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digitalmars.D.bugs - [Issue 12442] New: inefficient code with scope(exit)

reply d-bugmail puremagic.com writes:
https://d.puremagic.com/issues/show_bug.cgi?id=12442

           Summary: inefficient code with scope(exit)
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: performance
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: r.sagitario gmx.de


--- Comment #0 from Rainer Schuetze <r.sagitario gmx.de> 2014-03-23 01:55:50
PDT ---
Usage of scope(exit) comes with a significant performance cost, even if the
executed code is "nothrow":

/////////////////////////

uint fun() nothrow;

__gshared int recurse;

uint wrapper_scopeexit()
{
    recurse++;
    scope(exit) recurse--;
    return fun();
}

uint wrapper_linear()
{
    recurse++;
    uint rc = fun();
    recurse--;
    return rc;
}
//////////////////////////

This is the assembly for Win64 with "-O":

_D4test17wrapper_scopeexitFZk:
  0000000000000000: 55                 push        rbp
  0000000000000001: 48 8B EC           mov         rbp,rsp
  0000000000000004: 48 83 EC 18        sub         rsp,18h
  0000000000000008: 53                 push        rbx
  0000000000000009: 56                 push        rsi
  000000000000000A: 57                 push        rdi
  000000000000000B: 41 54              push        r12
  000000000000000D: 41 55              push        r13
  000000000000000F: 41 56              push        r14
  0000000000000011: 41 57              push        r15
  0000000000000013: FF 05 00 00 00 00  inc         dword ptr [_D4test7recursei]
  0000000000000019: 48 83 EC 20        sub         rsp,20h
  000000000000001D: E8 00 00 00 00     call        _D4test3funFNbZk
  0000000000000022: 48 83 C4 20        add         rsp,20h
  0000000000000026: 48 89 45 F8        mov         qword ptr [rbp-8],rax
  000000000000002A: 48 83 EC 08        sub         rsp,8
  000000000000002E: E8 28 00 00 00     call        000000000000005B
  0000000000000033: 48 83 C4 08        add         rsp,8
  0000000000000037: 48 8B 45 F8        mov         rax,qword ptr [rbp-8]
  000000000000003B: 41 5F              pop         r15
  000000000000003D: 41 5E              pop         r14
  000000000000003F: 41 5D              pop         r13
  0000000000000041: 41 5C              pop         r12
  0000000000000043: 5F                 pop         rdi
  0000000000000044: 5E                 pop         rsi
  0000000000000045: 5B                 pop         rbx
  0000000000000046: 48 8D 65 00        lea         rsp,[rbp]
  000000000000004A: 5D                 pop         rbp
  000000000000004B: C3                 ret
  000000000000004C: 48 83 EC 08        sub         rsp,8
  0000000000000050: E8 06 00 00 00     call        000000000000005B
  0000000000000055: 48 83 C4 08        add         rsp,8
  0000000000000059: EB 07              jmp         0000000000000062
  000000000000005B: FF 0D 00 00 00 00  dec         dword ptr [_D4test7recursei]
  0000000000000061: C3                 ret
  0000000000000062: 41 5F              pop         r15
  0000000000000064: 41 5E              pop         r14
  0000000000000066: 41 5D              pop         r13
  0000000000000068: 41 5C              pop         r12
  000000000000006A: 5F                 pop         rdi
  000000000000006B: 5E                 pop         rsi
  000000000000006C: 5B                 pop         rbx
  000000000000006D: 48 8D 65 00        lea         rsp,[rbp]
  0000000000000071: 5D                 pop         rbp
  0000000000000072: C3                 ret

_D4test14wrapper_linearFZk:
  0000000000000000: 55                 push        rbp
  0000000000000001: 48 8B EC           mov         rbp,rsp
  0000000000000004: FF 05 00 00 00 00  inc         dword ptr [_D4test7recursei]
  000000000000000A: 48 83 EC 20        sub         rsp,20h
  000000000000000E: E8 00 00 00 00     call        _D4test3funFNbZk
  0000000000000013: 48 83 C4 20        add         rsp,20h
  0000000000000017: FF 0D 00 00 00 00  dec         dword ptr [_D4test7recursei]
  000000000000001D: 5D                 pop         rbp
  000000000000001E: C3                 ret


For Win32, it is slightly worse because the exception frames are still set up:

_D4test17wrapper_scopeexitFZk    comdat
    assume    CS:_D4test17wrapper_scopeexitFZk
L0:        push    EBP
        mov    EBP,ESP
        mov    EDX,FS:__except_list
        push    0FFFFFFFFh
        push    offset _D4test17wrapper_scopeexitFZk[07Ch]
        push    EDX
        mov    FS:__except_list,ESP
        sub    ESP,8
        push    EBX
        push    ESI
        push    EDI
        inc    dword ptr _D4test7recursei
        mov    dword ptr -4[EBP],0
        call    near ptr _D4test3funFNbZk
        mov    dword ptr -4[EBP],0FFFFFFFFh
        push    EAX
        call    near ptr L5A
        pop    EAX
        mov    ECX,-0Ch[EBP]
        mov    FS:__except_list,ECX
        pop    EDI
        pop    ESI
        pop    EBX
        mov    ESP,EBP
        pop    EBP
        ret
        call    near ptr L5A
        jmp short    L68
L5A:        mov    dword ptr -4[EBP],0FFFFFFFFh
        dec    dword ptr _D4test7recursei
        ret
L68:        mov    ECX,-0Ch[EBP]
        mov    FS:__except_list,ECX
        pop    EDI
        pop    ESI
        pop    EBX
        mov    ESP,EBP
        pop    EBP
        ret
        mov    EAX,offset FLAT:_DATA
        jmp    near ptr __d_framehandler
_D4test17wrapper_scopeexitFZk    ends

_D4test14wrapper_linearFZk    comdat
    assume    CS:_D4test14wrapper_linearFZk
L0:        push    EAX
        inc    dword ptr _D4test7recursei
        call    near ptr _D4test3funFNbZk
        dec    dword ptr _D4test7recursei
        pop    ECX
        ret
_D4test14wrapper_linearFZk    ends

In addition, even if the code might throw, the usual path should inline the
code for scope(exit) instead of calling the exception handler.

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next sibling parent d-bugmail puremagic.com writes:
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--- Comment #1 from Rainer Schuetze <r.sagitario gmx.de> 2014-03-23 02:01:32
PDT ---
The same happens with RAII:

struct SCount
{
    this(bool) nothrow { recurse++; }
    ~this() nothrow { recurse--; }
}

uint wrapper_raii() nothrow
{
    SCount sc = SCount(true);
    return fun();
}

produces almost the same code as scope(exit).

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Andrej Mitrovic <andrej.mitrovich gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrej.mitrovich gmail.com


--- Comment #2 from Andrej Mitrovic <andrej.mitrovich gmail.com> 2014-03-23
09:24:36 PDT ---
Similar performance worry about scope(success);
http://forum.dlang.org/thread/mailman.493.1358378360.22503.digitalmars-d puremagic.com?page=1

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prev sibling parent d-bugmail puremagic.com writes:
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Martin Nowak <code dawg.eu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |code dawg.eu


--- Comment #3 from Martin Nowak <code dawg.eu> 2014-04-05 04:13:57 PDT ---
The inefficiency comes from the extra call for the finally block, but we need a
good idea how to make it cheaper.
Maybe it's somehow possible to inline the finally block for the normal control
flow.

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