Question

In my program this fragment:

  trace.log(
   String.Format("a= {0:R} b= {1:R} a<b= {2}", 
    b.GetPixel(447, 517).GetBrightness(), (100F / 255F),
    b.GetPixel(447, 517).GetBrightness() < (100F / 255F))
  );

outputs this in Debug\prog.exe:

 a= 0.392156869 b= 0.392156869 a<b= False

but this different result in Release\prog.exe:

 a= 0.392156869 b= 0.392156869 a<b= True

Can anyone explain why the same operands give a different comparision result? And recommend a remedy, ideally program-wide such as a compiler switch? Thanks.

EDIT: Clarification: the above results are from launching Debug\prog.exe and Release\prog.exe in Windows Explorer.

EDIT: Further info: executing from VS, “Start Debugging” gives False (i.e. the accurate result, same as WE-launched Debug\prog.exe), and “Start without debugging” gives True (i.e. the inaccurate result, same as WE-launched Release\prog.exe.

EDIT: Alternative test cases having literals substituted

These two cases

  trace.log(
   String.Format("a= {0:R} b= {1:R} a<b= {2}",
    0.392156869F, 0.392156869F, 
    0.392156869F < 0.392156869F)
  );
  trace.log(
   String.Format("a= {0:R} b= {1:R} a<b= {2}",
    0.392156869F, (100F / 255F),
    0.392156869F < (100F / 255F))
  );

show no discrepancy in Debug and Release output. This case:

  trace.log(
   String.Format("a= {0:R} b= {1:R} a<b= {2}",
    b.GetPixel(447, 517).GetBrightness(), 0.392156869F,
    b.GetPixel(447, 517).GetBrightness() < 0.392156869F)
  );

shows the same discrepancy (and inaccuracy in Release) as the original test case.

EDIT: Belated minimal test case demoing the problem

Color c = Color.FromArgb( 255, 100, 100, 100 );
trace.log(
 String.Format("a= {0} b= {1} a<b= {2}",
 c.GetBrightness(), 0.392156869F,
 c.GetBrightness() < 0.392156869F)
);

outputs this correct result in Debug\prog.exe:

 a= 0.392156869 b= 0.392156869 a<b= False

but this incorrect result in Release\prog.exe:

 a= 0.392156869 b= 0.392156869 a<b= True

EDIT: Remedies

1) From Peter’s answer below:

trace.log(
 String.Format("a= {0:R} b= {1:R} a<b= {2}",
  c.GetBrightness(), 0.392156869F, 
  c.GetBrightness().CompareTo(0.392156869F)<0)
);

2) From ChrisJJ, the questioner (UPDATED):

float comp = c.GetBrightness();
trace.log(
 String.Format("a= {0:R} b= {1:R} a<b= {2}", 
 comp, 0.392156869F,
 comp < 0.392156869F)
);

I think this adds to the evidence for a Release-mode compiler bug.

Answer 1

It’s most likely the result of different floating point rules between debug mode and release mode (see here and here).

Update:

Thanks to your new update to the question, I was able to reproduce the problem on my machine (64-bit Windows). This appears to happen only when setting the platform to X86; the X64 and AnyCPU platforms show the correct result in release mode. Probably, when the platform is X86, the common language runtime applies X86 emulation in 64-bit machines and apparently messed up in the comparison operator.

However, I found a possible workaround: Use CompareTo instead of the “<” and “>” operators, like this:

c.GetBrightness().CompareTo(0.392156869F)<0

On my machine, this will provide the same correct results in X86 as in X64 and AnyCPU.