[GHC] #12095: GHC and LLVM don't agree on what to do with byteSwap16#

21 May 2016

      #12095: GHC and LLVM don't agree on what to do with byteSwap16#
-------------------------------------+-------------------------------------
           Reporter:  thoughtpolice  |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:  8.0.2
          Component:  Compiler       |           Version:  8.0.1
  (LLVM)                             |
           Keywords:  codegen, llvm  |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider this test case (taken from [https://github.com/well-typed/binary-
 serialise-cbor/issues/67 here] and lightly modified to work on big/little
 endian machines):

 {{{#!hs
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE MagicHash    #-}
 {-# LANGUAGE CPP          #-}
 module Main
   ( main -- :: IO ()
   ) where

 #include "ghcconfig.h"

 import           GHC.Prim
 import           GHC.Word

 data T = T !Addr#

 t :: T
 #ifndef WORDS_BIGENDIAN
 t = T "\xcf\xb1"#
 #else
 t = T "\xb1\xcf"#
 #endif

 grabWord16 :: T -> Word64
 grabWord16 (T addr#) = W64# (byteSwap16# (indexWord16OffAddr# addr# 0#))

 trip :: Int
 trip = fromIntegral (grabWord16 t)

 main :: IO ()
 main = print trip
 }}}

 With GHC 7.10.3 using the NCG, the results given are correct:

 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.10.3
 $ ghc -Wall -fforce-recomp -O2 Issue67.hs && ./Issue67
 [1 of 1] Compiling Main             ( Issue67.hs, Issue67.o )
 Linking Issue67 ...
 53169
 }}}

 This also is the same on GHC 8.0.1 using the NCG, on both PowerPC and
 AMD64 as well. This answer is correct: `53169` is `0xCFB1` in hex, so the
 `byteSwap16#` primitive correctly works to decode the swapped-endian
 number.

 However, the story is not the same with GHC 7.10.3+LLVM 3.5, or GHC
 8.0.1+LLVM 3.7:

 {{{
 $ ghc --version
 The Glorious Glasgow Haskell Compilation System, version 7.10.3
 $ llc --version | head -2
 LLVM (http://llvm.org/):
   LLVM version 3.5.2
 $ ghc -Wall -fforce-recomp -O2 Issue67.hs -fllvm && ./Issue67
 [1 of 1] Compiling Main             ( Issue67.hs, Issue67.o )
 Linking Issue67 ...
 -12367
 }}}

 Note:

 {{{#!hs
 -12367 == (fromIntegral (53169 :: Word16) :: Int16)
 }}}

 The relevant snippet looks like this at the CMM level (GHC 7.10.3):

 {{{
 ==================== Output Cmm ====================
 [section "data" {
      Main.main2_closure:
          const Main.main2_info;
          const 0;
          const 0;
          const 0;
  },
  section "readonly" {
      c3rq_str:
          I8[] [207,177]
  },
  section "readonly" {
      c3rr_str:
          I8[] [207,177]
  },
  Main.main2_entry() //  [R1]
          { info_tbl: [(c3ru,
                        label: Main.main2_info
                        rep:HeapRep static { Thunk }),
                       (c3rD,
                        label: block_c3rD_info
                        rep:StackRep [])]
            stack_info: arg_space: 8 updfr_space: Just 8
          }
      {offset
        c3ru:
            ...
        c3ro:
            I64[Sp - 16] = stg_bh_upd_frame_info;
            I64[Sp - 8] = _c3rn::I64;
            (_c3rw::I64) = call MO_BSwap
 W16(%MO_UU_Conv_W16_W64(I16[c3rr_str]));
            I64[Sp - 24] = c3rD;
            R4 = GHC.Types.[]_closure+1;
            R3 = _c3rw::I64;
            R2 = 0;
            Sp = Sp - 24;
            call GHC.Show.$wshowSignedInt_info(R4,
                                               R3,
                                               R2) returns to c3rD, args:
 8, res: 8, upd: 24;
 ...
 }}}

 Pre-optimized LLVM basic block:

 {{{
 c3rB:
   %ln3sc = ptrtoint i8* @stg_bh_upd_frame_info to i64
   %ln3sb = load i64** %Sp_Var
   %ln3sd = getelementptr inbounds i64* %ln3sb, i32 -2
   store i64 %ln3sc, i64* %ln3sd, !tbaa !1
   %ln3sf = load i64* %lc3rA
   %ln3se = load i64** %Sp_Var
   %ln3sg = getelementptr inbounds i64* %ln3se, i32 -1
   store i64 %ln3sf, i64* %ln3sg, !tbaa !1
   %ln3sh = ptrtoint %c3rE_str_struct* @c3rE_str$def to i64
   %ln3si = inttoptr i64 %ln3sh to i16*
   %ln3sj = load i16* %ln3si, !tbaa !5
   %ln3sk = zext i16 %ln3sj to i64
   %ln3sl = trunc i64 %ln3sk to i16
   %ln3sm = call ccc i16 (i16)* @llvm.bswap.i16( i16 %ln3sl )
   %ln3sn = sext i16 %ln3sm to i64
   store i64 %ln3sn, i64* %lc3rJ
   %ln3sp = ptrtoint void (i64*, i64*, i64*, i64, i64, i64, i64, i64, i64,
 i64)* @c3rQ_info$def to i64
   %ln3so = load i64** %Sp_Var
   %ln3sq = getelementptr inbounds i64* %ln3so, i32 -3
   store i64 %ln3sp, i64* %ln3sq, !tbaa !1
   %ln3sr = ptrtoint i8* @ghczmprim_GHCziTypes_ZMZN_closure to i64
   %ln3ss = add i64 %ln3sr, 1
   store i64 %ln3ss, i64* %R4_Var
   %ln3st = load i64* %lc3rJ
   store i64 %ln3st, i64* %R3_Var
   store i64 0, i64* %R2_Var
   %ln3su = load i64** %Sp_Var
   %ln3sv = getelementptr inbounds i64* %ln3su, i32 -3
   %ln3sw = ptrtoint i64* %ln3sv to i64
   %ln3sx = inttoptr i64 %ln3sw to i64*
   store i64* %ln3sx, i64** %Sp_Var
   %ln3sy = bitcast i8* @base_GHCziShow_zdwshowSignedInt_info to void
 (i64*, i64*, i64*, i64, i64, i64, i64, i64, i64, i64)*
 }}}

 Post-optimized block (`opt --enable-tbaa=true -O2 out-llvm-orig.ll -o out-
 llvm.bc`):

 {{{
 c3rB:                                             ; preds = %c3rU
   %ln3s8 = ptrtoint i8* %ln3s7 to i64
   %ln3sd = getelementptr inbounds i64* %Sp_Arg, i64 -2
   store i64 ptrtoint (i8* @stg_bh_upd_frame_info to i64), i64* %ln3sd,
 align 8, !tbaa !5
   %ln3sg = getelementptr inbounds i64* %Sp_Arg, i64 -1
   store i64 %ln3s8, i64* %ln3sg, align 8, !tbaa !5
   store i64 ptrtoint (void (i64*, i64*, i64*, i64, i64, i64, i64, i64,
 i64, i64)* @"c3rQ_info$def" to i64), i64* %ln3rZ, align 8, !tbaa !5
   tail call cc10 void bitcast (i8* @base_GHCziShow_zdwshowSignedInt_info
 to void (i64*, i64*, i64*, i64, i64, i64, i64, i64, i64, i64)*)(i64*
 %Base_Arg, i64* %ln3rZ, i64* %Hp_Arg, i64 %R1_Arg, i64 0, i64 -12367, i64
 add (i64 ptrtoint (i8* @ghczmprim_GHCziTypes_ZMZN_closure to i64), i64 1),
 i64 undef, i64 undef, i64 %SpLim_Arg) #0
   ret void
 }}}

 Folds it right into a constant!

 I haven't spent time diagnosing this much further, yet.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12095
GHC http://www.haskell.org/ghc/
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