It seems the hypothesis is that the benchmark originally goes very fast due to dead-code analysis (the function 'cordicsincos' has been marked as dead code, and is therefore not executed).
That the test goes much slower when more dead code is added (code that in no way 'undeadens' the 'cordicsincos' function, indeed code that does nothing at all) implies that the dead-code analysis being done is either not really dead-code analysis at all but simply looking for this specific function (this would be the 'cheating' hypothesis), or, more charitably, the dead-code analysis could merely be extremely fragile.
That the test goes much slower when more dead code is added (code that in no way 'undeadens' the 'cordicsincos' function, indeed code that does nothing at all) implies that the dead-code analysis being done is either not really dead-code analysis at all but simply looking for this specific function (this would be the 'cheating' hypothesis), or, more charitably, the dead-code analysis could merely be extremely fragile.