py/asmxtensa: Emit prologue jump only when constants table is in use.

This commit simplifies native functions' prologue code by not emitting a
jump opcode that goes over the function's constants pool if the pool is
empty.

The original code assumed the constants pool is never empty as large
32-bits constants are commonly used, but for inline assembler functions
that may not be the case.  This meant that inline assembler functions
may start with an unneeded jump (along with its alignment byte), using
four bytes more than necessary.

This commit is limited to the "xtensa" target, as "xtensawin" doesn't
support inline assembler functions yet, so native functions' constant
pools are almost always guaranteed to hold one or more values.

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

py/asmxtensa.c

@@ -66,10 +66,12 @@ void asm_xtensa_end_pass(asm_xtensa_t *as) {
 }
 
 void asm_xtensa_entry(asm_xtensa_t *as, int num_locals) {
+    if (as->num_const > 0) {
         // jump over the constants
         asm_xtensa_op_j(as, as->num_const * WORD_SIZE + 4 - 4);
         mp_asm_base_get_cur_to_write_bytes(&as->base, 1); // padding/alignment byte
         as->const_table = (uint32_t *)mp_asm_base_get_cur_to_write_bytes(&as->base, as->num_const * 4);
+    }
 
     // adjust the stack-pointer to store a0, a12, a13, a14, a15 and locals, 16-byte aligned
     as->stack_adjust = (((ASM_XTENSA_NUM_REGS_SAVED + num_locals) * WORD_SIZE) + 15) & ~15;
@@ -183,6 +185,8 @@ size_t asm_xtensa_mov_reg_i32(asm_xtensa_t *as, uint reg_dest, uint32_t i32) {
     // store the constant in the table
     if (as->const_table != NULL) {
         as->const_table[as->cur_const] = i32;
+    } else {
+        assert((as->base.pass != MP_ASM_PASS_EMIT) && "Constants table was not built.");
     }
     ++as->cur_const;
     return loc;