Another Nios GCC Bug with -mflat

On reflection, I don't think this is a bug after all (or maybe just a a documentation bug). The description of the Nios stack frame in the "ABI Summary" section of the manual "User's Guide for Altera Nios" v1.0 is not as clear as it might be, and I think I misunderstood it. This became clear when I tried to figure out how things worked when there were more than 6 arguments to be passed to a function.

Stack space for the arguments to a function (including arguments passed in registers %o0 - %o5) is indeed allocated in the frame of the calling function. However, these arguments aren't pushed on to the stack - the space must be allocated by setting the %sp register to point 23 32-bit words below the top of the stack before making any calls (further if the calling a function with more than 6 arguments). Indeed, the startup code from Altera does this.

Thus, although the %fp and %sp registers mark the boundaries of the stack space a routine is responsible for allocating, they do not mark the boundaries of where a routine is allowed to write to. In addition to the stack space a routine has allocated, a routine is free to write to the stack space allocated by its caller for its argument save area. When setting up a new stack, this argument save area has to be provided before calling any routines.