `D2` as
 well. On the other hand, I don't see where we'd use those in the body of
 `p_entry`. And thus why we'd want to pass them in the first place?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/6084#comment:37
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