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Robert Griesemer a52c0a1992 math/big: make Rat.Denom side-effect free
A Rat is represented via a quotient a/b where a and b are Int values.
To make it possible to use an uninitialized Rat value (with a and b
uninitialized and thus == 0), the implementation treats a 0 denominator
as 1.

Rat.Num and Rat.Denom return pointers to these values a and b. Because
b may be 0, Rat.Denom used to first initialize it to 1 and thus produce
an undesirable side-effect (by changing the Rat's denominator).

This CL changes Denom to return a new (not shared) *Int with value 1
in the rare case where the Rat was not initialized. This eliminates
the side effect and returns the correct denominator value.

While this is changing behavior of the API, the impact should now be
minor because together with (prior) CL https://golang.org/cl/202997,
which initializes Rats ASAP, Denom is unlikely used to access the
denominator of an uninitialized (and thus 0) Rat. Any operation that
will somehow set a Rat value will ensure that the denominator is not 0.

Fixes #33792.
Updates #3521.

Change-Id: I0bf15ac60513cf52162bfb62440817ba36f0c3fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/203059
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2019-10-24 03:34:24 +00:00
.github
api api/go1.13: add debug/elf.Symbol fields added in CL 184099 2019-08-08 18:44:16 +00:00
doc doc/play: match the hello world snippet in x/website 2019-10-23 22:43:37 +00:00
lib/time lib/time: update tz data to 2019b 2019-07-03 23:08:27 +00:00
misc misc: delete benchcmp forwarding script 2019-10-23 21:41:15 +00:00
src math/big: make Rat.Denom side-effect free 2019-10-24 03:34:24 +00:00
test cmd/compile: recognize reflect.{Slice,String}Header for -d=checkptr 2019-10-21 20:51:06 +00:00
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