Add decimal128_t decoding methods and printer

Author: mborland

extra/decimal_printer.py

@@ -4,6 +4,7 @@
 
 from detail.decode_decimal32 import decode_decimal32
 from detail.decode_decimal64 import decode_decimal64
+from detail.decode_decimal128 import decode_decimal128
 import lldb
 
 def decimal32_summary(valobj, internal_dict):
@@ -34,9 +35,27 @@ def decimal64_summary(valobj, internal_dict):
     except Exception as e:
         return f"<invalid decimal64_t: {e}>"
 
+def decimal128_summary(valobj, internal_dict):
+    """
+    Custom summary for decimal128_t type
+    Displays in scientific notation with cohort preservation
+    """
+
+    try:
+        val = valobj.GetNonSyntheticValue()
+        bits = val.GetChildMemberWithName("bits_").GetValueAsUnsigned()
+        bits_high = bits.GetChildMemberWithName("high").GetValueAsUnsigned()
+        bits_low = bits.GetChildMemberWithName("low").GetValueAsUnsigned()
+        combined_bits = (bits_high << 64) | bits_low
+        return decode_decimal128(combined_bits, bits_high)
+
+    except Exception as e:
+        return f"<invalid decimal64_t: {e}>"
+
 def __lldb_init_module(debugger, internal_dict):
     decimal32_pattern = r"^(const )?(boost::decimal::decimal32_t|(\w+::)*decimal32_t)( &| \*)?$"
     decimal64_pattern = r"^(const )?(boost::decimal::decimal64_t|(\w+::)*decimal64_t)( &| \*)?$"
+    decimal128_pattern = r"^(const )?(boost::decimal::decimal128_t|(\w+::)*decimal128_t)( &| \*)?$"
 
     debugger.HandleCommand(
         f'type summary add -x "{decimal32_pattern}" -e -F decimal_printer.decimal32_summary'
@@ -56,6 +75,15 @@ def __lldb_init_module(debugger, internal_dict):
 
     print("decimal64_t printer loaded successfully")
 
+    debugger.HandleCommand(
+        f'type summary add -x "{decimal128_pattern}" -e -F decimal_printer.decimal128_summary'
+    )
+    debugger.HandleCommand(
+        f'type synthetic add -x "{decimal128_pattern}" -l decimal_printer.DecimalSyntheticProvider'
+    )
+
+    print("decimal128_t printer loaded successfully")
+
 class DecimalSyntheticProvider:
     def __init__(self, valobj, internal_dict):
         self.valobj = valobj

extra/detail/decode_decimal128.py

@@ -0,0 +1,57 @@
+# Copyright 2025 Matt Borland
+# Distributed under the Boost Software License, Version 1.0.
+# https://www.boost.org/LICENSE_1_0.txt
+
+def decode_decimal128(bits, bits_high):
+
+    d128_not_11_significand_mask = (562949953421311 << 64) | 18446744073709551615
+    d128_11_significand_mask = (140737488355327 << 64) | 18446744073709551615
+
+    sign = bits_high & 9223372036854775808 != 0
+    isnan = False
+
+    if bits_high & 8646911284551352320 == 8646911284551352320:
+
+        if bits_high & 9079256848778919936 == 9079256848778919936:
+            result = "-SNAN" if sign else "SNAN"
+            isnan = True
+        elif bits_high & 8935141660703064064 == 8935141660703064064:
+            result = "-SNAN" if sign else "QNAN"
+            isnan = True
+        elif bits_high & 8935141660703064064 == 8646911284551352320:
+            result = "-INF" if sign else "INF"
+        else:
+            raise ValueError("Unknown Finite Value")
+
+        if isnan:
+            payload = bits & d128_not_11_significand_mask
+            if payload > 0:
+                result += '(' + str(payload) + ')'
+
+    else:
+        # See decimal128_t::to_components()
+        if bits_high & 6917529027641081856 == 6917529027641081856:
+            implied_bit = 562949953421312 << 64
+            significand = implied_bit | (bits & d128_11_significand_mask)
+            exp = (bits_high & 2305702271725338624) >> 47
+        else:
+            significand = bits & d128_not_11_significand_mask
+            exp = (bits_high & 9222809086901354496) >> 49
+
+        exp -= 6176 # Bias Value
+
+        if significand == 0:
+            result = "0.0e+0"
+        else:
+            n_digits = len(str(abs(significand)))
+
+            # If there is no fractional component we want to remove the decimal point and zero
+            if n_digits > 1:
+                normalized = significand / (10 ** (n_digits - 1))
+                total_exp = exp + n_digits - 1
+            else:
+                normalized = significand
+                total_exp = exp
+            result = f"{'-' if sign else ''}{normalized:.{n_digits - 1}f}e{total_exp:+}"
+
+    return result