[RFC] Range Integer Types

considered/rfc-range-integer-types.rst

@@ -0,0 +1,106 @@
+- Feature Name: range_integer_types
+- Start Date: 2023-11-10
+- RFC PR:
+- RFC Issue:
+
+Summary
+=======
+
+RM-3.5.4(9) mandates a symmetric base range for signed integer types. This
+requirement often necessitates the use of larger data types for arithmetic
+operations, potentially introducing undesirable run-time overhead and
+performance penalties, particularly in embedded systems.
+
+This proposal introduces a new aspect for type definitions to explicitly
+enforce the use of an unsigned base type, alleviating these issues.
+
+Motivation
+==========
+
+Currently, an unsigned 64-bit integer type can be defined in Ada as:
+
+.. code-block:: ada
+
+  type Unsigned_64 is range 0 .. 2 ** 64 - 1 with Size => 64
+
+However, this type implicitly assumes a symmetric base range that exceeds 64 bits.
+Consequently, arithmetic operations on such a type are executed using 128-bit
+operations. This is misleading, as the Size aspect suggests that operations would
+utilize only 64 bits.
+
+The use of larger types for arithmetic operations adversely affects performance
+and, in some cases, necessitates software-based implementations in run-time
+libraries instead of hardware operations. This issue is especially problematic fo
+embedded applications. For instance, on a 64-bit architecture, a 64-bit
+multiplication can be performed with a single hardware instruction, whereas a
+128-bit multiplication requires multiple instructions and intermediate steps.
+
+By allowing unsigned types with unsigned base ranges, we can ensure that arithmetic
+operations are performed with the expected size, reducing performance overhead.
+
+The following enhancement could be used to explicitly indicate this intention:
+
+.. code-block:: ada
+
+  type Uns_64 is range 0 .. 2 ** 64 - 1 with Size => 64, Unsigned_Base_Range => True
+
+This ensures that arithmetic operations are carried out using 64 bits. Overflow
+checking is based on the processor's carry state flag.
+
+Guide-level explanation
+=======================
+
+Defining unsigned types with their expected ranges and bit widths should naturally
+result in arithmetic operations using the same bit width. This behavior aligns with
+the expectations of developers familiar with languages like C.
+
+Symmetric base ranges are advantageous for avoiding numeric overflows during
+intermediate calculations, when negative values arise from subtraction. However,
+symmetric ranges impose significant performance costs.
+
+Note that numeric overflow checking can be achieved without symmetric base ranges, as
+most hardware uses the processor's carry state flag to signal overflows.
+
+This proposal introduces a new explicit aspect for type declarations, which maintains
+upward compatibility while reducing run-time overhead.
+
+Reference-level explanation
+===========================
+
+For every subtype S of a signed integer type, S'Unsigned_Base_Range is an boolean
+attribute that indicates whether the base range is unsigned. The default falue is
+False.
+
+Rationale and alternatives
+==========================
+
+An alternative to this proposal is defining unsigned types using modular type
+definitions to avoid symmetric base ranges. However, modular types have fundamentally
+different semantics compared to integer types, which may not align with the intended
+use case.
+
+Drawbacks
+=========
+
+Using unsigned base ranges limits the applicability of these types in contexts where
+generic units expect regular signed integer types because potential assumptions on their
+base ranges are no longer valid.
+
+Prior art
+=========
+
+In C, unsigned types have a range that excludes negative values. Arithmetic operations
+on such types use the same number of bits as the defined width of the type. While their
+semantics differ, for example, unsigned types in C wrap around on overflow instead of
+raising an exception, this behavior ensures consistent bit-width usage during
+calculations. This proposal aligns Ada's behavior with such established practices.
+
+Unresolved questions
+====================
+
+None.
+
+Future possibilities
+====================
+
+None.