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spec: clarify implementation restrictions on untyped floats
Drop reference to "machine type." Specify that integer overflow must be an error. Drop requirement that exponent must be 128 bits--that's a lot. Clarify that floating point expressions may be rounded, including intermediate values. This is a reworking of https://golang.org/cl/5577068/ . Fixes #2789. R=r, rsc, r, gri, ken, ken, iant CC=golang-dev, remyoudompheng https://golang.org/cl/5655049
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@ -589,11 +589,33 @@ functions return and test for those values at run time.
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</p>
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<p>
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Implementation restriction: A compiler may implement numeric constants by choosing
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an internal representation with at least twice as many bits as any machine type;
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for floating-point values, both the mantissa and exponent must be twice as large.
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Implementation restriction: Although numeric constants have arbitrary
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precision in the language, a compiler may implement them using an
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internal representation with limited precision. That said, every
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implementation must:
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</p>
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<ul>
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<li>Represent integer constants with at least 256 bits.</li>
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<li>Represent floating-point constants, including the parts of
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a complex constant, with a mantissa of at least 256 bits
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and a signed exponent of at least 32 bits.</li>
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<li>Give an error if unable to represent an integer constant
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precisely.</li>
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<li>Give an error if unable to represent a floating-point or
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complex constant due to overflow.</li>
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<li>Round to the nearest representable constant if unable to
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represent a floating-point or complex constant due to limits
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on precision.</li>
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</ul>
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<p>
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These requirements apply both to literal constants and to the result
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of evaluating <a href="#Constant_expressions">constant
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expressions</a>.
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</p>
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<h2 id="Types">Types</h2>
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@ -3574,6 +3596,16 @@ int8(^1) // same as int8(-2)
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^int8(1) // same as -1 ^ int8(1) = -2
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</pre>
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<p>
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Implementation restriction: A compiler may use rounding while
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computing untyped floating-point or complex constant expressions; see
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the implementation restriction in the section
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on <a href="#Constants">constants</a>. This rounding may cause a
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floating-point constant expression to be invalid in an integer
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context, even if it would be integral when calculated using infinite
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precision.
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</p>
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