qemu: Emit Zcmt opcodes by default for RV32 boards.

This commit updates the configuration for RISC-V 32-bit boards to always
emit Zcmt opcodes when it comes to natively compiled functions.

QEMU's CPU configuration was changed accordingly, with Zcmt opcodes also
sharing opcode space with the Zcd instructions (just like Zcmp ones).

The entry point for the port will also perform the necessary
initialisation steps to create a suitable function table in RAM that can
be used for Zcmt opcodes, and also sets up the appropriate CPU registers
in order to make jump opcodes read the correct address.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti

ports/qemu/Makefile

@@ -105,14 +105,14 @@ GCC_VERSION = $(word 1, $(subst ., , $(shell $(CC) -dumpversion)))
 
 RV32_ABI = ilp32
 
-QEMU_ARGS += -bios none -cpu rv32,zcd=off,zcmp=on
+QEMU_ARGS += -bios none -cpu rv32,zcd=off,zcmp=on,zcmt=on
 
 # GCC 10 and lower do not recognise the Zicsr extension in the architecture name.
 ifeq ($(shell test $(GCC_VERSION) -le 10; echo $$?),0)
 RV32_ARCH ?= rv32imac
 else
 # Recent GCC versions explicitly require to declare extensions.
-RV32_ARCH ?= rv32imac_zicsr
+RV32_ARCH ?= rv32imac_zicsr_zifencei
 endif
 
 AFLAGS += -mabi=$(RV32_ABI) -march=$(RV32_ARCH)

ports/qemu/mcu/rv32/startup.c

@@ -29,13 +29,36 @@
 #include <stdlib.h>
 
 #include "uart.h"
+#include "mpconfigport.h"
 
 extern void set_interrupt_table(void);
 extern int main(int argc, char **argv);
 
+#if MICROPY_EMIT_RV32_ZCMT
+extern const uintptr_t mp_fun_table;
+
+static void jump_table_init(void) {
+    // Relocate the table address to always perform a jump and link (index has
+    // to be between 32 and 255), so that calling function index #32 will perform
+    // a jump-and-link to the real function at index #0 (see §28.14.5).
+    __asm volatile (
+        // Use the JVT CSR index directly for compatibility.
+        "csrw 0x017, %0 \n"
+        "fence.i        \n"
+        :
+        : "r" ((ptrdiff_t)&mp_fun_table - (ptrdiff_t)(32 * sizeof(uintptr_t)))
+        : "memory"
+        );
+}
+#endif
+
 void _entry_point(void) {
     // Set interrupt table
     set_interrupt_table();
+    #if MICROPY_EMIT_RV32_ZCMT
+    // Build native jump table.
+    jump_table_init();
+    #endif
     // Enable UART
     uart_init();
     // Now that we have a basic system up and running we can call main

ports/qemu/mpconfigport.h

@@ -44,6 +44,8 @@
 #define MICROPY_EMIT_RV32           (1)
 #define MICROPY_EMIT_RV32_ZBA       (1)
 #define MICROPY_EMIT_RV32_ZCMP      (1)
+#define MICROPY_EMIT_RV32_ZCMT      (1)
+#define MICROPY_NATIVE_FUN_TABLE_ALIGNMENT (64)
 #define MICROPY_EMIT_INLINE_RV32    (1)
 #elif (__riscv_xlen == 64)
 #define MICROPY_PERSISTENT_CODE_LOAD_NATIVE (1)