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I'm facing the bizarre situation that the same program on the same machine doing a Round() of the same floating point value does not always give the same result. At first I thought it has to be glitch because of a bit flip or something, but it keeps on coming back. Now it started to happen on a completely different machine as well. Bad results come up rather rarely, in the order of once a week. When they do, it seems they keep happening until the program is restarted. That might be a coincidence, however. So far, I've been unable to reproduce it at will. This is all happening on the main thread, by the way.

A check I consider adding is whether Round keeps pointing to the same memory address. Any other ideas?

2020-12-28 08:30:19.411 DBGrid1.Fields[17].AsString: 0,239999994635582
2020-12-28 08:30:19.411 FloatToStr(DBGrid1.Fields[17].AsFloat): 0,239999994635582
2020-12-28 08:30:19.411 FloatToStr(1000 * DBGrid1.Fields[17].AsFloat): 239,999994635582
2020-12-28 08:30:19.411 IntToStr(Round(1000 * DBGrid1.Fields[17].AsFloat)): 239
2020-12-28 08:30:19.411 FloatToStr(DBGrid1.Fields[17].AsSingle): 0,239999994635582
2020-12-28 08:30:19.411 FloatToStr(1000 * DBGrid1.Fields[17].AsSingle): 239,999994635582
2020-12-28 08:30:19.411 IntToStr(Round(1000 * DBGrid1.Fields[17].AsSingle)): 239
2020-12-28 08:30:19.411 FloatToStr(DBGrid1.Fields[17].AsExtended): 0,239999994635582
2020-12-28 08:30:19.412 FloatToStr(1000 * DBGrid1.Fields[17].AsExtended): 239,999994635582
2020-12-28 08:30:19.412 IntToStr(Round(1000 * DBGrid1.Fields[17].AsExtended)): 239
2020-12-28 08:30:19.412 CurrToStr(DBGrid1.Fields[17].AsCurrency): 0,2399
2020-12-28 08:30:19.412 CurrToStr(1000 * DBGrid1.Fields[17].AsCurrency): 239,9
2020-12-28 08:30:19.412 IntToStr(Round(1000 * DBGrid1.Fields[17].AsCurrency)): 239
2020-12-28 08:30:19.412 FloatToStr(Query1.FieldByName('FIELD').AsFloat): 0,239999994635582
2020-12-28 08:30:19.412 FloatToStr(1000 * Query1.FieldByName('FIELD').AsFloat): 239,999994635582
2020-12-28 08:30:19.412 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsFloat)): 239
2020-12-28 08:30:19.412 FloatToStr(Query1.FieldByName('FIELD').AsSingle): 0,239999994635582
2020-12-28 08:30:19.412 FloatToStr(1000 * Query1.FieldByName('FIELD').AsSingle): 239,999994635582
2020-12-28 08:30:19.413 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsSingle)): 239
2020-12-28 08:30:19.413 FloatToStr(Query1.FieldByName('FIELD').AsExtended): 0,239999994635582
2020-12-28 08:30:19.413 FloatToStr(1000 * Query1.FieldByName('FIELD').AsExtended): 239,999994635582
2020-12-28 08:30:19.413 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsExtended)): 239
2020-12-28 08:30:19.413 CurrToStr(Query1.FieldByName('FIELD').AsCurrency): 0,2399
2020-12-28 08:30:19.413 CurrToStr(1000 * Query1.FieldByName('FIELD').AsCurrency): 239,9
2020-12-28 08:30:19.413 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsCurrency)): 239
2020-12-28 08:30:19.413 CurrToStr(Query1.FieldByName('FIELD').AsCurrency): 0,2399
2020-12-28 08:30:19.413 CurrToStr(1000 * Query1.FieldByName('FIELD').AsCurrency): 239,9
2020-12-28 08:30:19.413 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsCurrency)): 239
2020-12-28 08:30:19.414 BinToHex(DBGrid1.Fields[17].AsFloat): 00000000008FC2F5FC3F

2020-12-28 08:40:46.461 DBGrid1.Fields[17].AsString: 0,239999994635582
2020-12-28 08:40:46.462 FloatToStr(DBGrid1.Fields[17].AsFloat): 0,239999994635582
2020-12-28 08:40:46.463 FloatToStr(1000 * DBGrid1.Fields[17].AsFloat): 239,999994635582
2020-12-28 08:40:46.463 IntToStr(Round(1000 * DBGrid1.Fields[17].AsFloat)): 240
2020-12-28 08:40:46.463 FloatToStr(DBGrid1.Fields[17].AsSingle): 0,239999994635582
2020-12-28 08:40:46.463 FloatToStr(1000 * DBGrid1.Fields[17].AsSingle): 239,999994635582
2020-12-28 08:40:46.463 IntToStr(Round(1000 * DBGrid1.Fields[17].AsSingle)): 240
2020-12-28 08:40:46.463 FloatToStr(DBGrid1.Fields[17].AsExtended): 0,239999994635582
2020-12-28 08:40:46.463 FloatToStr(1000 * DBGrid1.Fields[17].AsExtended): 239,999994635582
2020-12-28 08:40:46.463 IntToStr(Round(1000 * DBGrid1.Fields[17].AsExtended)): 240
2020-12-28 08:40:46.463 CurrToStr(DBGrid1.Fields[17].AsCurrency): 0,24
2020-12-28 08:40:46.463 CurrToStr(1000 * DBGrid1.Fields[17].AsCurrency): 240
2020-12-28 08:40:46.463 IntToStr(Round(1000 * DBGrid1.Fields[17].AsCurrency)): 240
2020-12-28 08:40:46.463 FloatToStr(Query1.FieldByName('FIELD').AsFloat): 0,239999994635582
2020-12-28 08:40:46.463 FloatToStr(1000 * Query1.FieldByName('FIELD').AsFloat): 239,999994635582
2020-12-28 08:40:46.463 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsFloat)): 240
2020-12-28 08:40:46.463 FloatToStr(Query1.FieldByName('FIELD').AsSingle): 0,239999994635582
2020-12-28 08:40:46.464 FloatToStr(1000 * Query1.FieldByName('FIELD').AsSingle): 239,999994635582
2020-12-28 08:40:46.464 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsSingle)): 240
2020-12-28 08:40:46.464 FloatToStr(Query1.FieldByName('FIELD').AsExtended): 0,239999994635582
2020-12-28 08:40:46.464 FloatToStr(1000 * Query1.FieldByName('FIELD').AsExtended): 239,999994635582
2020-12-28 08:40:46.464 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsExtended)): 240
2020-12-28 08:40:46.464 CurrToStr(Query1.FieldByName('FIELD').AsCurrency): 0,24
2020-12-28 08:40:46.464 CurrToStr(1000 * Query1.FieldByName('FIELD').AsCurrency): 240
2020-12-28 08:40:46.464 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsCurrency)): 240
2020-12-28 08:40:46.464 CurrToStr(Query1.FieldByName('FIELD').AsCurrency): 0,24
2020-12-28 08:40:46.464 CurrToStr(1000 * Query1.FieldByName('FIELD').AsCurrency): 240
2020-12-28 08:40:46.465 IntToStr(Round(1000 * Query1.FieldByName('FIELD').AsCurrency)): 240
2020-12-28 08:40:46.465 BinToHex(DBGrid1.Fields[17].AsFloat): 00000000008FC2F5FC3F
Thijs van Dien
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    Can you reproduce with a minimal test program? Can it be caused by the code executed before the call? Sometimes the 8087 control word is changed and not restored properly. – Uwe Raabe Jan 02 '21 at 21:58
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    Round is deterministic. Remember though that its input is its argument and the floating point control state. – David Heffernan Jan 02 '21 at 21:59
  • @UweRaabe It seems likely that some sequence of actions leads to this 'faulty' state. A simple test program just calling `Round` is not suddenly going to start giving different results, and changing the 8087 control word will (as it should), which makes me none the wiser. I will add logging to see if the control word is the cause rather than some memory corruption (in which case I'd expect more instability). The tricky thing will be finding the source. :/ – Thijs van Dien Jan 02 '21 at 22:19
  • It can't be memory corruption. The result of Round depends on its input and the control state. – David Heffernan Jan 02 '21 at 22:45
  • @DavidHeffernan By memory corruption I mean `Round` suddenly pointing to `Trunc` or something along those lines (however unlikely that would be). I agree we have a better explanation already. – Thijs van Dien Jan 02 '21 at 22:53
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    That isn't a thing that happens. If that could happen then we would all just have to give up. There have to be some rules! – David Heffernan Jan 02 '21 at 23:16
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    After these pointers, I found a library I recently started using more to leave the rounding mode altered under certain conditions. Should either of you turn your comment(s) into an answer, I'll accept that. Otherwise, I'll write one myself in a few days. – Thijs van Dien Jan 03 '21 at 05:11

1 Answers1

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Round is deterministic. However, its behaviour depends on the prevailing floating point rounding mode, as well as its argument. Something else in your program is changing the rounding mode.

David Heffernan
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