py/asmrv32: Emit C.LW opcodes only when necessary.

The RV32 emitter sometimes generated short load opcodes even when it
was not supposed to.  This commit fixes an off-by-one error in its
offset eligibility range calculation and corrects one case of offset
calculation, operating on the raw label index number rather than its
effective offset in the stack (C.LW assumes all loads are
word-aligned).

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

py/asmrv32.c

@@ -301,7 +301,7 @@ void asm_rv32_emit_call_ind(asm_rv32_t *state, mp_uint_t index) {
     mp_uint_t offset = index * ASM_WORD_SIZE;
     state->saved_registers_mask |= (1U << ASM_RV32_REG_RA);
 
-    if (IS_IN_C_REGISTER_WINDOW(REG_FUN_TABLE) && IS_IN_C_REGISTER_WINDOW(INTERNAL_TEMPORARY) && FIT_SIGNED(offset, 7)) {
+    if (IS_IN_C_REGISTER_WINDOW(REG_FUN_TABLE) && IS_IN_C_REGISTER_WINDOW(INTERNAL_TEMPORARY) && FIT_UNSIGNED(offset, 6)) {
         // c.lw   temporary, offset(fun_table)
         // c.jalr temporary
         asm_rv32_opcode_clw(state, MAP_IN_C_REGISTER_WINDOW(INTERNAL_TEMPORARY), MAP_IN_C_REGISTER_WINDOW(REG_FUN_TABLE), offset);
@@ -487,7 +487,7 @@ void asm_rv32_emit_mov_reg_local_addr(asm_rv32_t *state, mp_uint_t rd, mp_uint_t
 void asm_rv32_emit_load_reg_reg_offset(asm_rv32_t *state, mp_uint_t rd, mp_uint_t rs, mp_int_t offset) {
     mp_int_t scaled_offset = offset * sizeof(ASM_WORD_SIZE);
 
-    if (IS_IN_C_REGISTER_WINDOW(rd) && IS_IN_C_REGISTER_WINDOW(rs) && FIT_SIGNED(offset, 7)) {
+    if (scaled_offset >= 0 && IS_IN_C_REGISTER_WINDOW(rd) && IS_IN_C_REGISTER_WINDOW(rs) && FIT_UNSIGNED(scaled_offset, 6)) {
         // c.lw rd', offset(rs')
         asm_rv32_opcode_clw(state, MAP_IN_C_REGISTER_WINDOW(rd), MAP_IN_C_REGISTER_WINDOW(rs), scaled_offset);
         return;