http://d.puremagic.com/issues/show_bug.cgi?id=9991

           Summary: Optimizer Doesn't Enregister Static Array Elements
                    With Compile-Time Constant Index
           Product: D
           Version: D1 & D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: performance
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: dsimcha@yahoo.com


--- Comment #0 from David Simcha <dsimcha@yahoo.com> 2013-04-25 09:34:00 PDT ---
Can't static array elements where the index is known at compile time be treated just like regular stack variables?  GDC seems to treat them as such.

D Source code below.  I'd expect these two implementations to compile to the exact same ASM.  In GDC, the main loops do, though the pre-loop setup stuff does compile differently for reasons I don't understand.

int sum1(const int[] arr) {
  auto end = arr.ptr + arr.length - 3;
  int[4] sums;
  for(auto cur = arr.ptr; cur < end; cur += 4) {
    sums[0] += cur[0];
    sums[1] += cur[1];
    sums[2] += cur[2];
    sums[3] += cur[3];
  }

  return sums[0] + sums[1] + sums[2] + sums[3];
}

int sum2(const int[] arr) {
  auto end = arr.ptr + arr.length - 3;
  int sum0 = 0, sum1 = 0, sum2 = 0, sum3 = 0;
  for(auto cur = arr.ptr; cur < end; cur += 4) {
    sum0 += cur[0];
    sum1 += cur[1];
    sum2 += cur[2];
    sum3 += cur[3];
  }

  return sum0 + sum1 + sum2 + sum3;
}

ASM:

_D5test34sum1FxAiZi:
        push    RBP
        mov    RBP,RSP
        sub    RSP,020h
        push    RBX
        lea    RAX,-020h[RBP]
        xor    RCX,RCX
        mov    [RAX],RCX
        mov    8[RAX],RCX
        mov    R8,RSI
        mov    RDX,RSI
        mov    R9,RDI
        lea    R9,0FFFFFFF4h[R9*4][RDX]
        cmp    RDX,R9
        jae    L51
L2D:        mov    EBX,[R8]
        add    -020h[RBP],EBX
        mov    ESI,4[R8]
        add    -01Ch[RBP],ESI
        mov    EAX,8[R8]
        add    -018h[RBP],EAX
        mov    ECX,0Ch[R8]
        add    -014h[RBP],ECX
        add    R8,010h
        cmp    R8,R9
        jb    L2D
L51:        mov    EAX,-020h[RBP]
        add    EAX,-01Ch[RBP]
        add    EAX,-018h[RBP]
        add    EAX,-014h[RBP]
        pop    RBX
        mov    RSP,RBP
        pop    RBP
        ret
        nop

_D5test34sum2FxAiZi:
        push    RBP
        mov    RBP,RSP
        sub    RSP,010h
        push    RBX
        push    R12
        push    R13
        xor    R9D,R9D
        xor    R11D,R11D
        xor    EBX,EBX
        xor    R12D,R12D
        mov    R8,RSI
        mov    RAX,RSI
        mov    R13,RDI
        lea    R13,0FFFFFFF4h[R13*4][RAX]
        cmp    RAX,R13
        jae    L46
L2E:        add    R9D,[R8]
        add    R11D,4[R8]
        add    EBX,8[R8]
        add    R12D,0Ch[R8]
        add    R8,010h
        cmp    R8,R13
        jb    L2E
L46:        lea    EAX,[R11][R9]
        add    EAX,EBX
        add    EAX,R12D
        pop    R13
        pop    R12
        pop    RBX
        mov    RSP,RBP
        pop    RBP
        ret

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla@digitalmars.com
           Severity|normal                      |enhancement


--- Comment #1 from Walter Bright <bugzilla@digitalmars.com> 2013-04-25 12:41:56 PDT ---
The optimizer currently does not enregister variables that don't fit into registers. It could be enhanced to do it.

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timon.gehr@gmx.ch changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |timon.gehr@gmx.ch


--- Comment #2 from timon.gehr@gmx.ch 2013-04-25 12:48:21 PDT ---
(In reply to comment #1)
> The optimizer currently does not enregister variables that don't fit into registers. It could be enhanced to do it.

ints definitely fit into registers.

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--- Comment #3 from Walter Bright <bugzilla@digitalmars.com> 2013-04-25 13:44:14 PDT ---
(In reply to comment #2)
> ints definitely fit into registers.

sums is one variable, not 4.

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bearophile_hugs@eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs@eml.cc


--- Comment #4 from bearophile_hugs@eml.cc 2013-06-14 03:16:06 PDT ---
(In reply to comment #0)

> In GDC, the main loops do, though the pre-loop setup stuff does compile differently for reasons I don't understand.

ldc2 compiles them to the same asm (-release -profile-verifier-noassert -O5
-output-s):

__D4test4sum1FxAiZi:
    pushl    %ebx
    pushl    %edi
    pushl    %esi
    movl    16(%esp), %eax
    movl    20(%esp), %esi
    leal    -12(%esi,%eax,4), %ebx
    xorl    %ecx, %ecx
    xorl    %edx, %edx
    xorl    %edi, %edi
    xorl    %eax, %eax
    jmp    LBB0_2
    .align    16, 0x90
LBB0_1:
    addl    12(%esi), %eax
    addl    8(%esi), %edi
    addl    4(%esi), %edx
    addl    (%esi), %ecx
    addl    $16, %esi
LBB0_2:
    cmpl    %ebx, %esi
    jb    LBB0_1
    addl    %edi, %eax
    addl    %edx, %eax
    addl    %ecx, %eax
    popl    %esi
    popl    %edi
    popl    %ebx
    ret    $8


__D4test4sum2FxAiZi:
    pushl    %ebx
    pushl    %edi
    pushl    %esi
    movl    16(%esp), %eax
    movl    20(%esp), %esi
    leal    -12(%esi,%eax,4), %ebx
    xorl    %ecx, %ecx
    xorl    %edx, %edx
    xorl    %edi, %edi
    xorl    %eax, %eax
    jmp    LBB1_2
    .align    16, 0x90
LBB1_1:
    addl    12(%esi), %ecx
    addl    8(%esi), %edx
    addl    4(%esi), %edi
    addl    (%esi), %eax
    addl    $16, %esi
LBB1_2:
    cmpl    %ebx, %esi
    jb    LBB1_1
    addl    %edi, %eax
    addl    %edx, %eax
    addl    %ecx, %eax
    popl    %esi
    popl    %edi
    popl    %ebx
    ret    $8

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