[GHC] #8922: GHC unnecessarily sign/zero-extends C call arguments

[GHC]

#8922: GHC unnecessarily sign/zero-extends C call arguments
-------------------------------------+----------------------------------
        Reporter:  simonmar          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.10.1
       Component:  Compiler (NCG)    |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #7684
-------------------------------------+----------------------------------

Comment (by tibbe):

 Note this exchange in the linked email thread above. I'm not sure if
 Richard (the editor) is saying that zero extending in the caller is an
 optimization:

 >>> How?  You aren't allowed to access the bits outside the specified
 argument
 >>> type (which must match on caller and callee side), so you can't
 observe
 >>> them, so it's not required to specify their content.
 >>
 >> OK, thanks.  It's clear now.
 >>
 >> The problem is that LLVM assumes that values are extended at a call.
 GCC
 >> does that, but libffi doesn't.  So, calls via libffi to LLVM don't work
 >> correctly.
 >
 > It's an optimization to do so to avoid partial register stalls.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8922#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler