[GHC] #14753: Assembly for stg_enter_info starts with a dead 'mov'

[GHC]

#14753: Assembly for stg_enter_info starts with a dead 'mov'
-------------------------------------+-------------------------------------
           Reporter:  alpmestan      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.4.1-alpha1
  (CodeGen)                          |
           Keywords:                 |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  None/Unknown
  (amd64)                            |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In the course of my investigation in #14675, I happened to look at the
 x86_64 assembly for `stg_enter_info`, through gdb:

 {{{#!asm
 Breakpoint 2, stg_enter_info () at rts/HeapStackCheck.cmm:166
 166     {
 (gdb) disassemble
 Dump of assembler code for function stg_enter_info:
 => 0x0000000002cf5900 <+0>:     mov    0x0(%rbp),%rax
    0x0000000002cf5904 <+4>:     mov    0x8(%rbp),%rax
    0x0000000002cf5908 <+8>:     test   $0x7,%al
    0x0000000002cf590a <+10>:    jne    0x2cf593d <stg_enter_info+61>
    0x0000000002cf590c <+12>:    mov    (%rax),%rbx
    0x0000000002cf590f <+15>:    cmpl   $0x1a,-0x8(%rbx)
    0x0000000002cf5913 <+19>:    jb     0x2cf5947 <stg_enter_info+71>
    0x0000000002cf5915 <+21>:    cmpl   $0x1c,-0x8(%rbx)
    0x0000000002cf5919 <+25>:    jb     0x2cf592d <stg_enter_info+45>
    0x0000000002cf591b <+27>:    cmpl   $0x1d,-0x8(%rbx)
    0x0000000002cf591f <+31>:    jb     0x2cf5933 <stg_enter_info+51>
    0x0000000002cf5921 <+33>:    mov    %rbx,%rcx
    0x0000000002cf5924 <+36>:    mov    %rax,%rbx
    0x0000000002cf5927 <+39>:    add    $0x10,%rbp
    0x0000000002cf592b <+43>:    jmpq   *%rcx
    0x0000000002cf592d <+45>:    cmpl   $0x1b,-0x8(%rbx)
    0x0000000002cf5931 <+49>:    jb     0x2cf5921 <stg_enter_info+33>
    0x0000000002cf5933 <+51>:    mov    0x8(%rax),%rax
    0x0000000002cf5937 <+55>:    mov    %rax,0x8(%rbp)
    0x0000000002cf593b <+59>:    jmp    0x2cf5908 <stg_enter_info+8>
    0x0000000002cf593d <+61>:    mov    %rax,%rbx
    0x0000000002cf5940 <+64>:    add    $0x10,%rbp
    0x0000000002cf5944 <+68>:    jmpq   *0x0(%rbp)
    0x0000000002cf5947 <+71>:    cmpl   $0xf,-0x8(%rbx)
    0x0000000002cf594b <+75>:    jb     0x2cf5969 <stg_enter_info+105>
    0x0000000002cf594d <+77>:    cmpl   $0x19,-0x8(%rbx)
    0x0000000002cf5951 <+81>:    jb     0x2cf595d <stg_enter_info+93>
    0x0000000002cf5953 <+83>:    mov    %rax,%rbx
    0x0000000002cf5956 <+86>:    add    $0x10,%rbp
    0x0000000002cf595a <+90>:    jmpq   *0x0(%rbp)
    0x0000000002cf595d <+93>:    movslq -0x8(%rbx),%rcx
    0x0000000002cf5961 <+97>:    cmp    $0x17,%rcx
    0x0000000002cf5965 <+101>:   jne    0x2cf5921 <stg_enter_info+33>
    0x0000000002cf5967 <+103>:   jmp    0x2cf5953 <stg_enter_info+83>
    0x0000000002cf5969 <+105>:   cmpl   $0x8,-0x8(%rbx)
    0x0000000002cf596d <+109>:   jb     0x2cf5921 <stg_enter_info+33>
    0x0000000002cf596f <+111>:   movslq -0x8(%rbx),%rbx
    0x0000000002cf5973 <+115>:   add    $0xfffffffffffffff8,%rbx
    0x0000000002cf5977 <+119>:   jmpq   *0x2e36d90(,%rbx,8)
 }}}

 Notice the first two instructions:

 {{{#!asm
 mov    0x0(%rbp),%rax
 mov    0x8(%rbp),%rax
 }}}

 We successively write two different things to `rax`, making the first
 `mov` effectively useless. I'm not quite sure the impact is of any
 significance but it still seems worth writing this down and addressing it
 at some point in the future, as there's just no way that first instruction
 will ever be useful.

 I think the assembly is generated from
 [https://github.com/ghc/ghc/blob/master/rts/HeapStackCheck.cmm#L164 this
 code], which in turns call
 [https://github.com/ghc/ghc/blob/master/includes/Cmm.h#L301 `ENTER`],
 which in turns calls
 [https://github.com/ghc/ghc/blob/master/includes/Cmm.h#L272 `LOAD_INFO`],
 which appears to rcorrespond to the first few instructions in the assembly
 I pasted, if we ignore the extra `mov`.

 (Assembly generated by ghc 8.4.1 built from the same commit as the alpha1
 for 8.4.1 from hvr's PPA, in a 64bits Ubuntu VM. Not sure we generate a
 dead `mov` with the other codegens, haven't tested.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14753>
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