[GHC] #7308: Better code for top-level indirections

[GHC]

#7308: Better code for top-level indirections
---------------------------------+------------------------------------------
    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
---------------------------------+------------------------------------------
 GHC generates lots of top-level indirections looking like this:
 {{{
 foo :: T
 foo = bar `cast` (some coercion)
 }}}
 There are good reasons for this. Moreover `foo` will almost never be
 evaluated, because GHC will inline any call to `foo` at its call site.

 '''But''' GHC generates rather a lot of code for this one (almost-vacuous)
 bindings:
 {{{
         .align 4,0x90
         .long   _Swx_srt-(_Foo_zdfTypeableT_info)+4
         .long   0
         .long   65558
 .globl _Foo_zdfTypeableT_info
 _Foo_zdfTypeableT_info:
 .LcwI:
         movl %esi,%eax
         leal -8(%ebp),%ecx
         cmpl 84(%ebx),%ecx
         jb .LcwP
         addl $8,%edi
         cmpl 92(%ebx),%edi
         ja .LcwR
         movl $_stg_CAF_BLACKHOLE_info,-4(%edi)
         movl 100(%ebx),%ecx
         movl %ecx,0(%edi)
         leal -4(%edi),%ecx
         pushl %ecx
         pushl %eax
         pushl %ebx
         movl %eax,76(%esp)
         call _newCAF
         addl $12,%esp
         testl %eax,%eax
         je .LcwK
         movl $_stg_bh_upd_frame_info,-8(%ebp)
         leal -4(%edi),%eax
         movl %eax,-4(%ebp)
         movl $_Foo_zdfTypeableTzuzdctypeOf_closure+1,%esi
         addl $-8,%ebp
         jmp *(%ebp)
 .LcwK:
         movl 64(%esp),%eax
         jmp *(%eax)
 .LcwR:
         movl $8,116(%ebx)
 .LcwP:
         movl %eax,%esi
         jmp *-12(%ebx)
 }}}
 Whereas all we really need for `foo` is code that says "jump to `bar`".
 No need to update that top-level thunk. It's just an indirection and (if
 you have `-O` on) one that will never even be used.

 In short, I think that top-level indirections could do with a tiny special
 case in the code generator.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7308>
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