Compiler validation used to be a big thing; a NIST quarterly validated products list could run to nearly 150 pages, and approaching 1,000 products (not all were compilers).
Why did compiler validation stop being a thing?
Running a compiler validation service (NIST was also involved with POSIX, graphics, and computer security protocols validation) costs money. If there are enough people willing to pay (NIST charged for the validation service), the service pays for itself.
The 1990s was a period of consolidation, lots of computer manufacturers went out of business and Micro Focus grew to dominate the Cobol compiler business. The number of companies willing to pay for validation fell below the number needed to maintain the service; the service was terminated in 1998.
The source code of the Cobol, Fortran and SQL + others tests that vendors had to pass (to appear for 12 months in the validated products list) is still available; the C validation suite costs money. But passing these tests, then paying NIST’s fee for somebody to turn up and watch the compiler pass the tests, no longer gets your product’s name in lights (or on the validated products list).
At the time, those involved lamented the demise of compiler validation. However, compiler validation was only needed because many vendors failed to implement parts of the language standard, or implemented them differently. In many ways, reducing the number of vendors is a more effective means of ensuring consistent compiler behavior. Compiler monoculture may spell doom for those in the compiler business (and language standards), but is desirable from the developers’ perspective.
How do we know whether today’s compilers implement the requirements contained in the corresponding ISO language standard? You could argue that this is a pointless question, i.e., gcc and llvm are the language standard; but let’s pretend this is not the case.
Fuzzing is good for testing code generation. Checking language semantics still requires expert human effort, and lots of it. People have to extract the requirements contained in the language specification, and write code that checks whether the required behavior is implemented. As far as I know, there are only commercial groups doing this, i.e., nothing in the open source world; pointers welcome.