With --cpu-dispatch, an x86-64 executable carries two copies of each
program function: the baseline code and a copy compiled for x86-64-v3.
main reads CPUID and XGETBV once. Calls between the program's functions
go to the caller's copy. A function that is also named outside the
copies (a function value, a call from main, a runtime callback or a
coroutine body) keeps a dispatcher under its own name, which tests the
level and tail-calls one copy. Functions without such a use get no
dispatcher, so their copies have the callers the original had and LLVM
inlines them the same way.
The executable still runs on any x86-64 CPU. SCRIPTC_CPU_LEVEL caps the
level at run time. The option is off by default, applies to executables
without a --target-cpu level, and is part of the executable cache key.
bench:runtime takes --baseline-cpu-dispatch and --candidate-cpu-dispatch,
and the LLVM differential suite runs each program at levels 0 and 3 with
SCRIPTC_CPU_DISPATCH=1.
Chris Tate
07c5e79ffd