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.
projects / gnulib.git / blobdiff