Correct decoding of esp32s2 negative LD/ST offsets

The ESP32-S2/S3 support a negative offset in ST/LD instructions.
Those offsets are two's-complement encoded into a field that is
11-bits wide.

This change corrects the decoding of negative offsets given the
field width of just 11-bits, rather than relying on the 32 or
64 bits of a MicroPython `int`.

Note 1: Negative offsets used in JUMP instructions are encoded
differently (sign bit + positive value), and their decoding is
already done correctly.

Note 2: The LD/ST instructions in of the ESP32 do not support
negative offsets (according to Espressif tests), so their
decoding remains as is.

tests/fixtures/all_opcodes-v.esp32s2.lst

@@ -1,8 +1,8 @@
 header
 ULP magic    : b'ulp\x00' (0x00706c75)
 .text offset : 12 (0x0c)
-.text size   : 244 (0xf4)
-.data offset : 256 (0x100)
+.text size   : 260 (0x104)
+.data offset : 272 (0x110)
 .data size   : 8 (0x08)
 .bss size    : 0 (0x00)
 ----------------------------------------
@@ -527,7 +527,45 @@ ULP magic    : b'ulp\x00' (0x00706c75)
                  label      =   0
                  upper      =   0
                  wr_way     =   0
+00f4  09e01fd0  LD r1, r2, -8
+                 dreg       =   1
+                 offset     =  -8 (0x7f8)
+                 opcode     =  13 (0x0d)
+                 sreg       =   2
+                 unused1    =   0
+                 unused2    =   0
+                 rd_upper   =   0
+00f8  09e01fd8  LDH r1, r2, -8
+                 dreg       =   1
+                 offset     =  -8 (0x7f8)
+                 opcode     =  13 (0x0d)
+                 sreg       =   2
+                 unused1    =   0
+                 unused2    =   0
+                 rd_upper   =   1
+00fc  89e11f68  ST r1, r2, -8
+                 dreg       =   2
+                 offset     =  -8 (0x7f8)
+                 opcode     =   6
+                 sreg       =   1
+                 sub_opcode =   4
+                 unused1    =   0
+                 unused2    =   0
+                 label      =   0
+                 upper      =   0
+                 wr_way     =   3
+0100  c9e11f68  STH r1, r2, -8
+                 dreg       =   2
+                 offset     =  -8 (0x7f8)
+                 opcode     =   6
+                 sreg       =   1
+                 sub_opcode =   4
+                 unused1    =   0
+                 unused2    =   0
+                 label      =   0
+                 upper      =   1
+                 wr_way     =   3
 ----------------------------------------
 .data
-00f4  00000000  <empty>
-00f8  fecadec0  <non-empty>
+0104  00000000  <empty>
+0108  fecadec0  <non-empty>

tests/fixtures/all_opcodes.esp32s2.S

@@ -89,3 +89,9 @@ STI32 R1, R2, 0
 STI32 R1, R2, 1
 
 STO   0x20
+
+LDL R1, R2, -0x20
+LDH R1, R2, -0x20
+
+STL R1, R2, -0x20
+STH R1, R2, -0x20

tests/fixtures/all_opcodes.esp32s2.lst

@@ -60,6 +60,10 @@
 00e8  09000062  STI32 r1, r2, 0
 00ec  19000062  STI32 r1, r2, 1
 00f0  00200064  STO 8
+00f4  09e01fd0  LD r1, r2, -8
+00f8  09e01fd8  LDH r1, r2, -8
+00fc  89e11f68  ST r1, r2, -8
+0100  c9e11f68  STH r1, r2, -8
 .data
-00f4  00000000  <empty>
-00f8  fecadec0  <non-empty>
+0104  00000000  <empty>
+0108  fecadec0  <non-empty>

tools/decode_s2.py

@@ -66,7 +66,7 @@
     opcodes.OPCODE_LD: (
         'LD/LDH',
         opcodes._ld,
-        lambda op: '%s r%s, r%s, %s' % ('LDH' if op.rd_upper else 'LD', op.dreg, op.sreg, op.offset)
+        lambda op: '%s r%s, r%s, %s' % ('LDH' if op.rd_upper else 'LD', op.dreg, op.sreg, twos_comp(op.offset, 11))
     ),
     opcodes.OPCODE_ST: ('ST', opcodes._st, {
         opcodes.SUB_OPCODE_ST_AUTO: (
@@ -79,14 +79,14 @@
         opcodes.SUB_OPCODE_ST_OFFSET: (
             'STO',
             opcodes._st,
-            lambda op: 'STO %s' % op.offset
+            lambda op: 'STO %s' % twos_comp(op.offset, 11)
         ),
         opcodes.SUB_OPCODE_ST: (
             'ST/STH/ST32',
             opcodes._st,
-            lambda op: '%s r%s, r%s, %s, %s' % ('STH' if op.upper else 'STL', op.sreg, op.dreg, op.offset, op.label) if op.wr_way and op.label
-                else '%s r%s, r%s, %s' % ('STH' if op.upper else 'ST', op.sreg, op.dreg, op.offset) if op.wr_way
-                else 'ST32 r%s, r%s, %s, %s' % (op.sreg, op.dreg, op.offset, op.label)
+            lambda op: '%s r%s, r%s, %s, %s' % ('STH' if op.upper else 'STL', op.sreg, op.dreg, twos_comp(op.offset, 11), op.label) if op.wr_way and op.label
+                else '%s r%s, r%s, %s' % ('STH' if op.upper else 'ST', op.sreg, op.dreg, twos_comp(op.offset, 11)) if op.wr_way
+                else 'ST32 r%s, r%s, %s, %s' % (op.sreg, op.dreg, twos_comp(op.offset, 11), op.label)
         )
     }),
     opcodes.OPCODE_RD_REG: (
@@ -103,6 +103,16 @@
 }
 
 
+def twos_comp(val, bits):
+    """
+    compute the correct value of a 2's complement
+    based on the number of bits in the source
+    """
+    if (val & (1 << (bits - 1))) != 0:  # if sign bit is set e.g., 8bit: 128-255
+        val = val - (1 << bits)         # compute negative value
+    return val
+
+
 def decode_instruction(i):
     if i == 0:
         raise Exception('<empty>')
@@ -167,6 +177,9 @@ def get_instruction_fields(ins):
                 extra = ' (%s)' % bs_cmp_ops[val]
             else:
                 extra = ' (%s)' % cmp_ops[val]
+        elif field == 'offset':
+            if ins.opcode in (opcodes.OPCODE_ST, opcodes.OPCODE_LD):
+                val = twos_comp(val, 11)
 
         field_details.append((field, val, extra))