This happens in a large function, which has a relatively complicated algorithm.

Simply speaking, its structure is as such:
___________________________________________________________
some basic calculation
------------------------------

y = expression;

-------------------------------
____________________________________________________________
some operations
--------------------------------

--------------------------------

if {
block 1;
}

else{
if { block 2; }
else { block 3; }
}

The structure of blocks 2 and 3 will change the way in which y is calculated. The result is wrong. I check it through gdb. The five results above are obtained at the same environment except for the changes in the two blocks

In many executions, y is always equal to 0. Otherwise, it will be far larger than a right value.

But when either or each of the two blocks is removed, y will obtain a normal value.

I can not understand