23 May
2007
23 May
'07
6:33 p.m.
On Wed, May 23, 2007 at 10:00:38PM +0200, Fons Adriaensen wrote:
On Wed, May 23, 2007 at 03:25:26PM -0400, Paul Davis
wrote:
$ apt-get source ardour # current unstable
...
$ find . -type f -print0 | xargs -0 grep SYSLIB
./SConstruct: BoolOption('SYSLIBS', 'USE AT YOUR OWN RISK: CANCELS ALL
SUPPORT FROM ARDOUR AUTHORS: Use existing system versions of various libraries instead of
internal ones', 0),
./SConstruct:if env['SYSLIBS']:
./debian/rules: SYSLIBS=yes \
$
I am very disappointed to hear that the Debian
package is using
SYSLIBS=1.
This gives me two occasions to show my ignorance instead of just
the one I was going to post :-)
1. What does SYSLIBS=1 imply ? 2. (Original reason for this post)
I've been chasing a bug for hours this afternoon, and the conclusion
is that apparently g++, under some conditions, is not consistent
in the way it rounds floats to ints in a expression such as
float a, b, x;
int y;
y = (int)((x - a) * b + 0.5f);
I have this calculation twice, in the same source file. In one
case it's within a for loop, in the second case it's just a single
calculation. Both use exactly the same values for a, b, and x, yet
the result is different (by 1). The value before the cast its something
like 115.50424, and that gets rounded up in one case and down in the
other.
This is with -O3. Same with -O2, -O1, but OK without optimisation, and
also OK if I use -O3 -march=pentium4.
Is this to be expected, and should I use floorf() to avoid it, or is it
something that should not happen ?
It might not be a bug. You are not guaranteed of the same order of
evaluation of arithmetic expressions in C. If you want that you have
to use FORTRAN or assembler (maybe some other language has it also).
You might get around it with parantheses and/or temporary variables to
force the compiler to do the plusses etc. in a fixed order.
Beware that floating point values (in numerical circles) are considered
to be inexact, also, you might have better luck if you are using
doubles.
Regards,
/Karl
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