[GHC] #7307: Share top-level code for strings

[GHC]

#7307: Share top-level code for strings
---------------------------------+------------------------------------------
    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:                    |  
---------------------------------+------------------------------------------
 A string constant in GHC turns into
 {{{
 foo :: String
 foo = unpackCString# "the-string'
 }}}
 This is a top-level thunk, and it expands into rather a lot of code like
 this
 {{{
 .text
         .align 4,0x90
         .long   0
         .long   22
 .globl _Foo_zdfTypeableTzuds1_info
 _Foo_zdfTypeableTzuds1_info:
 .LcvI:
         movl %esi,%eax
         leal -12(%ebp),%ecx
         cmpl 84(%ebx),%ecx
         jb .LcvQ
         addl $8,%edi
         cmpl 92(%ebx),%edi
         ja .LcvS
         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 .LcvL
         movl $_stg_bh_upd_frame_info,-8(%ebp)
         leal -4(%edi),%eax
         movl %eax,-4(%ebp)
         movl $_cvJ_str,-12(%ebp)
         addl $-12,%ebp
         jmp _ghczmprim_GHCziCString_unpackCStringzh_info
 .LcvL:
         movl 64(%esp),%eax
         jmp *(%eax)
 .LcvS:
         movl $8,116(%ebx)
 .LcvQ:
         movl %eax,%esi
         jmp *-12(%ebx)
 }}}
 That's rather a lot of goop for one thunk!  Of course we can share this,
 by making a 2-word thunk like this:
 {{{
 ------------------------------
 | TopUnpack_info  |   -------|-----> "the-string"#
 ------------------------------
 }}}
 where `TopUnpack_info` is a shared RTS info-table and code that embodies
 the code fragment above.

 This would save useless code bloat for every constant string.  (This came
 up when looking at the code generated by `deriving(Typeable)`.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7307>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler