# Fixed: Loop optimization is less than VB
**URL:** https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652
**Category:** Mercury
**Created:** [November 4, 2021, 5:51pm UTC](https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652 "2021-11-04T17:51:20Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Theo69](https://talk.remobjects.com/u/Theo69)
#### Post date: [November 4, 2021, 5:51pm UTC](https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652/1 "2021-11-04T17:51:20Z")
I have the following code:
```
Function StringBuilderModification(s As String) As String
Dim b = New System.Text.StringBuilder(10000)
For i = 0 To s.Length - 1 Step 5
b.Append(s, i, 5)
b.Append("X")
Next
Return b.ToString
End Function
```
When I call this with a string of 10.000 chars long, it runs for 4741 ticks on VB and for 23471 ticks on Mercury. So Mercury takes 5 times longer somehow.
The IL generated by VB:
```
.method public static string StringBuilderModification (
string s
) cil managed
{
.locals init (
[0] class [mscorlib]System.Text.StringBuilder b,
[1] int32 V_1,
[2] int32 i
)
IL_0000: ldc.i4 10000
IL_0005: newobj instance void [mscorlib]System.Text.StringBuilder::.ctor(int32)
IL_000a: stloc.0
IL_000b: ldarg.0
IL_000c: callvirt instance int32 [mscorlib]System.String::get_Length()
IL_0011: ldc.i4.1
IL_0012: sub.ovf
IL_0013: stloc.1
IL_0014: ldc.i4.0
IL_0015: stloc.2
IL_0016: br.s IL_0032
.loop
{
IL_0018: ldloc.0
IL_0019: ldarg.0
IL_001a: ldloc.2
IL_001b: ldc.i4.5
IL_001c: callvirt instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(string, int32, int32)
IL_0021: pop
IL_0022: ldloc.0
IL_0023: ldstr "X"
IL_0028: callvirt instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(string)
IL_002d: pop
IL_002e: ldloc.2
IL_002f: ldc.i4.5
IL_0030: add.ovf
IL_0031: stloc.2
IL_0032: ldloc.2
IL_0033: ldloc.1
IL_0034: ble.s IL_0018
}
IL_0036: ldloc.0
IL_0037: callvirt instance string [mscorlib]System.Text.StringBuilder::ToString()
IL_003c: ret
}
```
IL generated by Mercury:
.method public hidebysig static string StringBuilderModification (
string s
) cil managed
{
.locals init (
[0] class [mscorlib]System.Text.StringBuilder V\_0,
[1] int32 V\_1
)
```
IL_0000: ldc.i4 10000
IL_0005: newobj instance void [mscorlib]System.Text.StringBuilder::.ctor(int32)
IL_000a: stloc.0
IL_000b: ldc.i4.0
IL_000c: stloc.1
IL_000d: br.s IL_0029
.loop
{
IL_000f: ldloc.0
IL_0010: ldarg.0
IL_0011: ldloc.1
IL_0012: ldc.i4.5
IL_0013: callvirt instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(string, int32, int32)
IL_0018: pop
IL_0019: ldloc.0
IL_001a: ldstr "X"
IL_001f: callvirt instance class [mscorlib]System.Text.StringBuilder [mscorlib]System.Text.StringBuilder::Append(string)
IL_0024: pop
IL_0025: ldloc.1
IL_0026: ldc.i4.5
IL_0027: add
IL_0028: stloc.1
IL_0029: ldloc.1
IL_002a: ldarg.0
IL_002b: callvirt instance int32 [mscorlib]System.String::get_Length()
IL_0030: ldc.i4.1
IL_0031: sub
IL_0032: ble.s IL_000f
}
IL_0034: ldloc.0
IL_0035: callvirt instance string [mscorlib]System.Text.StringBuilder::ToString()
IL_003a: ret
}
```
When I look at the decompiled VB code, I see an optimization that is not done in Mercury:
VB:
Mercury:
What means that s.Length - 1 is calculated in every loop again for the Mercury code, while it is only done once with the VB code.
---
### Author:  [@Theo69](https://talk.remobjects.com/u/Theo69)
#### Post date: [November 4, 2021, 6:07pm UTC](https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652/2 "2021-11-04T18:07:00Z")
This recalculation that is done in Mercury is indeed - as Mohammed Hamdy pointed out - a breaking change; VB only calculates the loop once and does not change it when the loop variable changes.
---
### Author:  [@RemObjectsSoftware](https://talk.remobjects.com/u/RemObjectsSoftware)
#### Post date: [November 4, 2021, 6:35pm UTC](https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652/3 "2021-11-04T18:35:40Z")
Logged as [bugs://E25494](https://www.remobjects.com/portal/bugs/?id=E25494).
---
### Author:  [@mh](https://talk.remobjects.com/u/mh)
#### Post date: [November 4, 2021, 6:36pm UTC](https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652/4 "2021-11-04T18:36:21Z")
Agreed, that’s wrong. A for loop should determine the end expression once, opposed to a while loop
---
### Author:  [@RemObjectsSoftware](https://talk.remobjects.com/u/RemObjectsSoftware)
#### Post date: [November 9, 2021, 9:55am UTC](https://talk.remobjects.com/t/fixed-loop-optimization-is-less-than-vb/25652/5 "2021-11-09T09:55:38Z")
[bugs://E25494](https://www.remobjects.com/portal/bugs/?id=E25494) was closed as fixed.