[GHC] #10676: silly assembly for comparing the result of comparisons that return Int# against 0#

23 Jul 2015

      #10676: silly assembly for comparing the result of comparisons that return Int#
against 0#
-------------------------------------+-------------------------------------
              Reporter:  rwbarton    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
  (CodeGen)                          |
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:  x86_64      |   Type of failure:  Runtime
  (amd64)                            |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Consider the module
 {{{
 {-# LANGUAGE MagicHash #-}
 module Min where

 import GHC.Exts

 fgood :: Int# -> Int# -> Int
 fgood x# y# = case isTrue# (x# <# y#) of
   False -> I# y#
   True  -> I# x#

 fbad :: Int# -> Int# -> Int
 fbad x# y# = case x# <# y# of
   0# -> I# y#
   _  -> I# x#
 }}}

 The code for `fgood` looks fine:
 {{{
 0000000000000130 :
  130:   49 83 c4 10             add    $0x10,%r12
  134:   4d 3b a5 58 03 00 00    cmp    0x358(%r13),%r12
  13b:   77 1a                   ja     157 
  13d:   49 39 f6                cmp    %rsi,%r14
  140:   7c 29                   jl     16b 
  142:   49 c7 44 24 f8 00 00    movq   $0x0,-0x8(%r12)
  149:   00 00
                         147: R_X86_64_32S
 ghczmprim_GHCziTypes_Izh_con_info
  14b:   49 89 34 24             mov    %rsi,(%r12)
  14f:   49 8d 5c 24 f9          lea    -0x7(%r12),%rbx
  154:   ff 65 00                jmpq   *0x0(%rbp)
  157:   49 c7 85 88 03 00 00    movq   $0x10,0x388(%r13)
  15e:   10 00 00 00
  162:   bb 00 00 00 00          mov    $0x0,%ebx
                         163: R_X86_64_32        Min_fgood_closure
  167:   41 ff 65 f8             jmpq   *-0x8(%r13)
  16b:   49 c7 44 24 f8 00 00    movq   $0x0,-0x8(%r12)
  172:   00 00
                         170: R_X86_64_32S
 ghczmprim_GHCziTypes_Izh_con_info
  174:   4d 89 34 24             mov    %r14,(%r12)
  178:   49 8d 5c 24 f9          lea    -0x7(%r12),%rbx
  17d:   ff 65 00                jmpq   *0x0(%rbp)
 }}}

 But the code for `fbad` has several problems:
 {{{
 0000000000000018 :
   18:   48 8d 45 f0             lea    -0x10(%rbp),%rax
   1c:   4c 39 f8                cmp    %r15,%rax
   1f:   72 3a                   jb     5b 
   21:   48 89 f0                mov    %rsi,%rax
   24:   4c 89 f3                mov    %r14,%rbx
   27:   49 39 f6                cmp    %rsi,%r14
   2a:   0f 9c c1                setl   %cl
   2d:   0f b6 c9                movzbl %cl,%ecx
   30:   48 85 c9                test   %rcx,%rcx
   33:   75 51                   jne    86 
   35:   49 83 c4 10             add    $0x10,%r12
   39:   4d 3b a5 58 03 00 00    cmp    0x358(%r13),%r12
   40:   0f 87 aa 00 00 00       ja     f0 
   46:   49 c7 44 24 f8 00 00    movq   $0x0,-0x8(%r12)
   4d:   00 00
                         4b: R_X86_64_32S
 ghczmprim_GHCziTypes_Izh_con_info
   4f:   49 89 04 24             mov    %rax,(%r12)
   53:   49 8d 5c 24 f9          lea    -0x7(%r12),%rbx
   58:   ff 65 00                jmpq   *0x0(%rbp)
   5b:   bb 00 00 00 00          mov    $0x0,%ebx
                         5c: R_X86_64_32 Min_fbad_closure
   60:   41 ff 65 f8             jmpq   *-0x8(%r13)
         ...

 ; c1Sm_info is the other case with its own heap check and GC entry code
 ; c1Su_info is another GC entry
 ; in total, another 160 bytes of code
 }}}

 For some reason, the heap checks were moved into the alternatives, which
 was not a good decision in this case. But the silly thing here is the
 `cmp/setl/movzbl/test/jne` sequence in `Min_fbad_info`, which should be
 replaced by a `cmp/jl` as in `Min_fgood_info`.

 Same behavior in 7.8 and HEAD.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10676
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