[GHC] #14359: C-- pipeline/NCG fails to optimize simple repeated addition

[GHC]

#14359: C-- pipeline/NCG fails to optimize simple repeated addition
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           Reporter:  bgamari        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While debugging #14346 I noticed some rather abhorrent code in a
 disassembly of the `newPinnedByteArray#` primop:
 {{{
 Dump of assembler code for function stg_newPinnedByteArrayzh:
    0x00000000004a8518 <+0>:     mov    0x378(%r13),%rax
    0x00000000004a851f <+7>:     cmpq   $0x0,0x10(%rax)
    0x00000000004a8524 <+12>:    je     0x4a8593
 <stg_newPinnedByteArrayzh+123>
    0x00000000004a8526 <+14>:    mov    0x4f5730,%rax
    0x00000000004a852e <+22>:    mov    0x38(%rax),%rax
    0x00000000004a8532 <+26>:    cmp    0x4f5718,%rax
    0x00000000004a853a <+34>:    jae    0x4a8593
 <stg_newPinnedByteArrayzh+123>
    0x00000000004a853c <+36>:    mov    %rbx,%rax
    0x00000000004a853f <+39>:    lea    0x7(%rax),%rcx
    0x00000000004a8543 <+43>:    shr    $0x3,%rcx
    0x00000000004a8547 <+47>:    add    $0x10,%rax       <--- starts here
    0x00000000004a854b <+51>:    add    $0xf,%rax
    0x00000000004a854f <+55>:    add    $0x7,%rax
    0x00000000004a8553 <+59>:    shr    $0x3,%rax
    0x00000000004a8557 <+63>:    mov    $0x49d820,%ecx
 }}}
 That is three successive `add` instructions; surely those should be
 collapsed into one by the Cmm-to-Cmm pipeline.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14359>
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