[GHC] #8336: Sinking pass does not

20 Sep 2013


      #8336: Sinking pass does not
------------------------------+--------------------------------------------
       Reporter:  jstolarek   |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  Compiler    |           Version:  7.7
       Keywords:              |  Operating System:  Unknown/Multiple
   Architecture:              |   Type of failure:  Runtime performance bug
  Unknown/Multiple            |         Test Case:
     Difficulty:  Unknown     |          Blocking:
     Blocked By:              |
Related Tickets:              |
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 Compiling this program:

 {{{
 {-# LANGUAGE BangPatterns, MagicHash, CPP #-}
 {-# OPTIONS_GHC -O2 -funbox-strict-fields #-}
 module HashStr where

 import Foreign.C
 import GHC.Exts
 import Data.Word

 #define hASH_TBL_SIZE          4091

 hashStr  :: Ptr Word8 -> Int -> Int
  -- use the Addr to produce a hash value between 0 & m (inclusive)
 hashStr (Ptr a#) (I# len#) = loop 0# 0#
    where
     loop h n | n GHC.Exts.==# len# = I# h
              | otherwise  = loop h2 (n GHC.Exts.+# 1#)
           where !c = ord# (indexCharOffAddr# a# n)
                 !h2 = (c GHC.Exts.+# (h GHC.Exts.*# 128#)) `remInt#`
                       hASH_TBL_SIZE#
 }}}
 produces following Cmm code for `hashStr` function:
 {{{
 {offset
   cut:
       goto cux;
   cux:
       _stC::I64 = R3;
       _stB::I64 = R2;
       _stF::I64 = 0;
       _stE::I64 = 0;
       goto stD;
   stD:
       if (_stF::I64 == _stC::I64) goto cuH; else goto cuI;
   cuH:
       R1 = _stE::I64;
       call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
   cuI:
       _stM::I64 = %MO_S_Rem_W64(%MO_UU_Conv_W8_W64(I8[_stB::I64 +
 _stF::I64]) + (_stE::I64 << 7),
                                 4091);
       _stF::I64 = _stF::I64 + 1;
       _stE::I64 = _stM::I64;
       goto stD;
 }
 }}}
 The problem here is that three last assignments:
 {{{
 _stM::I64 = %MO_S_Rem_W64(%MO_UU_Conv_W8_W64(I8[_stB::I64 + _stF::I64]) +
 (_stE::I64 << 7), 4091);
 _stF::I64 = _stF::I64 + 1;
 _stE::I64 = _stM::I64;
 }}}
 could be optimized as:
 {{{
 _stE::I64 = %MO_S_Rem_W64(%MO_UU_Conv_W8_W64(I8[_stB::I64 + _stF::I64]) +
 (_stE::I64 << 7), 4091);
 _stF::I64 = _stF::I64 + 1;
 }}}
 We should improve sinking pass so that it can optimize such cases. See
 Note [dependent assignments] in CmmSink.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8336
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