Move proposal specific to split register-file

chap-architecture.tex

@@ -3173,22 +3173,6 @@ \section{Potential Future Changes to the CHERI Architecture}
   represents, and the caller might want to restrict these permissions before
   passing the sealed capability to another subsystem.
 
-\item
-  Add instructions for copying non-capability data from a capability register
-  into a general-purpose integer register. A use case is when a function is called
-  with a parameter whose type is the union of a pointer and a non-pointer type,
-  such as an int. This parameter must be passed in a capability register, because
-  the tag needs to be preserved when it holds a capability. If the body of
-  the function accesses the non-capability branch of the union, it needs to
-  get the non-capability bits out of the capability register and into a general
-  purpose register. This can be done by spilling the capability register to the
-  stack and then reading it back into a general-purpose integer register, but a register
-  to register copy would be faster. (We need to investigate whether this happens
-  often enough for the optimization to be worthwhile).
-
-  With compressed 128-bit capabilities, two instructions are needed (to get
-  the upper and lower 64 bits of the capability register).
-
 \item
   Add an instruction that is like \insnref{CSetBounds} except that it
   sets \cbase{} to the current \cbase{} $+$ \coffset{} and the new length

chap-rationale.tex

@@ -261,6 +261,18 @@ \section{Capability Register File}
 might be quite different; effectively pinning the two to one another would
 reduce the efficiency of both.
 
+With a split register file, it may be useful for the architecture to add
+instructions for copying non-capability data from a capability register
+into a general-purpose integer register. A use case is when a function is called
+with a parameter whose type is the union of a pointer and a non-pointer type,
+such as an int. This parameter must be passed in a capability register, because
+the tag needs to be preserved when it holds a capability. If the body of
+the function accesses the non-capability branch of the union, it needs to
+get the non-capability bits out of the capability register and into a general
+purpose register. This can be done by spilling the capability register to the
+stack and then reading it back into a general-purpose integer register, but a register
+to register copy would be faster.
+
 CHERI-RISC-V (Chapter~\ref{chap:cheri-riscv}),
 Morello, and CHERI-x86-64 (Chapter~\ref{chap:cheri-x86-64}) use merged register files.