[GHC] #9431: integer-gmp small Integer multiplication does two multiplications on x86

[GHC]

#9431: integer-gmp small Integer multiplication does two multiplications on x86
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       Reporter:  rwbarton           |                   Owner:
           Type:  feature request    |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 `timesInteger` begins thusly:

 {{{
 timesInteger :: Integer -> Integer -> Integer
 timesInteger (S# i) (S# j)         = if isTrue# (mulIntMayOflo# i j ==#
 0#)
                                      then S# (i *# j)
                                      else -- ...
 }}}

 The x86 backend implements `mulIntMayOflo#` as a (word, word) -> double
 word multiplication, followed by bit manipulation to test for overflow of
 the low word. Then, if there was no overflow, on the next line we multiply
 the operands again.

 We should be able to do better here. We need a new primop that combines
 `mulIntMayOflo#` with the actual multiplication result, at least in the
 non-overflow case (though with some more work we might be able to turn the
 double word result directly into a large Integer), and then we need to
 update `timesInteger` to use it.

 The LLVM backend probably has the same behavior, though it might be smart
 enough to notice that the multiplication is repeated; I haven't checked
 its assembly output.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9431>
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