[Issue 9991] New: Optimizer Doesn't Enregister Static Array Elements With Compile-Time Constant Index

[d-bugmail at puremagic.com]

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 at puremagic.com
        ReportedBy: dsimcha at yahoo.com


--- Comment #0 from David Simcha <dsimcha at 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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