[hayley-patton]

You can't seriously say that, just because one targets a well-specified machine, that the language being used is well-specified. The determinism of a Clojure-on-JVM program would also be dependent on the particular code the Clojure compiler generates. The code has to remain semantically identical between compiler versions, and it should not introduce any new non-determinism, e.g. that presented in [1]. In Common Lisp there is the Armed Bear Common Lisp implementation, which runs on a JVM. Does it benefit from JVM determinism or not? It probably does not, because the JVM is simply not aware of undefined behaviour that ABCL or Clojure are implicitly defining.

When it comes to having different platforms, it would also be necessary for any other compilers to generate semantically identical code. Different Clojure systems do _not_ do that. For example, arithmetic in ClojureScript uses JS floats where Clojure-on-JVM and others use integers of some size.

In my experience, writing a non-conforming CL program is hard, and much harder than writing a program without undefined behaviour in C. I am not sure why, other than the UB being more "localised" in some vague way. But there is also a modification of the ANSI standard being worked on, which attempts to eliminate undefined behaviour <https://github.com/s-expressionists/wscl>.

[1] Hans Boehm, Threads cannot be implemented as a library <https://web.stanford.edu/class/cs240/readings/p261-boehm.pdf>