Re: [GHC] #9308: nofib fannkuch-redux runs perpetually with -fllvm

11 Aug 2014

      #9308: nofib fannkuch-redux runs perpetually with -fllvm
-------------------------------------+-------------------------------------
              Reporter:  jrp         |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (LLVM)                             |         Keywords:  fannkuch-redux
            Resolution:              |     Architecture:  x86_64 (amd64)
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Incorrect   |  Related Tickets:  #5567
  result at runtime                  |
             Test Case:  fannkuch-   |
  redux                              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Changes (by rwbarton):

 * failure:  Runtime performance bug => Incorrect result at runtime
 * component:  NoFib benchmark suite => Compiler (LLVM)
 * os:  MacOS X => Unknown/Multiple
 * related:   => #5567

Comment:

 I can reproduce this with LLVM 3.3 on Linux x86_64. This one is a TBAA
 issue. The infinite loop goes away with `-fno-llvm-tbaa` or with this
 patch applied (from #9125):
 {{{
 diff --git a/compiler/llvmGen/LlvmCodeGen/Regs.hs
 b/compiler/llvmGen/LlvmCodeGen/Regs.hs
 index 0048659..d837d13 100644
 --- a/compiler/llvmGen/LlvmCodeGen/Regs.hs
 +++ b/compiler/llvmGen/LlvmCodeGen/Regs.hs
 @@ -101,11 +101,6 @@ stgTBAA
      , (heapN,  fsLit "heap",  Just topN)
      , (rxN,    fsLit "rx",    Just heapN)
      , (baseN,  fsLit "base",  Just topN)
 -    -- FIX: Not 100% sure about 'others' place. Might need to be under
 'heap'.
 -    -- OR I think the big thing is Sp is never aliased, so might want
 -    -- to change the hieracy to have Sp on its own branch that is never
 -    -- aliased (e.g never use top as a TBAA node).
 -    , (otherN, fsLit "other", Just topN)
      ]

  -- | Id values
 @@ -115,7 +110,7 @@ stackN = getUnique (fsLit "LlvmCodeGen.Regs.stackN")
  heapN  = getUnique (fsLit "LlvmCodeGen.Regs.heapN")
  rxN    = getUnique (fsLit "LlvmCodeGen.Regs.rxN")
  baseN  = getUnique (fsLit "LlvmCodeGen.Regs.baseN")
 -otherN = getUnique (fsLit "LlvmCodeGen.Regs.otherN")
 +otherN = topN

  -- | The TBAA metadata identifier
  tbaa :: LMString
 }}}

 The issue is Cmm code that looks like this:
 {{{
 /*
         let go !acc = do
                 k <- peekElemOff xs 0
                 if k == 0 then f acc else flop (fromIntegral k) xs >> go
 (acc+1)
 */
        c7b3:
            _s6xW::I64 = I64[Sp + 8];
            _s6yp::I64 = %MO_UU_Conv_W8_W64(I8[_s6xW::I64]);   // read from
 xs via _s6xW: TBAA "other"
            if (_s6yp::I64 != 0) goto c7ca; else goto c7cb;
        c7ca:
            I64[Sp - 8] = block_c7bd_info;
            R3 = _s6xW::I64 + _s6yp::I64;
            R2 = _s6xW::I64;
            Sp = Sp - 8;
            call $wa1_r6tm_info(R3, R2) args: 8, res: 8, upd: 8;
 ...

 /*
 flop k xs = flopp xs (xs `advancePtr` k)
  where flopp i j = when (i < j) $ swap i j >> flopp (incPtr i) (decPtr j)
        swap i j = do
         a <- peek i
         b <- peek j
         poke j a
         poke i b
 */
  $wa1_r6tm_entry() //  [R3, R2]
 ...
      {offset
        c6Wl:
            if (R2 < R3) goto c6Ws; else goto c6Wt;
        c6Ws:
            _s6uQ::I64 = %MO_UU_Conv_W8_W64(I8[R3]);
            I8[R3] = I8[R2];                                   // read from
 xs via R2: TBAA "rx"
            I8[R2] = %MO_UU_Conv_W64_W8(_s6uQ::I64);
            R3 = R3 - 1;
            R2 = R2 + 1;
            call $wa1_r6tm_info(R3, R2) args: 8, res: 0, upd: 8;
 }}}

 "rx" and "other" are incomparable in our TBAA tree. Apparently LLVM treats
 this as undefined behavior and, in this case, emits an infinite loop.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9308#comment:2
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