Is it possible you're confusing LLVM with JIT compilation?

Clang targeting LLVM, despite the name, produces binaries compiled 100% ahead-of-time.

The "LLVM" is just the intermediate representation used by the compiler. GCC uses GIMPLE and RTL.

> In C... it is still possible to bake what is placed in "register" a priori

By what mechanism? Even ancient GCC circa 1989 ignored the "register" keyword/storage-class. Automatic register allocation algorithms have been the default for a long time. You have to go back to the K&R days to find a compiler willing to honor it (or find an esoteric compiler like AZTEC C for the 6502).