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## Integer/Word Conversions Explained This note describes an approach to representing and optimizing the conversions between integers (and words) of different sizes in Standard ML. ### Background Because different SML implementations can have different sizes of integer and word types, the designers of the SML Basis Library had to design a API for conversions that would work for any SML implementation. The approach is based on the idea that one can convert from `IntN.int` to `IntM.int` by first converting from `IntN.int` to `LargeInt.int` and then converting from `LargeInt.int` to `IntM.int`. While this approach works for source code, inside a given compiler we want to have direct conversions. ### Primitive conversion operations All integer/word conversion operations are expressed using five primitive conversion operators. Algebraic equations over these operators are easy to define and can be used to simplify composition of conversion operations. The five basic conversion operators are (in all cases, we assume that (n >= m): TEST(n,m) -- map an n-bit, 2's complement signed value to an m-bit, 2's complement signed value; raise Overflow if the value is too large. TESTU(n,m) -- map an unsigned n-bit value to an m-bit 2's complement value; raise Overflow if the value is too large. EXTEND(m,n) -- sign extend an m-bit value to a n-bit value TRUNC(n,m) -- truncate an n-bit value to an m-bit value. COPY(m,n) -- copy (i.e., zero-extend) an m-bit value to an n-bit value. The operation COPY(n,n) is the identity function on n-bit values. TEST, TESTU, and TRUNC are used to go from large values to small ones, and EXTEND and COPY are used to go from small values to large. The operators EXTEND, TRUNC, and COPY are "pure," while TEST and TESTU may raise Overflow. We use `*` for m or n to denote arbitrary precision integers (IntInf.int). Conversions where the sizes are the same can be simplified to copies: TEST(n,n) == COPY(n,n) EXTEND(n,n) == COPY(n,n) Note: this does not apply to TESTU TRUNC(n,n) == COPY(n,n) ### Examples Assuming that LargeInt is aribitrary precision and the default Int and Word types are 31-bits, then the translation of conversion operations in the Word32 structure is given by: toLargeInt => TESTU(32,*) toLargeIntX => EXTEND(32,*) = COPY(32,32) fromLargeInt => TESTU(*,32) toInt => TESTU(32,31) toIntX => TEST(32,31) fromInt => EXTEND(31,32) toLargeWord => COPY(32,32) toLargeWordX => EXTEND(32,32) = COPY(32,32) fromLargeWord => TRUNC(32,32) = COPY(32,32) And if LargeInt was Int32, then the operations in Word8 would be toLargeInt => COPY(8,32) toLargeIntX => EXTEND(8,32) fromLargeInt => TRUNC(32,8) toInt => COPY(8,31) toIntX => EXTEND(8,31) fromInt => TRUNC(31,8) toLargeWord => COPY(8,32) toLargeWordX => EXTEND(8,32) fromLargeWord => TRUNC(32,8) ### Rewrites These operations allow for simplification via algebraic rewrites. Each operator composed with itself is itself, but with different parameters: TEST(n,m) o TEST(p,n) == TEST(p,m) TESTU(n,m) o TESTU(p,n) == TESTU(p,m) EXTEND(n,m) o EXTEND(p,n) == EXTEND(p,m) TRUNC(n,m) o TRUNC(p,n) == TRUNC(p,m) COPY(n,m) o COPY(p,n) == COPY(p,m) The composition of different operators can be described by a simple algebra. EXTEND(n,m) o COPY(p,n) == COPY(p,m) if (n > p) == EXTEND(p,m) if (n = p) COPY(n,m) o EXTEND(p,n) == EXTEND(p,m) if (n = m) TRUNC(n,m) o COPY(p,n) == COPY(p,m) if (m >= p) == TRUNC(p,m) if (m < p) COPY(n,m) o TRUNC(p,n) == TRUNC(p,m) if (n = m) COPY(n,m) o TEST(p,n) == TEST(p,m) if (n = m) TEST(n,m) o COPY(p,n) == COPY(p,m) if (m >= p) == TEST(p,m) if (m < p) TESTU(n,m) o COPY(p,n) == COPY(p,m) if (m >= p) == TESTU(p,m) if (m < p) COPY(n,m) o TESTU(p,n) == TESTU(p,m) if (n = m) TRUNC(n,m) o EXTEND(p,n) == EXTEND(p,m) if (m >= p) == TRUNC(p,m) if (m < p) TEST(n,m) o EXTEND(p,n) == EXTEND(p,m) if (m >= p) == TEST(p,m) if (m < p) TESTU(n,m) o EXTEND(p,n) == EXTEND(p,m) if (m >= p) == TESTU(p,m) if (m < p) For example, consider: Word.toInt o Word.fromLargeWord o Word8.toLargeWord This translates to: TESTU(31,31) o TRUNC(32,31) o COPY(8,32) and simplifies to: TESTU(31,31) o COPY(8,31) This expression further simplifies to: COPY(8, 31) Since both 8-bit and 31-bit quantities are tagged the same way, this can be translated to a MOVE. With a smart register allocator that MOVE can be eliminated.

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