Is t4 / x29 preserved across calls?
No. t0-t6 are temporary registers and are not preserved across calls.
Register Guide
RISC-V t4 / x29 Register: Temporary
Temporary register 4 (x29). The psABI classifies t4 as a temporary register for caller-managed short-lived values; its value is not preserved across calls.
ABI name: t4; physical register: x29.
Save rule: Caller.
Read it through its psABI role first, then inspect how each instruction reads or writes it.
Physical Name x29ABI Name t4Save Rule Caller
Role
Temporary
Convention
Caller-saved
Remember This First
t4 is a caller-saved temporary register, so callees may overwrite it.
psABI Reference
Temporary register
The psABI marks t0-t6 as temporary registers. They are not callee-saved; callers must save any value that needs to survive a call.
Preserved across calls: No
RISC-V psABI integer register conventionQuick Understanding & Search Notes
t4 / x29 belongs to the t0-t6 temporary-register group. The psABI marks it not preserved across calls; callers must save any value that needs to survive a call.
t0-t6 are temporary registers and are not preserved across calls.
If a t4 value is still needed across a call, the caller must save it explicitly.
When It Fits Best
- - Hold short-lived temporary computation results.
- - Use it for local computations that do not need to survive calls.
- - Use it as a caller-managed scratch register.
When Not To Use It This Way
- - Do not keep values in it across a call unless you save them yourself first.
- - Do not expect callees to restore it for you.
What Happens Around A Call
1
t4 is a caller-saved temporary register, so callees may overwrite it.
2
If the caller still needs the t4 value after a call, it must save it in advance.
3
Values that must survive calls should usually live in s* registers or on the stack.
Save t4 Before A Call If Needed
Examples explain the rule, not a complete programaddi t4, a0, 1
# If t4 is needed after the call, the caller must save it to an ABI-valid location first
call foo
# restore t4 from that saved location after the call returnsFAQ
What if a t4 value is needed after a call?
The caller must save it before the call, usually to the stack or another suitable location, and restore it afterward.