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

16 Oct 2017


      #14359: C-- pipeline/NCG fails to optimize simple repeated addition
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             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:                    |
-------------------------------------+-------------------------------------
Description changed by bgamari:

Old description:
...
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

   0x00000000004a8526 <+14>:    mov    0x4f5730,%rax
   0x00000000004a852e <+22>:    mov    0x38(%rax),%rax
   0x00000000004a8532 <+26>:    cmp    0x4f5718,%rax
   0x00000000004a853a <+34>:    jae    0x4a8593

   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.
New description:

 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
 
    0x00000000004a8526 <+14>:    mov    0x4f5730,%rax
    0x00000000004a852e <+22>:    mov    0x38(%rax),%rax
    0x00000000004a8532 <+26>:    cmp    0x4f5718,%rax
    0x00000000004a853a <+34>:    jae    0x4a8593
 
    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#comment:1
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