How to describe this bug?

Simon Marlow marlowsd at
Tue Jul 10 14:33:46 CEST 2012

On 10/07/2012 12:21, Aleksey Khudyakov wrote:
> On Tue, Jul 10, 2012 at 3:06 PM, Sönke Hahn <shahn at> wrote:
>> I've attached the code. The code does not make direct use of
>> unsafePerformIO. It uses QuickCheck, but I don't think, this is a
>> QuickCheck bug. The used Eq-instance is the one for Float.
>> I've only managed to reproduce this bug on 32-bit-linux with ghc-7.4.2
>> when compiling with -O2.
> It's expected behaviour with floats. Calculations in FPU are done in
> maximul precision available.  If one evaluation result is kept in registers
> and another has been moved to memory and rounded and move back to registers
> number will be not the same indeed.
> In short. Never compare floating point number for equality unless you
> really know
> what are you doing.

I consider it a bug, because as the original poster pointed out it is a 
violation of referential transparency.  What's more, it is *not* an 
inherent property of floating point arithmetic, because if the compiler 
is careful to do all the operations at the correct precision then you 
can get determinstic results.  This is why GHC has the 
-fexcess-precision flag: you have to explicitly ask to break referential 

The bug is that the x86 native code generator behaves as if 
-fexcess-precision is always on.  I seriously doubt that we'll ever fix 
this "bug": you can get correct behaviour by enabling -msse2, or using a 
64-bit machine.  I don't off-hand know what the LLVM backend does here, 
but I would guess that it has the same bug.


More information about the Glasgow-haskell-users mailing list