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[PATCH] * README: Modernize discussion of signed integers.
From: |
Paul Eggert |
Subject: |
[PATCH] * README: Modernize discussion of signed integers. |
Date: |
Sun, 24 Jul 2011 01:41:45 -0700 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.18) Gecko/20110617 Thunderbird/3.1.11 |
Assuming overflow wraparound is no longer safe.
Mention ones' complement and signed magnitude.
---
ChangeLog | 6 ++++++
README | 36 ++++++++++++------------------------
2 files changed, 18 insertions(+), 24 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index 53a5121..4571ed4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2011-07-24 Paul Eggert <address@hidden>
+
+ * README: Modernize discussion of signed integers.
+ Assuming overflow wraparound is no longer safe.
+ Mention ones' complement and signed magnitude.
+
2011-07-22 Bruno Haible <address@hidden>
select tests, pselect tests: Refactor.
diff --git a/README b/README
index ef58c0d..921cfe7 100644
--- a/README
+++ b/README
@@ -242,30 +242,18 @@ than 'long'. POSIX 1003.1-2001 and the GNU coding
standards both
require 'int' to be at least 32 bits wide, so Gnulib code assumes this
as well. Gnulib code makes the following additional assumptions:
- * With one exception noted below, signed integer arithmetic is two's
- complement, without runtime overflow checking. This is the
- traditional behavior, and is supported by C99 implementations that
- conform to ISO/IEC 10967-1 (LIA-1) and that define signed integer
- types as being modulo.
-
- The exception is signed loop indexes. Here, the behavior is
- undefined if any signed expression derived from the loop index
- overflows. For example, the following code contains two such
- overflows (the "i++" and the "i + 1") and therefore has undefined
- behavior:
-
- int i;
- for (i = INT_MAX - 10; i <= INT_MAX; i++)
- if (i + 1 < 0)
- {
- report_overflow ();
- break;
- }
-
- This exception is a concession to modern optimizing compilers,
- which can turn the above loop into code that executes the loop body
- 11 times, even though wraparound arithmetic would cause the loop to
- iterate forever.
+ * Signed integer arithmetic is two's complement.
+
+ Previously, gnulib code sometimes assumed that signed integer
+ arithmetic wraps around, but modern compiler optimizations
+ sometimes do not guarantee this, and gnulib code with this
+ assumption is now considered to be questionable. For more, please
+ see the file doc/intprops.texi.
+
+ Some gnulib modules contain explicit support for the other signed
+ integer representations allowed by C99 (ones' complement and signed
+ magnitude), but these modules are the exception rather than the rule.
+ All practical gnulib targets use two's complement.
* There are no "holes" in integer values: all the bits of an integer
contribute to its value in the usual way.
--
1.7.4.4
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