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spec: split shift examples into groups for 32- and 64-bit ints

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In the current (pre-CL) version of the spec, the 2nd last shift
example appears to be using the array declared in the last example.
On a 32-bit platform, that array would have length 0, which would
lead to a panic in the 2nd last example. Also, if this code were
inside a function, it wouldn't compile (array declared after use).

Use an explicitly declared array for that specific shift example.
Also, split out all cases that produce different results for 32-
vs 64-bit ints.

Fixes #41835.

Change-Id: Ie45114224509e4999197226f91f7f6f934449abb
Reviewed-on: https://go-review.googlesource.com/c/go/+/260398
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2020-10-07 11:33:11 -07:00
parent 256d729c0b
commit 2b9b2720b8
1 changed files with 26 additions and 16 deletions
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42
doc/go_spec.html
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@ -1,6 +1,6 @@
<!--{
"Title": "The Go Programming Language Specification",
"Subtitle": "Version of Sep 24, 2020",
"Subtitle": "Version of Oct 7, 2020",
"Path": "/ref/spec"
}-->
@ -3594,23 +3594,33 @@ replaced by its left operand alone.
</p>
<pre>
var a [1024]byte
var s uint = 33
var i = 1&lt;&lt;s // 1 has type int
var j int32 = 1&lt;&lt;s // 1 has type int32; j == 0
var k = uint64(1&lt;&lt;s) // 1 has type uint64; k == 1&lt;&lt;33
var m int = 1.0&lt;&lt;s // 1.0 has type int; m == 0 if ints are 32bits in size
var n = 1.0&lt;&lt;s == j // 1.0 has type int32; n == true
var o = 1&lt;&lt;s == 2&lt;&lt;s // 1 and 2 have type int; o == true if ints are 32bits in size
var p = 1&lt;&lt;s == 1&lt;&lt;33 // illegal if ints are 32bits in size: 1 has type int, but 1&lt;&lt;33 overflows int
var u = 1.0&lt;&lt;s // illegal: 1.0 has type float64, cannot shift
var u1 = 1.0&lt;&lt;s != 0 // illegal: 1.0 has type float64, cannot shift
var u2 = 1&lt;&lt;s != 1.0 // illegal: 1 has type float64, cannot shift
var v float32 = 1&lt;&lt;s // illegal: 1 has type float32, cannot shift
var w int64 = 1.0&lt;&lt;33 // 1.0&lt;&lt;33 is a constant shift expression
var x = a[1.0&lt;&lt;s] // 1.0 has type int; x == a[0] if ints are 32bits in size
var a = make([]byte, 1.0&lt;&lt;s) // 1.0 has type int; len(a) == 0 if ints are 32bits in size
</pre>
// The results of the following examples are given for 64-bit ints.
var i = 1&lt;&lt;s // 1 has type int
var j int32 = 1&lt;&lt;s // 1 has type int32; j == 0
var k = uint64(1&lt;&lt;s) // 1 has type uint64; k == 1&lt;&lt;33
var m int = 1.0&lt;&lt;s // 1.0 has type int; m == 1&lt;&lt;33
var n = 1.0&lt;&lt;s == j // 1.0 has type int; n == true
var o = 1&lt;&lt;s == 2&lt;&lt;s // 1 and 2 have type int; o == false
var p = 1&lt;&lt;s == 1&lt;&lt;33 // 1 has type int; p == true
var u = 1.0&lt;&lt;s // illegal: 1.0 has type float64, cannot shift
var u1 = 1.0&lt;&lt;s != 0 // illegal: 1.0 has type float64, cannot shift
var u2 = 1&lt;&lt;s != 1.0 // illegal: 1 has type float64, cannot shift
var v float32 = 1&lt;&lt;s // illegal: 1 has type float32, cannot shift
var w int64 = 1.0&lt;&lt;33 // 1.0&lt;&lt;33 is a constant shift expression; w == 1&lt;&lt;33
var x = a[1.0&lt;&lt;s] // panics: 1.0 has type int, but 1&lt;&lt;33 overflows array bounds
var b = make([]byte, 1.0&lt;&lt;s) // 1.0 has type int; len(b) == 1&lt;&lt;33
// The results of the following examples are given for 32-bit ints,
// which means the shifts will overflow.
var mm int = 1.0&lt;&lt;s // 1.0 has type int; mm == 0
var oo = 1&lt;&lt;s == 2&lt;&lt;s // 1 and 2 have type int; oo == true
var pp = 1&lt;&lt;s == 1&lt;&lt;33 // illegal: 1 has type int, but 1&lt;&lt;33 overflows int
var xx = a[1.0&lt;&lt;s] // 1.0 has type int; xx == a[0]
var bb = make([]byte, 1.0&lt;&lt;s) // 1.0 has type int; len(bb) == 0
</pre>
<h4 id="Operator_precedence">Operator precedence</h4>
<p>