missed optimization with "complex" condition

[1-Olivier]

This is similar to #1146 but likely has a different root cause. Consider this shader:

vector foo(float x, string a)
{
    vector result = 0;
    if( x >= -1 && a != "" )
    {
        result = 1;
    }
    return result;
}

surface simple(string s = "")
{
    P += foo(u, s);
}

Once again, testshade --debug simple.oso yields:

Globals written: (1)  P

Making the condition simpler:

vector foo(float x, string a)
{
    vector result = 0;
    if( a != "" )
    {
        result = 1;
    }
    return result;
}

is enough to get proper optimization:

Globals written: (0)

As that second condition can clearly be proven false, the first one should be easy to prove false also. So I would expect it to be optimized the same way.

Versions

• OSL branch/version: 1.10.9 (testshade output above) / 3d0a284 (where I originally found the issue in a larger network)
• OS: linux
• LLVM version: 9.0.1 / 6.0.0

[1-Olivier]

It gets more interesting: if I swap the condition around, to if( a != "" && x >= -1 ), the optimizer is able to figure it out.

[1-Olivier]

More interesting trivia: swapping the condition to fix the optimization made a scene with some particularly ugly shader networks render 2.75 times faster.

[lgritz]

I will investigate