Re: [GHC] #9944: Performance issue re: simple loop

31 Dec 2014

      #9944: Performance issue re: simple loop
-------------------------------------+-------------------------------------
        Reporter:  clinton           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  low               |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Linux             |            Architecture:  x86_64
 Type of failure:  Runtime           |  (amd64)
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Old description:
...
The runtime of the following code actually decreases if the number
2147483647 (2^31^-1) is increased to 2147483648 (2^31^).
{{{#!hs
f n = go 1 0 where
  go i c = if i == n then c + i else go (i+1) (c+i)
main = print $ f (2147483647 :: Int)
}}}
I've attached two dumps from ghc-core, core7 is the 2147483647 case and
core8 is the 2147483648 case, however the main differences are below:
'''2147483647 case:'''
{{{
_c3Qg:
        cmpq $2147483647,%r14
        jne _c3Q9
_c3Qa:
        leaq 2147483647(%rsi),%rbx
        jmp *(%rbp)
_c3Q9:
        addq %r14,%rsi
        incq %r14
        jmp _c3Qg
}}}
'''2147483648 case:'''
{{{
_c3Qg:
        movl $2147483648,%eax
        cmpq %rax,%r14
        jne _c3Q9
_c3Qa:
        movl $2147483648,%eax
        movq %rsi,%rbx
        addq %rax,%rbx
        jmp *(%rbp)
_c3Q9:
        addq %r14,%rsi
        incq %r14
        jmp _c3Qg
}}}
Despite the extra instructions, the latter approach seems faster for  my
PC.
New description:

 The runtime of the following code actually decreases if the number
 2147483647 (2^31^-1) is increased to 2147483648 (2^31^). In addition,
 adding "module Main where" at the top of the file improves performance to
 the fast case regardless of what number is picked.

 {{{#!hs
 f n = go 1 0 where
   go i c = if i == n then c + i else go (i+1) (c+i)

 main = print $ f (2147483647 :: Int)
 }}}

 I've attached two dumps from ghc-core, core7 is the 2147483647 case and
 core8 is the 2147483648 case, however the main differences are below:

 '''2147483647 case:'''
 {{{
 _c3Qg:
         cmpq $2147483647,%r14
         jne _c3Q9
 _c3Qa:
         leaq 2147483647(%rsi),%rbx
         jmp *(%rbp)
 _c3Q9:
         addq %r14,%rsi
         incq %r14
         jmp _c3Qg
 }}}

 '''2147483648 case:'''
 {{{
 _c3Qg:
         movl $2147483648,%eax
         cmpq %rax,%r14
         jne _c3Q9
 _c3Qa:
         movl $2147483648,%eax
         movq %rsi,%rbx
         addq %rax,%rbx
         jmp *(%rbp)
 _c3Q9:
         addq %r14,%rsi
         incq %r14
         jmp _c3Qg
 }}}

 Despite the extra instructions, the latter approach seems faster for  my
 PC.

--

Comment (by clinton):

 Just "time". Difference for me is between 1.8 secs and 1.6 secs. Not a
 huge difference.

 Tripped over it because

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9944#comment:4
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