# Checking the Floating-point Stack State

On systems based on the IA-32 architecture, when an application calls a function that returns a floating-point value, the returned floating-point value is supposed to be on the top of the floating-point stack. If the return value is not used, the compiler must pop the value off of the floating-point stack in order to keep the floating-point stack in the correct state.

On systems based on Intel® 64 architecture, floating-point values are usually returned in the `xmm0` register. The floating-point stack is used only when the return value is a long double on Linux* and macOS* systems.

If the application calls a function without defining or incorrectly defining the function's prototype, the compiler cannot determine if the function must return a floating-point value. Consequently, the return value is not popped off the floating-point stack if it is not used. This can cause the floating-point stack to overflow.

The overflow of the stack results in two undesirable situations:

- A NaN value gets involved in the floating-point calculations

- The program results become unpredictable; the point where the program starts making errors can be arbitrarily far away from the point of the actual error.

For systems based on the IA-32 and Intel® 64 architectures, the [Q]fp-stack-check option checks whether a program makes a correct call to a function that should return a floating-point value. If an incorrect call is detected, the option places a code that marks the incorrect call in the program. The [Q]fp-stack-check option marks the incorrect call and makes it easy to find the error.

### Note

The [Q]fp-stack-check option causes significant code generation after every function/subroutine call to ensure that the floating-point stack is maintained in the correct state. Therefore, using this option slows down the program being compiled. Use the option only as a debugging aid to find floating point stack underflow/overflow problems, which can be otherwise hard to find.

Parent topic: Understanding Floating-Point Operations

## See Also

fp-stack-check, Qfp-stack-check compiler option