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Procedures

Procedures are very important for writing reusable and maintainable code in assembly and high-level languages. How are they implemented? Application Binary Interfaces Calling Conventions Recursive Calls Examples Reference: PowerPC Embedded ABI. Procedures. General Concepts.

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Procedures

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  1. Procedures are very important for writing reusable and maintainable code in assembly and high-level languages. How are they implemented? • Application Binary Interfaces • Calling Conventions • Recursive Calls • Examples • Reference: PowerPC Embedded ABI Procedures

  2. General Concepts • Caller: The calling procedure • Callee: The procedure called by the caller • … int mult(x, y) • prod = mult (a, b) … • … return (x * y) • Caller and callee must agree on: • How to pass parameters • How to return the return value(s), if any • How to maintain relevant information across calls • PowerPC architecture does not define “agreement”. Instead, common policies are defined by convention.

  3. PowerPC Features • The PowerPC ISA provides the following features to support procedure/function calls: • link register (p. 2-12) • bl: branch and link (p. 4-41) • blr: branch to link register (Table F-4)

  4. A Very Simple Calling Convention • Passing arguments • Use GPRs r3 to r10 in order • Use stack in main memory if more than 8 arguments • Passing return value • Leave result in r3

  5. Example int func(int a, int b) { return (a + b); } int func2(int a, int b) { return func(b , a); } main { … func2(5,6); … }

  6. The Stack • Information for each function invocation (e.g. link register) is saved on the call stack or simply stack. • Each function invocation has its own stack frame (a.k.a. activation record ). high address stack frame func2 func stack frame stack pointer low address

  7. What Goes into Stack Frame? • link register • passing parameters (optional) • return value (optional) • what else? stack pointer

  8. Using the Stack main … … bl func2 … func2 … … bl func … ?? … func … … ?? • Describe the stack and LR contents • right before the first bl • right after the first bl • right after the second bl • after each procedure returns

  9. Application Binary Interface (ABI) • Application Binary Interface (ABI): Defines everything needed for a binary object file to run on a system (CPU and operating system), so that it can • call system library routines • call (and be called by) code generated by other people and other compilers • The ABI specifies: • file format • rules for linker operation • procedure calling conventions • register usage conventions • PowerPC has different but very similar ABIs for MacOS, AIX, embedded systems (EABI), Unix, Windows.

  10. PowerPC Conventions • Stack pointer is r1 • Stack pointer is double-word aligned (8 bytes) • Stack “grows” down, toward address 0 • r1 points to lowest stack address (bottom of current stack frame) • First item in stack frame (offset 0 from r1) is address of previous stack frame (a.k.a. frame pointer ) • Second item (offset 4) is saved link register • Minimum stack frame is 8 bytes • Stack frame optional for leaf functions • Always use update addressing mode to allocate stack frame automatically

  11. Register Usage Convention • Rules about who gets to use/save which registers • Caller-save: r0, r3-r12, LR, CTR, CR0, CR1, CR5-7 • Callee-save: r14-r31, CR2-4 (non-volatile registers whose values must be preserved across calls and must thus be restored by callee before returning to caller) • Dedicated: r1, r2, r13

  12. EABI Stack Frame highest address FPRs (optional, var size) GPRs (optional, var size) CR (optional, 1 word) local variables (optional, var size) function param’s (optional, var size) Padding (make size mult of 8 bytes) frame header Link Register lowest address Back Chain

  13. Entry Code (prologue) FuncX: mflr r0 ; Get Link register stwu r1, -88(r1) ; Save Back chain and move SP stw r0, 92(r1) ; Save Link register stmw r28, 72(r1) ; Save 4 non-volatiles r28-r31

  14. Exit Code (epilogue) lwz r0, 92(r1) ; Get saved Link register mtlr r0 ; Restore Link register lmw r28, 72(r1) ; Restore non-volatiles addi r1, r1, 88 ; Remove frame from stack blr ; Return to calling function

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