This work proves functional equivalence, so no test suite is necessary.
Imperfect or not, optimizing compilers do not require an objective function at all. The majority of computer programs do not have an objective function.
How does this claim mesh with this quote from the paper:
"""Stoke (Schkufza et al., 2013) performs black-box optimization of a cost function on the space of programs, represented as a series of instructions. Each instruction is composed of an opcode, specifying what to execute, and some operands, specifying the corresponding registers. Each given input program T defines a cost function. For a candidate program R called rewrite, the goal is to optimize the following cost function"""