Does a1 / x11 keep its old argument value after a call?
Do not assume so. a0-a7 are not preserved across calls, and the callee may overwrite them.
Register Guide
RISC-V a1 / x11 Register: Argument / Return value
One of the a0-a7 integer argument registers; the psABI also uses a1 for return values. It is not preserved across calls.
ABI name: a1; physical register: x11.
Save rule: Not preserved across calls.
Read it through its psABI role first, then inspect how each instruction reads or writes it.
Physical Name x11ABI Name a1Save Rule Not preserved across calls
Role
Argument / Return value
Convention
Not preserved across calls
Remember This First
The psABI marks a0-a7 as not preserved across calls.
psABI Reference
Argument/return-value register
The standard psABI uses a0-a7 as argument registers, with only a0-a1 also used for return values. They are not preserved across calls.
Preserved across calls: No
RISC-V psABI integer register conventionQuick Understanding & Search Notes
a1 / x11 belongs to the a0-a7 argument-register group. The psABI uses a0-a7 for arguments and a0-a1 for return values; these registers are not preserved across calls.
a0-a7 are argument registers, and a0-a1 are also used for return values.
a1 is not preserved across calls, so a callee may overwrite it.
When It Fits Best
- - a1 is one of the psABI a0-a7 integer argument registers.
- - a1 is one of the first two argument registers that the psABI also uses for return values.
- - Read a0-a7 first as function argument registers under the psABI.
When Not To Use It This Way
- - Do not assume a* registers keep their old values after a call.
- - If the original a* value is needed after a call, do not leave it in a* across that call.
What Happens Around A Call
1
The psABI marks a0-a7 as not preserved across calls.
2
a0 / a1 are the first two argument registers that the psABI also uses for return values; after a call, do not assume they retain old arguments.
3
For a scalar of at most XLEN bits, the psABI passes it by value on the stack when no argument register is available.
Arguments And Return Values
Examples explain the rule, not a complete programaddi a1, zero, 3 # write a1
# The psABI also uses a1 for return valuesFAQ
Which a registers carry return values?
The psABI uses a0-a1 for return values as well as arguments.