[Libreoffice-bugs] [Bug 129256] New: ReDim is slow when array custom type is explicitly known

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=129256

            Bug ID: 129256
           Summary: ReDim is slow when array custom type is explicitly
                    known
           Product: LibreOffice
           Version: unspecified
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Keywords: perf
          Severity: normal
          Priority: medium
         Component: BASIC
          Assignee: libreoffice-bugs at lists.freedesktop.org
          Reporter: mikekaganski at hotmail.com

With this code:

> Type TCustomType
>   m_arr(20) As Long
>   m_str As String
> End Type
> 
> Sub CustomReDim1(ByRef A() As TCustomType, newU As Long)
>   ReDim Preserve A(newU)
> End Sub
> 
> Sub CustomReDim2(ByRef A(), newU As Long)
>   Dim oldU As Long, emptyVal As TCustomType, i As Long
>   oldU = UBound(A)
>   ReDim Preserve A(newU)
>   For i = oldU + 1 To newU
>     A(i) = emptyVal
>   Next i
> End Sub
> 
> Sub ReDimTest
>   Dim A1(0) As TCustomType, A2(0) As TCustomType, A3(0) As TCustomType, time1 As Date, i As Long, msg As String
>   time1 = Now()
>   Const maxIter = 2000
>   For i = 1 To maxIter
>     ReDim Preserve A1(i)
>   Next i
>   msg = "Testing ReDim Preserve: " & Format(now() - time1, "[s]"" s""") & Chr(13)
>   time1 = Now()
>   For i = 1 To maxIter
>     CustomReDim1(A2, i)
>   Next i
>   msg = msg & "Testing CustomReDim1: " & Format(now() - time1, "[s]"" s""") & Chr(13)
>   time1 = Now()
>   For i = 1 To maxIter
>     CustomReDim2(A3, i)
>   Next i
>   MsgBox msg & "Testing CustomReDim2: " & Format(now() - time1, "[s]"" s""")
> End Sub

I get this on current master Version: 6.5.0.0.alpha0+ (x64)
Build ID: 08615ce879927bc58924e9591985f27fe59a11fa
CPU threads: 12; OS: Windows 10.0 Build 18363; UI render: Skia/Raster; VCL:
win; 
Locale: ru-RU (ru_RU); UI-Language: en-US
Calc: CL

44 s
44 s
5 s

See how much faster the result is when ReDim gets an array without the element
type known in the function, and then assigning an empty value as it would
happen when the type is known.

Of course, that is debug version, and thus slow, but it shows how inefficient
the ReDim is for custom types, and in the same time, it can be much more
efficient if ReDim doesn't know the custom type - still providing the same end
result.

The code is affected by tdf#129227, so it would result in even greater
difference in releases 5.3 to 6.3.3, so better test in master with the
mentioned bug fixed. Note that the problem discussed in this issue was present
in to v.5.2 and older.

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