[GHC] #11792: Optimised unsafe FFI call can get wrong argument

[GHC]

#11792: Optimised unsafe FFI call can get wrong argument
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        Reporter:  Szunti            |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hsyl20):

 Replying to [comment:5 osa1]:
 > {{{
 > third arg:  [   movl $7457,%edx]
 > fourth arg: [   movl $255,%vI_n4I8,     movq 7(%rbx),%vI_n4Ia,
 >                 imulq %vI_s4pW,%vI_n4Ia,        movl %vI_n4Ia,%eax,
 xorq %rdx,%rdx,
 > }}}
 >
 > These look correct to me. Since the code mostly uses virtual registers
 `%edx`
 > isn't overridden at this point.

 It is overriden: `%rdx` is `xor`ed before each `divq`. With -ddump-asm-
 liveness, we can see that `%edx` (`%r3`) born in the `movl` is
 overwritten.

 {{{
                 movl $7457,%edx
                     # born:    %r3
                     # w_dying: %r3

                 movl $255,%vI_n413
                     # born:    %vI_n413

                 movq 7(%rbx),%vI_n415
                     # born:    %vI_n415
                     # r_dying: %r1

                 imulq %vI_s3Mw,%vI_n415

                 movl %vI_n415,%eax
                     # born:    %r0
                     # r_dying: %vI_n415

                 xorq %rdx,%rdx
                     # born:    %r3

                 divq %vI_n413
                     # r_dying: %vI_n413
                     # w_dying: %r3
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11792#comment:6>
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