Not a compiler author, but have some experience with finding bugs in compilers. If I had to venture a guess, I'd pick the testsuite (and other tools like fuzzin

Not a compiler author, but have some experience with finding bugs in compilers. If I had to venture a guess, I'd pick the testsuite (and other tools like fuzzing and mass user base), since the complexity is quite high. In our experience with the EMI project (https://web.cs.ucdavis.edu/~su/publications/emi.pdf https://web.cs.ucdavis.edu/~su/publications/emi.pdf), some of the miscompilations were caused by one pass of the compiler emitting code that was considered to have contained undefined behavior by the next pass (see Figure 1(b)). I'd say the code reviewer will catch issues within a single pass, but the interaction between them are less likely to be caught by a human reviewer as the complexity grows.