Re: [Bug-glpk] Different generated model on ARMv7 and x86-64

[Andrew Makhorin]

Hi Heinrich,

Thank you for your help.

> The following model is sufficient to demonstrate it:
> 
> param xt := Uniform01();
> param pt := Uniform01();
> 
> var y, >= 0;
> minimize obj : y + pt + xt;
> 
> solve;
> 
> printf "%f ", pt;
> printf "%f\n", xt;
> end;
> 
> 
> 
> x86: (32-bit LSB executable, i686, gcc 4.9.2)
> 0.009597 0.128312
> 
> x86-64: (64-bit LSB, x86-64, gcc 6.3.0)
> 0.009597 0.128312
> 
> armv7 (32-bit LSB, ARM, gcc 6.3.0)
> 0.128312 0.009597
> 
> armv8 (64-bit LSB, ARM aarch64, gcc 6.4.0)
> 0.128312 0.009597
> 
> mips (ELF 32-bit MSB, gcc 4.9.2)
> 0.128312 0.009597
> 
> Random numbers are generated in this sequence on all architectures:
> 
> printf {1..2} "%16.15f\n", Uniform01();
> 
> 0.128311804030091
> 0.009596675168723
> 

Good news: there is no bug.

In the first two cases evaluation of pt precedes evaluation of xt while
in the last three cases xt is evaluated before pt. At the moment I
cannot say why this happens, however, this is obviously not a bug,
because the order of evaluation of subexpressions is not defined (like
in C). Thus, if evaluation of a subexpression has a side effect (as in
case of using Uniform01), the result is indeterminate.


Best regards,

Andrew Makhorin