Compilers, as one of the most important infrastructure of today's digital world, are expected to be trustworthy. Dif-ferent testing techniques are developed for testing compilers automatically. However, it is unknown so far how these test-ing techniques compared to each other in terms of testing effectiveness: how many bugs a testing technique can find within a time limit. In this paper, we conduct a systematic and comprehensive empirical comparison of three compiler testing techniques, namely, Randomized Differential Testing (RDT), a variant of RDT|Different Optimization Levels (DOL), and Equiv-alence Modulo Inputs (EMI). Our results show that DOL is more effective at detecting bugs related to optimization, whereas RDT is more effective at...