Is it a Bug in g++ compiler`s -O flag??

The output is different for the following two codes when compiled as : "g++ -O foo.cpp"
1st COde :

Code:

#include<iostream.h>
#include<math.h>
int main(){
	double d=log(8)/log(2);
	cout<<"d="<<d<<"\t floor(d)="<<floor(d);        //<<"\t"<<floor(10.0)<<"\n";
	return 0;
}

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2nd COde :

Code:

#include<iostream.h>
#include<math.h>
int main(){
	double d=log(8)/log(2);
	cout<<"d="<<d<<"\t floor(d)="<<floor(d)<<"\t"<<floor(10.0)<<"\n";
	return 0;
}

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Expected output : d=3 floor(d)=3 10
But The First code give : d=3 floor(d)=2 10
while the second gives the right output.
Could nyone plz explain whats going wrong!!

Nikhil