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Changes to TS 18661 Part 3 Interchange and extended types

Changes to TS 18661 Part 3 Interchange and extended types. WG 14 N1796 2014-04-07. Part 3 draft N1796. TS 18661 Part 3 is C support for new IEC 60559 formats N1796 updates N1758 discussed in C hicago Goal: show changes, consider for WG 14 review in preparation for PDTS ballot.

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Changes to TS 18661 Part 3 Interchange and extended types

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  1. Changes to TS 18661 Part 3Interchange and extended types WG 14 N1796 2014-04-07

  2. Part 3 draft N1796 • TS 18661 Part 3 is C support for new IEC 60559 formats • N1796 updates N1758 discussed in Chicago • Goal: show changes, consider for WG 14 review in preparation for PDTS ballot

  3. Changes (1) • Page 7: added explanation of type structure changes • Page 10: defined binary floating types for types _FloatN and _FloatNx • Page 10: included types _DecimalNx in decimal floating types • Pages 14-15: extended definitions of FLT_EVAL_METHOD and DEC_EVAL_METHOD to cover new types • Pages 17-18: extended specification of conversions between integer and floating to cover new types

  4. Changes (2) • Pages 19-21: extended specification of operation constraints to cover new types • Pages 22-23: added definitions for _t types corresponding to new interchange types • Pages 26-27: enhanced specification of rounding mode controls to cover new types • Pages 36-37: enhanced specification of frexp, ldexp, logb, scalbn, and scalbln to cover new types • Corrections and clarifications

  5. Type structure additions interchange floating types: _FloatN, _DecimalN extended floating types: _FloatNx, _DecimalNx real floating types standard floating types: float, double, long double binary floating types: _FloatN, _FloatNx decimal floating types: _DecimalN, _DecimalNx complex types float _Complex, double _Complex, long double _Complex _FloatN _Complex,_FloatNx _Complex Imaginary types float _Imaginary, double _Imaginary, long double _Imaginary _FloatN _Imaginary, _FloatNx_Imaginary

  6. Type structure unchanged floating types real floating types complex types imaginary types real types integer types real floating types arithmetic types integer types floating types

  7. Background slides …

  8. TS organization • Conformance to Part 3 requires conformance to Part 1 or Part 2 (or both) • Specification is cumulative: C11 (+ TC 1) + Part 1 + Part 2 + Part 3 • Changes in Part 3 are applied to C11 + Part 1 + Part 2 • Part 3 decimal specification generalizes Part 2, so decimal floating types include all _DecimalN and _DecimalNx types • Identifiers controlled by WANT macros listed in header clauses

  9. Interchange formats • IEC 60559-2011 introduced a "tower" of interchange formats • Arbitrarily large widths (32x) • Precision and range determined by width • binary16, for GPU data etc. • For exchange of FP data • May or may not be arithmetic

  10. Extended formats • IEC 60559-2011 specifies extended formats that extend its basic formats (binary32|64|128 and decimal64|128) • Have at least a specified precision and range • For explicit wide evaluation • Not for data exchange

  11. IEC 60559 format support • IEC 60559 formats: Interchange formats • Arithmetic • Non-arithmetic Extended formats Extendable formats • Arithmetic interchange and extended formats fully supported as floating types • Non-arithmetic interchange formats supported without additional types • Extendable formats not covered

  12. Non-arithmetic interchange formats • Supported as encodings, not types • Encodings stored in unsigned char arrays • Required conversion operations provided by library functions • Arithmetic interchange formats are supported as encodings and as types

  13. Requirements • Types are distinct and not compatible • Requires interchange and extended floating types whose formats must already be supported because of conformance to Part 1 or 2 • Requires support for binary16 format, at least as an encoding (if Part 1 is supported) • Allows support for other interchange floating types and encodings • Requires complex (and imaginary) types for supported binary interchange and extended floating types

  14. Example 1 Assume • Part 1 conformance • long double has common IEEE 80-bit extended format And complex (and imaginary) types for all of above Required binary encoding widths: 16, 32, 64

  15. Example 2 Assume • Part 1 conformance • long double has IEEE binary128 format And complex (and imaginary) types for all of above Required binary encoding widths: 16, 32, 64, 128

  16. Example 3 Assume • Part 2 conformance Required decimal encoding widths: 32, 64, 128

  17. Encoding functions For all supported interchange floating types … • Encode – type-to-encoding (same format) • Decode – encoding-to-type (same format) For all supported IEC 60559 encodings … • Encoding-to-encoding conversions • String-to-encoding conversions • String-from-encoding conversions • Each decimal type and encoding requires two sets of encoding functions, one for each decimal encoding scheme

  18. Example 4 Assume • Part 1 conformance • long double has common IEEE 80-bit extended format • binary16 supported only as an encoding To convert binary16 encoding stored in unsigned char e16[2]; to _Float32 f32; use unsigned char e32[4]; f32encf16(e32, e16); decodef32(&f32, e32);

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