RISC-V: Remove the CSub instruction.

It was no longer needed after the removal of split register file
support and CHERI LLVM no longer emits it.  It remains in the ISA
quick reference summary table with a stub entry marking the encoding
as a removed instruction.

app-isaquick-riscv.tex

@@ -114,13 +114,6 @@ \chapter{CHERI-RISC-V ISA Quick Reference}
 
 		\rvcheriisaquick{CFromPtr}
 
-		\jwnote{We do not need CSub, since a standard Sub will return the difference between two capabilities.}
-
-		\jrtcnote{We do need a separate CSub with a split register file though,
-		so we define one that should be used even with a merged register file.}
-
-		\rvcheriisaquick{CSub}
-
 		\rvcheriisaquick{CMove}
 
 		%\dcnote{We probably shouldn't have these, because RISC-V doesn't have equivalents for integer registers - or we should define these as working on all registers.}
@@ -760,10 +753,12 @@ \chapter{CHERI-RISC-V ISA Quick Reference}
 	\vspace{1em}
 
 	{\rvcherienctablefontsize
-	\def\rvcherireservedfootnotemark{$^\dagger$}
+	\def\rvcheriremovedfootnotemark{$^\dagger$}
+	\def\rvcherireservedfootnotemark{$^\ddagger$}
 	\rvcherienctablesrcsrcdest\\\\
 	\footnotesize
-	$^\dagger$Reserved for future use.
+	$^\dagger$Previously used by a removed instruction.\\
+	$^\ddagger$Reserved for future use.
 	}
 
 	\subsection*{Stores encoding allocation  (rd field)}

app-versions.tex

@@ -567,7 +567,7 @@ \section{Detailed CHERI ISA Version Change History}
   \insnnoref{CEQ} instruction checked only that their offsets pointed at the
   same location.
 
-  A new capability instruction, \insnref{CSUB}, allows the implementation
+  A new capability instruction, \insnnoref{CSUB}, allows the implementation
   of C-language pointer subtraction semantics with the atomicity properties
   required for garbage collection.
 
@@ -724,7 +724,7 @@ \section{Detailed CHERI ISA Version Change History}
   \insnref{CUnseal} instruction, and an explanation added.
 
   Opcodes have now been specified for the \insnref{CSetBoundsExact} and
-  \insnref{CSub} instructions.
+  \insnnoref{CSub} instructions.
 
   To improve code generation when constructing a \PCC{}-relative capability as
   a jump target, a new \insnnoref{CGetPCCSetOffset} instruction has been

chap-architecture.tex

@@ -1631,9 +1631,9 @@ \section{Capability-Aware Instructions}
 These support efficient hybrid code, in which use of integer pointers and
 capabilities are intermixed.
 
-\item[Capability pointer comparison and arithmetic]
-These instructions provide pointer comparison and
-subtraction behavior: \insnref{CSetEqualExact}, \insnref{CSub}, and
+\item[Capability pointer comparison]
+These instructions provide pointer comparison:
+\insnref{CSetEqualExact} and
 \insnref{CTestSubset}.
 
 \item[Load or store via a capability]
@@ -3044,7 +3044,7 @@ \section{Potential Future Changes to the CHERI Architecture}
   they have $base=0$ and $length=2^{64}$ for the purposes of the
   instructions where this matters, namely \insnref{CGetBase},
   \insnref{CGetOffset}, \insnref{CIncOffset},
-  \insnref{CGetLen} and \insnref{CSub}.
+  and \insnref{CGetLen}.
   However, there is also a desire that \insnref{CSetOffset} should preserve
     the values of $T$ and $B$ for debugging purposes, where possible.
 

chap-cheri-riscv.tex

@@ -1420,11 +1420,10 @@ \subsection{Separate Capability Register File}
   obtained by reading the aliased general-purpose integer register.
   Note that \insnref{CGetAddr} is currently still defined.
 
-\item A separate \insnref{CSub} instruction to compute the difference
+\item A separate \insnnoref{CSub} instruction to compute the difference
   between addresses in two capability registers.  In a ``merged''
   register file this can be computed via the existing \insnnoref{SUB}
-  instruction on the aliased general-purpose integer registers.  Note
-  that \insnref{CSub} is currently still defined.
+  instruction on the aliased general-purpose integer registers.
 
 \item Dirty bits were defined in a the per-mode CSR \xccsr{}. This bit
   was set anytime a general-purpose capability register was changed.

chap-isaref-riscv.tex

@@ -287,7 +287,6 @@ \section{CHERI-RISC-V Instructions}
 \input{insn-riscv/csethigh}
 \input{insn-riscv/csetoffset}
 \input{insn-riscv/cspecialrw}
-\input{insn-riscv/csub}
 \input{insn-riscv/ctestsubset}
 \input{insn-riscv/ctoptr}
 \input{insn-riscv/cunseal}

def-riscv-insns.tex

@@ -49,7 +49,7 @@
 
 \rvcherisrcsrcdest[name=CToPtr]{12}{rd}{cs1}{cs2}
 \rvcherisrcsrcdest[name=CFromPtr]{13}{cd}{cs1}{rs2}
-\rvcherisrcsrcdest[name=CSub]{14}{rd}{cs1}{cs2}
+\rvcherisrcsrcdest[name=CSub,noref,tablesuffix=\rvcheriremovedfootnotemark]{14}{rd}{cs1}{cs2}
 \rvcherisrcdest[name=CMove]{A}{cd}{cs1}
 
 \rvcherisrcsrcdest[name=CTestSubset]{20}{rd}{cs1}{cs2}