Strange C++ compilation error with header file due to makefile setting?

I have the following defined in the header file:

some function ...
            if (table2[i] != ""){
                new_table2[hash2(table2[i])] = table2[i];       
                table2[i] = "";
                }       
            // std::cout << " first i : " << i  << "\n";
        }


        for (int i=0; i < 150; ++i){
            if (new_table1[i] != "" && !try_insert(new_table1[i])){
                std::cout << " bad rehash : " << i << " " << new_table1[i]  << "\n"; 
                rehash();
            }
        }

Makefile looks like this:

CC=g++
CFLAGS=-Wall -Wextra -O3.
DEPS = somehash.h hashfunctions.h 
OBJ = main.o

%.o: %.c $(DEPS)
    $(CC) -c -o $@ $< $(CFLAGS)

main: $(OBJ)
    g++ -o $@ $^ $(CFLAGS)


.PHONY: clean

clean:
    rm -f *.o main 

After I do make clean, make, and ./main, "first i" is still printed even though it is commented out. When I comment out " bad rehash ", then both of them are not printed. It seems the compiler did some auto optimization that causes this to be cached? How can I fix it?