Stop building JIT as altjit or not altjit

[BruceForstall]

Currently, a JIT is built with the ALT_JIT define if it is intended to be used as an "altjit". Only when this is defined can a JIT define opts.altJit to true. This alters what functions are compiled by the JIT, and what COMPlus variables apply to the JIT versus the non-altjit.

This is problematic because we would like to be able to use any JIT in "altjit position" without this special build.

Proposal: the determination about whether a JIT is an altjit should be made via the JIT/EE interface. When the VM invokes the JIT, such as invokeCompileMethodHelper, it knows it is invoking an altjit:

 ret = jitMgr->m_alternateJit->compileMethod( comp,
                                              info,
                                              CORJIT_FLAGS::CORJIT_FLAG_CALL_GETJITFLAGS,
                                              nativeEntry,
                                              nativeSizeOfCode);

It should either tell the JIT (probably by a new method in the ICorJitCompiler interface; we typically can't change the compileMethod signature), or the JIT should use a callback on the JIT/EE interface to ask if it is an altjit.

This should help scenarios such as crossgen2, where all the cross-compilers could be built as "normal" JITs, and used both for crossgen2 cross-compilation as well as in altjit position for SuperPMI, for example. See #41126.

category:implementation
theme:altjit
skill-level:beginner
cost:small

[AndyAyersMS]

Seems like this could just be toggled via a new jit flag?

Phase 1 would be to enable the jit to internally behave as an altjit or regular jit depending on a runtime provided value; initially this could just be hardwired in the jit to true/false.

Phase 2 would be to modify the interface (add flag, say) update the various jit hosts to set it appropriately, and update the jit to use the value of the flag.

[AndyAyersMS]

@BruceForstall is this done?

[BruceForstall]

Yes, fixed with #44565 and follow-ups