Tcl Source Code

% set x -2147483647; expr {abs($x)}
2147483647
% set x -2147483648; expr {abs($x)}
integer value too large to represent
% set x -2147483649; expr {abs($x)}
2147483649

This is a practical problem as we use -2147483648 
(0x80000000, MIN_INT) to express missing data.

Seen in 8.4.1 and 8.5a2 Tclkit, Windows XP and Linux.

[email protected]

File Added - 159527: tcl-abs.patch

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I just committed the attached patch to the 8.4 branch.

It assumes that MIN_LONG is defined by one of the headers
that are being included already. If there is any platform
where this isn't true, a compatibility definition needs to
be added.

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Hmm - my patch fixes 32bit platform, but it breaks 64bit :(

How about using MIN_INT from limits.h for checking this
special case, and providing compatibility definitions for
platforms that don't have limits.h?

File Added - 159278: tcl-abs.patch

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This fix depends on implementation defined behaviour of the
compiler and fails with gcc 4.1.0. What happens is, that the
compiler decides that the negation of a negative number can
never have a negative result, and so it optimizes away the
special case for INT_MIN as dead code.

There are two solutions for this: either rewriting the code
so that it doesn't depend on the assumption that -MIN_INT ==
MIN_INT, or use the -fwrapv parameter to gcc, so that it
guarantees the wrapping behaviour of integers.

See also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25329

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Right you are.  Although in 8.4.11, you have to do
expr {int(-2147483648)} to tickle the bug.  Anyway, abs()
is fixed.

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Problem is it that it hits the "FIXME" case in ExprAbsFunc
(in tclBasic.c in the HEAD.)