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 an internal 80-bit floating-point data type on Linux* and OS X* 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.