RV64: Relax AUIPC+JALR to C.JAL

[rui314]

Currently, we relax AUIPC+JALR only to JAL, but if the C extension is available and the displacement is within its range, we should optimize it further to C.JAL. Doing this should save extra one byte for each instruction.

[rui314]

It looks like most if not all Unix-like systems assume that the C extension is supported, so we may not have to detect if C.JAL is allowed. Instead, we may want to assume that C.JAL is always usable.

[ksco]

It seems that C.JAL is an RV32IC-only instruction, so this optimization may not work with RV64.

[rui314]

Are you sure? I thought that it's also a part of the RV64 C extension.

[ksco]

Yes, C.ADDIW is an RV64-only instruction, they share the same opcode.

Copied from the Spec:

C.JAL is an RV32C-only instruction that performs the same operation as C.J, but additionally writes the address of the instruction following the jump (pc+2) to the link register, x1. C.JAL expands to jal x1, offset.

[rui314]

Oh, that's true. Thank you for pointing it out!