From 9126c6570ce293761a4e5eefd61427902f291263 Mon Sep 17 00:00:00 2001
From: Ian Lance Taylor
Date: Mon, 13 Feb 2012 11:25:56 -0800
Subject: [PATCH] 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
---
doc/go_spec.html | 38 +++++++++++++++++++++++++++++++++++---
1 file changed, 35 insertions(+), 3 deletions(-)
diff --git a/doc/go_spec.html b/doc/go_spec.html
index 27c37c2ce1..7d4862f50d 100644
--- a/doc/go_spec.html
+++ b/doc/go_spec.html
@@ -589,11 +589,33 @@ functions return and test for those values at run time.
-Implementation restriction: A compiler may implement numeric constants by choosing
-an internal representation with at least twice as many bits as any machine type;
-for floating-point values, both the mantissa and exponent must be twice as large.
+Implementation restriction: Although numeric constants have arbitrary
+precision in the language, a compiler may implement them using an
+internal representation with limited precision. That said, every
+implementation must:
+
+ - Represent integer constants with at least 256 bits.
+ - 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.
+
+ - Give an error if unable to represent an integer constant
+ precisely.
+
+ - Give an error if unable to represent a floating-point or
+ complex constant due to overflow.
+
+ - Round to the nearest representable constant if unable to
+ represent a floating-point or complex constant due to limits
+ on precision.
+
+
+These requirements apply both to literal constants and to the result
+of evaluating constant
+expressions.
+
Types
@@ -3574,6 +3596,16 @@ int8(^1) // same as int8(-2)
^int8(1) // same as -1 ^ int8(1) = -2
+
+Implementation restriction: A compiler may use rounding while
+computing untyped floating-point or complex constant expressions; see
+the implementation restriction in the section
+on constants. 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.
+
+