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spec: clarify implementation restrictions on untyped floats

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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
This commit is contained in:
Ian Lance Taylor 2012-02-13 11:25:56 -08:00
parent 9a4487458a
commit 9126c6570c
1 changed files with 35 additions and 3 deletions
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38
doc/go_spec.html
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@ -589,11 +589,33 @@ functions return and test for those values at run time.
</p> </p>
<p> <p>
Implementation restriction: A compiler may implement numeric constants by choosing Implementation restriction: Although numeric constants have arbitrary
an internal representation with at least twice as many bits as any machine type; precision in the language, a compiler may implement them using an
for floating-point values, both the mantissa and exponent must be twice as large. internal representation with limited precision. That said, every
implementation must:
</p> </p>
<ul>
<li>Represent integer constants with at least 256 bits.</li>
<li>Represent floating-point constants, including the parts of
a complex constant, with a mantissa of at least 256 bits
and a signed exponent of at least 32 bits.</li>
<li>Give an error if unable to represent an integer constant
precisely.</li>
<li>Give an error if unable to represent a floating-point or
complex constant due to overflow.</li>
<li>Round to the nearest representable constant if unable to
represent a floating-point or complex constant due to limits
on precision.</li>
</ul>
<p>
These requirements apply both to literal constants and to the result
of evaluating <a href="#Constant_expressions">constant
expressions</a>.
</p>
<h2 id="Types">Types</h2> <h2 id="Types">Types</h2>
@ -3574,6 +3596,16 @@ int8(^1) // same as int8(-2)
^int8(1) // same as -1 ^ int8(1) = -2 ^int8(1) // same as -1 ^ int8(1) = -2
</pre> </pre>
<p>
Implementation restriction: A compiler may use rounding while
computing untyped floating-point or complex constant expressions; see
the implementation restriction in the section
on <a href="#Constants">constants</a>. This rounding may cause a
floating-point constant expression to be invalid in an integer
context, even if it would be integral when calculated using infinite
precision.
</p>
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<p> <p>
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