[Cfp-interest 1640] TS3 missing conversions

[Jim Thomas]

This is an idea for discussion at the CFP meeting this Wednesday.

The current TS3 annex does not support IEC 60559 required conversions from binary format to decimal format where the source or destination ia a non-arithmetic interchange format (see table below). The following change enhances conversions from string to decimal format so that they accept hexadecimal input. With the change, the missing conversions would be done by converting the binary format input exactly to a floating number string in the hexadecimal form, then converting the hexadecimal string to decimal format.

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In X.2.2, append:

[] For implementations that supports both binary and decimal floating types and a (binary or decimal) non-arithmetic interchange format, the strtodN and strtodNx functions shall accept subject sequences that have the form of hexadecimal floating numbers and otherwise meet the requirements of subject sequences (7.22.1.6). Then the decimal results shall be correctly rounded if the subject sequence has at most M significant hexadecimal digits, where M >= ceil((P-1)/4) + 1 is implementation defined, and P is the maximum precision of the supported binary floating types and non-arithmetic formats. If all subject sequences of hexadecimal form are correctly rounded, M may be regarded as infinite. If the subject sequence has more than M significant hexadecimal digits, the implementation may first round to M significant hexadecimal digits according to the applicable rounding direction mode, signaling exceptions as though converting from a wider format, then correctly round the result of the shortened hexadecimal input to the result type.

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IEC 60559 requires conversions between all pairs of the following: 

Binary interchange type

Binary extended type

Binary non-arithmetic format

Decimal interchange type

Decimal extended type

Decimal non-arithmetic format

String

The table below shows the conversions as supported in TS3-as-annex: 

From\to

Binary interchange type

Binary extended type

Binary non-arithmetic format

Decimal interchange type

Decimal extended type

Decimal non-arithmetic format

string

Binary interchange type

Assignment, cast

Assignment, cast

Encode + encoding-to-encoding

Assignment, cast

Assignment, cast

1

Strfrom type

Binary extended type

Assignment, cast

Assignment, cast

Exact strfrom type + strto encoding

Assignment, cast

Assignment, cast

1

Strfrom type

Binary non-arithmetic format

Encoding-to-encoding + decode

Exact strfrom encoding + strto type

Encoding-to-encoding

1

1

1

Strfrom encoding

Decimal interchange type

Assignment, cast

Assignment, cast

Exact strfrom type + strto encoding

Assignment, cast

Assignment, cast

Encode + encoding-to-encoding

Strfrom type

Decimal extended type

Assignment, cast

Assignment, cast

Exact strfrom type + strto encoding

Assignment, cast

Assignment, cast

Exact strfrom type + strto encoding

Strfrom type

Decimal non-arithmetic format

Exact strfrom encoding + strto type

Exact strfrom encoding + strto type

Exact strfrom encoding + strto encoding

Encoding-to-encoding + decode

Exact strfrom encoding + strto type

Encoding-to-encoding

Strfrom encoding

string

Strto type

Strto type

Strto encoding

Strto type

Strto type

Strto encoding

NA

 

1) missing conversions 
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