24 May
2007
24 May
'07
9:42 a.m.
fons(a)kokkinizita.net:
1. There is absolutely no ambiguity in order of execution of the statement
above.
- subtract a from x
- multiply by b
- add 0.5
- cast to int
No, but as others has discussed, you have used an optimiser.
Also, what are the values of x, a, b and f, or, what is the numerical
error of the sum? If it is greater than 0.004, you are basically
looking at noise. That is the reason it suggested using doubles.
On Wed, May 23, 2007 at 10:33:12PM +0200, Karl Hammar
wrote:
> On Wed, May 23, 2007 at 10:00:38PM +0200, Fons Adriaensen wrote:
...
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). 2. In both cases the floating point result was the
same. Only the case to int
was different.
Which cases?
Regards,
/Karl
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