[GHC] #9638: Speed up Data.Char.isDigit

[GHC]

#9638: Speed up Data.Char.isDigit
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:  Runtime
  hour)                              |  performance bug
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 `isDigit` is currently defined like this:

 {{{#!hs
 isDigit                 :: Char -> Bool
 isDigit c               =  c >= '0' && c <= '9'
 }}}

 This will short-circuit the right way if you're looking for digits mixed
 with spaces, but the wrong way if you're looking for digits mixed with
 letters. It also requires a conditional jump to do that short-circuiting
 (confirmed by inspecting the assembly). It should be better to use an
 unsigned comparison instead:

 {{{#!hs
 isDigit                 :: Char -> Bool
 isDigit c               = (fromIntegral (ord c) :: Word) - 48 <= 9
 }}}

 The interesting section looks like this

 {{{
         movq 7(%rbx),%rax
         addq $-48,%rax
         cmpq $9,%rax
         setbe %al
         movzbl %al,%eax
         shlq $3,%rax
         movq ghczmprim_GHCziTypes_Bool_closure_tbl(%rax),%rbx
         addq $8,%rbp
         jmp *(%rbp)
 }}}

 or like this with -fllvm:

 {{{
         movq    7(%rbx), %rax
         addq    $-48, %rax
         cmpq    $10, %rax
         sbbq    %rax, %rax
         andq    $8, %rax
         movq    ghczmprim_GHCziTypes_Bool_closure_tbl(%rax), %rbx
         movq    8(%rbp), %rax
         addq    $8, %rbp
         jmpq    *%rax  # TAILCALL
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9638>
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