Why 1.99 suddenly no longer equals 1.99
A strange problem suddenly occurs in all my Delphi 7 and Delphi 2009 unit tests.
I used DUnit CheckEquals to compare two Currency values, like
CheckEquals(1.99, SomeCurrencyValue);
This worked fine in the IDE and in our build scripts, which run using the same version of DUnit for two years. Now, all of a sudden, all tests fail with strange error messages like
Expected: <1.99> - Found: <1.99>
With some newsgroup help I found that something has changed a processor flag which is responsible for floating point operations. Digging deeper in my unit tests, I found the root cause in the IBX component TIBDatabase, which seems to modify this flag.
TIBDataBase.Create(nil) changes the value of Get8087CW from $1372 to $1272, a very strange value.
procedure TForm2.Button1Click(Sender: TObject);
begin
Memo1.Lines.Add(Format('FPU CW = $%4.4X',[Get8087CW]));
TIBDataBase.Create(nil);
Memo1.Lines.Add(Format('FPU CW = $%4.4X',[Get8087CW]));
end;
Tested on two systems running Windows 2000 SP 4, InterBase 7.5.1 and 2007 libraries (gds32.dll), Delphi 7 and Delphi 2009
I did some more checks with Delphi 2007 and InterBase 2007: the failure does not occur.
But: if I manually call Set8087CW with the values you mention, I can reproduce.
The difference between $1372 (default) and $1272 (failure) is the internal mantissa precision. It changes from 64 bits to 53 bits, which is enough to make 1.99 not equal to 1.99.
I have seen behaviour like this in the past with some networking stacks in the Turbo Pascal 7 era, with some C++ DLL’s in the Delphi 1-3 era, and some printer drivers in the Delphi 5-7 era.
What kind of system are you using? (Processor, OS, 32-bit/64-bit, drivers, etc)?
–jeroen
jpluimers
May 5, 2009 at 10:50 pm
[...] 1.7 of the Habari Active MQ Client components) posted a blog entry about a strange circumstance when 1.99 would not compare equally to 1.99. He tracked it down to the 8087 (more formally: Intel FPU) Control Word being hosed on his [...]
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