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[appendix]
== Calling Convention (Not authoritative - Placeholder Only)

NOTE: This Appendix is only a placeholder to help explain the
conventions used in the code examples, and is not considered frozen or
part of the ratification process.  The official RISC-V psABI document
is being expanded to specify the vector calling conventions.

In the RISC-V psABI, the vector registers `v0`-`v31` are all caller-saved.
The `vl` and `vtype` CSRs are also caller-saved.

Procedures may assume that `vstart` is zero upon entry.  Procedures may
assume that `vstart` is zero upon return from a procedure call.

NOTE: Application software should normally not write `vstart` explicitly.
Any procedure that does explicitly write `vstart` to a nonzero value must
zero `vstart` before either returning or calling another procedure.

The `vxrm` and `vxsat` fields of `vcsr` have thread storage duration.

Executing a system call causes all caller-saved vector registers
(`v0`-`v31`, `vl`, `vtype`) and `vstart` to become unspecified.

NOTE: This scheme allows system calls that cause context switches to avoid
saving and later restoring the vector registers.

NOTE: Most OSes will choose to either leave these registers intact or reset
them to their initial state to avoid leaking information across process
boundaries.

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提交于 2021-09-14 02:32 . Clarified the calling convention section is only a placeholder until t…

Appendix A: Calling Convention (Not authoritative - Placeholder Only)

Note	This Appendix is only a placeholder to help explain the conventions used in the code examples, and is not considered frozen or part of the ratification process. The official RISC-V psABI document is being expanded to specify the vector calling conventions.

In the RISC-V psABI, the vector registers v0-v31 are all caller-saved. The vl and vtype CSRs are also caller-saved.

Procedures may assume that vstart is zero upon entry. Procedures may assume that vstart is zero upon return from a procedure call.

Note	Application software should normally not write `vstart` explicitly. Any procedure that does explicitly write `vstart` to a nonzero value must zero `vstart` before either returning or calling another procedure.

The vxrm and vxsat fields of vcsr have thread storage duration.

Executing a system call causes all caller-saved vector registers (v0-v31, vl, vtype) and vstart to become unspecified.

Note	This scheme allows system calls that cause context switches to avoid saving and later restoring the vector registers.

Note	Most OSes will choose to either leave these registers intact or reset them to their initial state to avoid leaking information across process boundaries.

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