Derive xlen values from a logarithmic constant

The motivation for this is that it makes deriving CHERI constants (field widths etc.) which vary between RV32 and RV64 a lot easier.

Makefile

@@ -18,6 +18,7 @@ else
  $(error '$(ARCH)' is not a valid architecture, must be one of: RV32, RV64)
 endif
 
+SAIL_XLEN += riscv_xlen.sail
 SAIL_FLEN := riscv_flen_D.sail
 SAIL_VLEN := riscv_vlen.sail
 

model/riscv_xlen.sail

@@ -0,0 +1,11 @@
+/*=======================================================================================*/
+/* This Sail RISC-V architecture model, comprising all files and */
+/* directories except where otherwise noted is subject the BSD */
+/* two-clause license in the LICENSE file. */
+/* */
+/* SPDX-License-Identifier: BSD-2-Clause */
+/*=======================================================================================*/
+
+type xlen_bytes : Int = 2 ^ log2_xlen_bytes
+type xlen : Int = xlen_bytes * 8
+type xlenbits = bits(xlen)

model/riscv_xlen32.sail

@@ -6,8 +6,7 @@
 /* SPDX-License-Identifier: BSD-2-Clause */
 /*=======================================================================================*/
 
-/* Define the XLEN value for the architecture. */
-
-type xlen : Int = 32
-type xlen_bytes : Int = 4
-type xlenbits = bits(xlen)
+// Define the XLEN value for the architecture.
+// This is done using the smallest/most logarithmic possible value since Sail's
+// type system works well for multiply and 2^ but not divide and log2.
+type log2_xlen_bytes : Int = 2

model/riscv_xlen64.sail

@@ -6,8 +6,7 @@
 /* SPDX-License-Identifier: BSD-2-Clause */
 /*=======================================================================================*/
 
-/* Define the XLEN value for the architecture. */
-
-type xlen : Int = 64
-type xlen_bytes : Int = 8
-type xlenbits = bits(xlen)
+// Define the XLEN value for the architecture.
+// This is done using the smallest/most logarithmic possible value since Sail's
+// type system works well for multiply and 2^ but not divide and log2.
+type log2_xlen_bytes : Int = 3