Just for reference atm. Currently I think the impact isn't big enough for the added complexity. If we would be able to determine a double can't be a NaN it could simplify some codegeneration a bit and improve speed. The case I'm looking at is: raytrace.js:223 initialize : function(x, y, z) { this.x = (x ? x : 0); this.y = (y ? y : 0); this.z = (z ? z : 0); }, We can determine that x, y and z are never undefined in the benchmark. Actually the type is always double in this benchmark, but we can specialize the code to: initialize : function(x, y, z) { this.x = x; this.y = y; this.z = z; }, because NaN also returns false and need a 0. In this testcase NaN never occurs and removing the "if" would give a 1% increase on v8-raytrace. Another way to solve this, is to decide that when NaN occurs in TestDAndBranch we bailout. Because NaN shouldn't be the main path and it doesn't matter if that path is slow ...