tail — tail-call a symbol

tail symbol

tail transfers control without creating a new return address. The current function’s caller remains the eventual return destination. Its canonical relocation-aware form is:

auipc t1, %pcrel_hi(symbol)
jalr  x0, %pcrel_lo(symbol)(t1)

GNU-compatible assemblers reserve t1 (x6) as the scratch register for this macro. The call relocation on the emitted pair carries the exact linker semantics; the explicit modifiers above show the address calculation.

Is it always one instruction?

No. It normally starts as two instructions. The linker may relax it to jal x0, symbol, or to an applicable compressed jump, when the final target and enabled ISA allow it. Symbol binding, distance, enabled extensions, and relaxation can therefore change the final sequence; .option pic does not select a different canonical tail form.

ABI consequences

tail deliberately does not write ra, but it clobbers t1. Before using it, the function must restore any stack frame and callee-saved registers just as it would before returning. Arguments must already be arranged according to the calling convention.