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Implementing non-leaf methods/functions using the program stack

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• Implementing a non-lead method/function using the program stack
  • What we have discovered in the previous discussion are:

    A non-leaf method/function must save its return address (in the lr register (because otherwise the return address will be overwritten) The most suitable data structure used to save return addresses is a stack (because the call/return ordering of methods is FILO (first in last out))

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    Once you understand this, it's very easy to solve the problem so you don't lose the return address.

    We start with the structure of a leaf method that was discussed prviously:

    leafMethod: .... .... (instructions in the method) .... /* ------------------------------------------- End of method, we need to return to caller ------------------------------------------- */ mov pc, lr

    We must save the return address by pushing the lr register on the stack.

    And: when the method returns, we must restore the return address using the value in the stack.

    So the structure of a non-leaf method is:

    nonLeafMethod: push {lr} // Save return address on stack .... .... (instructions in the method) .... /* ------------------------------------------- End of method, we need to return to caller ------------------------------------------- */ pop {lr} // Restore return address mov pc, lr // Return to caller

    Instead of popping the top of the stack into the lr register and then moving the lr register into the pc register, we can simply pop the return address directly into the pc register !!!

    So a better solution is:

    nonLeafMethod: push {lr} // Save return address on stack .... .... (instructions in the method) .... /* ------------------------------------------- End of method, we need to return to caller ------------------------------------------- */ pop {pc} // Return to caller !!!

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• The non-leaf method example revisited
  • Example:

    public static void main(String[] args) { A( ); } public static void A( ) { ... B( ); // Calls B( ) ... } public static void B( ) { ... // Does not call any method/function ... }

    Solution:

    main: mov r0, #1111 mov r1, #1111 bl A mov r2, #2222 mov r3, #2222 bl A mov r4, #3333 mov r5, #3333 Stop: A: push {lr} // Save return address mov r0, #7777 mov r1, #7777 bl B // Overwrites lr, it's OK, we saved it !!! mov r2, #8888 mov r3, #8888 pop {pc} // A returns to main !!! B: mov r0, #9999 mov r1, #9999 mov pc, lr // B can return to A

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/8-sub/bl+pop.s

    How to run the program:

    To compile:   as255 bl+pop          To run: use EGTAPI