Hi,
With tcc 0.9.25 on x86_64, x == x is regarded as true, even for NaN.
I've looked at the generated code with "tcc -c" and "objdump -d" for:
int main(void)
{
volatile double x = 0.0/0.0;
return x == x ? 1 : 0;
}
It appears:
[...]
37: 66 0f 2e 45 f8 ucomisd -0x8(%rbp),%xmm0
3c: 0f 85 05 00 00 00 jne 47 <main+0x47>
42: e9 0a 00 00 00 jmpq 51 <main+0x51>
47: b8 00 00 00 00 mov $0x0,%eax
4c: e9 05 00 00 00 jmpq 56 <main+0x56>
51: b8 01 00 00 00 mov $0x1,%eax
56: e9 00 00 00 00 jmpq 5b <main+0x5b>
5b: c9 leaveq
5c: c3 retq
According to
http://www.sesp.cse.clrc.ac.uk/html/SoftwareTools/vtune/users_guide/mergedProjects/analyzer_ec/mergedProjects/reference_olh/mergedProjects/instructions/instruct32_hh/vc314.htm
(found with Google), one has:
UNORDERED: ZF,PF,CF <- 111;
GREATER_THAN: ZF,PF,CF <- 000;
LESS_THAN: ZF,PF,CF <- 001;
EQUAL: ZF,PF,CF <- 100;
If I understand correctly (I've never worked with x86/x86_64 asm
code), the jne tests the ZF flag, but this flag is set also for
unordered (the case of NaN). Hence the incorrect result.
--
Vincent Lefèvre <[email protected]> - Web: <http://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)
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