Free the x87 FPU Stack (ia32)

Question

At my university we were just introduced to IA32 x87 FPU. But we weren't informed how to clear the FPU-Stack of no longer demanded elements.

Imagine we're performing a simple calculation like (5.6 * 2.4) + (3.9 * 10.3).

.data
        value1: .float 5.6
        value2: .float 2.4
        value3: .float 3.8
        value4: .float 10.3

        output: .string "The result is: %f\n"

.text
.global main

main:
        fld     value1          # Load / Push 5.6 into FPU
        fmul    value2          # Multiply FPU's top (5.6) with 2.4
        fld     value3          # Load / Push 3.8 into FPU
        fmul    value4          # Multiply the top element of the FPU's Stacks with 10.3
        fadd    %st(1)          # Add the value under the top element to the top elements value

.output:
        # Reserve memory for a float (64 Bit)
        subl $8, %esp
        # Pop the FPU's top element to the program's Stack
        fstpl (%esp)
        # Push the string to the stack
        pushl $output
        # Call printf function with the both parameters above
        call printf
        # Free the programs stack from the parameters for printf
        addl $12, %esp

.exit:
        movl $1, %eax
        int $0x80

The problem is: After popping the FPU's top element which holds the calculation's result. How do I free the FPU's stack from the now remaining newly top element which holds the result of (5.6*2.4).

The only way I'm capable to imagine is freeing some more program stack and popping elements from the FPU's stack until all no longer demanded elements are removed.

Is there a way to directly manipulate the top pointer?

Answer 1

To accomplish that you don't have any garbadge on the stack you need to use the FADDP and FMULP and similar instructions.

Answer 2

In case someone like me comes here searching for the best way to clear stack I found this simple solution to be the best:

fstp ST(0) ; just pops top of the stack

Answer 3

emms can also be used to mark every member of the f.p. stack as free. This has the advantage over finit that it doesn't change any flags in the f.p. control or status words (exception masks, etc.)

Answer 4

There are several instructions that can perform operations as the one you're seeking. FDECSTP decrements the stack pointer (without doing anything else), FFREE marks a slot as empty (without touching the stack pointer though). The solution mentioned above with the FADDP or FMULP is often nicer, though.

You should consider downloading the Intel Architecture Manuals. They contain the full instruction set of the Intel CPU family.