Register Guide
RISC-V vl / vl Register: Vector Length CSR
vl is the Vector Length register, holding the number of elements to be processed by the current vector instruction, set by vsetvl/vsetvli or updated by fault-only-first loads.
ABI name: vl; physical register: vl.
Save rule: Caller.
Read it through its psABI role first, then inspect how each instruction reads or writes it.
Physical Name vlABI Name vlSave Rule Caller
Role
Vector Length CSR
Convention
Caller-saved
Remember This First
t* registers are caller-saved, so callees may overwrite them.
psABI Reference
ABI register
vl is the Vector Length register, holding the number of elements to be processed by the current vector instruction, set by vsetvl/vsetvli or updated by fault-only-first loads.
Preserved across calls: Caller
RISC-V psABI integer register conventionWhen It Fits Best
- - Hold short-lived temporary computation results.
- - Use them for local computations that do not need to survive calls.
- - Use them as caller-managed scratch registers.
When Not To Use It This Way
- - Do not keep values in t* across a call unless you save them yourself first.
- - Do not expect callees to restore t* for you.
What Happens Around A Call
1
t* registers are caller-saved, so callees may overwrite them.
2
If the caller still needs a t* value after a call, it must save it in advance.
Protect Temporary Values Before A Call
Examples explain the rule, not a complete programaddi t0, a0, 1
# If t0 is needed after the call, save it to an ABI-valid location first
call foo
# restore t0 from that saved location after the call returns