| Summary: | Floating point comparison failure | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Ian Macky <ian> |
| Component: | c | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED DUPLICATE | ||
| Severity: | normal | CC: | 166255, 36876, adam, AlekM, anoullez, benoit.sibaud, bernert, birger.b, brad_atcheson, brooks, cognot, csk, CycleTimeChart, debian-gcc, denis.nagorny, deshpand, doko, dsell, dyang, eda-qa, fang, gbburkhardt, gcc-bugs, gcc-erikd, gcczilla1, Graham.Murphy, green, gsinai, gtalbot, have, hjl.tools, ian, ich, ismail, jmurray, konstantin, lani, macracan, mbrudka, mehta, neff.kevin, nicos, nikos42, nouiz, npr1, olcios, P.Schaffnit, paul_blankenbaker, pepalogik, petr.savicky, piaget, pr2345, preciseflight, purnnam1, roebel, rohit.x.tripathi, rozenman, sk2alexa, sliwa, sunjoong, themis_hv, tjf, tterribe, tydeman, u.strempel, vincent-gcc, whaley, wirawan0, xiaoyi_wu, ywei |
| Priority: | P3 | ||
| Version: | 4.5.0 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Known to work: | ||
| Known to fail: | Last reconfirmed: | ||
pr323? As a general rule: never compare floating points for equality, use abs(a-b)<epsilon. As an even more general rule, remember to always specify your target: in this case, for example, I can't reproduce at all the behavior on x86_64 -m64, only with -m32. *** This bug has been marked as a duplicate of 323 *** *** This bug has been marked as a duplicate of 323 *** i?86 is a FLT_EVAL_METHOD 2 target, so for strict C compliance all floating
operations and constants are supposed to be evaluated in the precision
of long double. The assignment of the constant to a double var or explicit
cast to double cause rounding to happen, so your testcase is evaluated as:
int main()
{
double z = 3.14159265358979323846L;
puts((long double) z == 3.14159265358979323846L ? "==" : "!=");
return 0;
}
and of course (double) 3.14159265358979323846L != 3.14159265358979323846L.
You can use -fexcess-precision=fast (the default unless -std=c99/-std=c89
is requested) for the old behavior, or you can add explicit cast,
z == (double) M_PI, or (as usually a good idea) avoid floating point
equality/non-equality comparisons, instead check whether difference is
smaller than some epsilon.
Thanks Jakub. Subject: Re: Floating point comparison failure Thanks everyone. I usually do fuzzy floating-point comparison, except in certain special circumstances. I will switch to using double constants; I'm trying for a code that is maximally portable, so having to worry about what exact compiler switches to use is anathema. And might I add: you people are super-fast! I'm very impressed. |
After upgrading to gcc 4.5.0 from 3.3.4, some of my floating point code fails. Searched for and could not find a matching bug. It boils down to this very simple example: #include <stdio.h> #define MY_PI 3.14159265358979323846 int main() { double z = MY_PI; puts(z == MY_PI ? "==" : "!="); return 0; } If this is compiled "gcc -o bug -Wall bug.c" it works: there is equality. Doesn't matter if optimization is used or not (-g, -O, -O2 all give same results). But if compiled with -ansi or -std=c99, then the equality fails, again regardless of optimization!! The preprocessed code looks as expected: int main() { double z = 3.14159265358979323846; puts(z == 3.14159265358979323846 ? "==" : "!="); return 0; } Cannot see how this is correct behavior since the exact same expression was used to initialize the variable and to test for equality. Do not see anything in my ANSI/ISO C reference that sheds any light. I can work around this by using actual double constants instead of preprocessor expressions ("double my_pi_2 = MY_PI_2" and setting/testing with my_pi_2, etc), but this should work as is!