Re: [Boost-users] [Interval] Help debugging compiler optimization

Subject: Re: [Boost-users] [Interval] Help debugging compiler optimization
From: Ilja Honkonen (ilja.honkonen_at_[hidden])
Date: 2018-03-08 08:49:04

Hello
> My (somewhat naive) view of compilers is that they can only do 
> optimization that has no visible effects on the behavior of the program.

I wouldn't be surprised if ieee floating point is an exception to that, 
especially with optimizations enabled.

> What I meant is:
> double v1 = 1.0/3.0;
> printf("%f", v1);
> double computation = v1 * 3;
> Then the print statement will show the user some value for v1 that fits 
> in a double. So I would expect that it is then no longer free to treat 
> v1 as exactly 1/3, because the use has already observed a different value.

I think in order to show the number to the user it would have to be 
copied to memory or at least a regular register but further computation 
could continue to use the more accurate floating point register. And the 
compiler might still optimize the computation away and just show you the 
result of printf(..., 1.0/3.0). I don't know if writing constexpr double 
v1 = 1.0/3.0 would change anything, at least if particular optimizations 
are enabled and/or disabled.

Ilja