CS A331 Programming Language Concepts

Transcription

1 CS A331 Programming Language Concepts Lecture 9 Subroutines March 17, 2014 Sam Siewert

2 Purpose of a Subroutine Control Abstraction perform a well defined operation E.g. transform a color image into a graymap Compute the root of a function by Regula Falsi method Sort a list records by key numbers Compared to Data Abstraction represent a collection of information E.g. Structure or record in a list, graph or tree abstract data type Array of intensity data for a color image Look-up table Use of Stack for local variables (function scope) and passing of parameters mapping actual to formal arguments Sam Siewert 2

3 Subroutine Stack Fundamentals Recall Program Use of Memory Code segment, constants generally read-only Data segment, Global memory global variables, arrays, structures File or module scope Application global (frowned upon) Lives in Data Segment Heap memory allocation during run time E.g. C malloc or API to Heap Manager E.g. C++ new object instantiation Stack segment local variables in a subroutine, parameters Recall scoping rules for local/global variables Pass by value and pass by reference Sam Siewert 3

4 Subroutine Stack Visibility Subroutines Can Be Module/file global Or, Nested and Local to Another Subroutine Sam Siewert 4

5 Simple C Local Function Even C Has Simple Locally Visible Functions Modules and Object Orientation Provide More Sophisticated Scoping and Function/Method Visibility Link Error if We Call localfunct() in main() #include <stdio.h> #include <string.h> void globalfunct(void) { } void localfunct(void) { printf("local function\n"); } printf("global function\n"); localfunct(); void main(void) { globalfunct(); // not visible here, so can't call, // but global can // localfunct(); Sam Siewert 5 }

6 Recall ASM and Machine Frame/Stack Stack address (sp, r13) Program Counter (pc, r15) Frame address (fp) Link Register (lr, r14) Support ARM ABI gcda: Copy Stack Ptr Push return info Argument #1 Argument #2.text.align 2.global gcda.type gcda, %function mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4 sub sp, sp, #8 str r0, [fp, #-16] str r1, [fp, #-20] Intra- Procedure Call (ip, r12) Pop return info Return from function END2: sub sp, fp, #12 ldmfd sp, {fp, sp, lr} bx lr Sam Siewert 6

7 ARM Prolog and Epilog EABI r15 is the program counter. r14 is the link register. (The BL instruction, used in a subroutine call, stores the return address in this register). r13 is the stack pointer. r12 is the Intra-Procedure-call scratch register. r4 to r11: used to hold local variables. r0 to r3: used to hold argument values passed to a subroutine, and also hold results returned from a subroutine. If the type of value returned is too large to fit in r0 to r3,, then the caller must allocate space for that value at run time, and pass a pointer in r0. The ARM calling convention mandates using a full-descending stack. In the prolog, push r4 to r11 to the stack to save if used, and push the return address in r14, to the stack. (This can be done with a single STM instruction). copy any passed arguments (in r0 to r3) to the local scratch registers (r4 to r11) or use allocate other local variables to the remaining local scratch registers (r4 to r11) if needed Compute and call other sub-routines using BL instruction (note r0 r3 and r12, r14 are not preserved) Put result in r0 In the epilog, pull r4 to r11 from the stack to restore if needed, and pull the return address to the program counter r15. (This can be done with a single LDM instruction). Sam Siewert 7

8 Underlying ISA Supports Stack Parameters to subroutines (pass by value or reference) Local variables Temporary variables Scoping Operating System, PL Runtime, and API Libraries Using Globally Reserved Memory Support Heap Runtime Allocation Must Free at Runtime or Count on Garbage Collection Sam Siewert 8

9 PLP Stack Layout Contents of a stack frame bookkeeping return PC (dynamic link) saved registers line number --- FOR DEBUGGING ( g ) saved display entries [avoid chaining for non-local variables in nested subroutines] Page 383, bottom Page 387, Maintaining the Static Chain Page 389 In More Depth static link arguments and returns local variables Temporaries Static Chain Page 385, Figure 8.1, Examples 8.3 and 8.4 Sam Siewert 9

10 C Does Not Support Subroutine Nesting PLP, Page 389, In More Depth, Last sentence p385.c We can define functions inside functions, but what are the visibility and scoping rules? GCC Does Support Nested Functions for C (not C++) - Visual Studio Does not try same code in VS2010 1> nesting-simple.cpp 1>c:\users\ssiewert\documents\visual studio 2010\projects\nesting-simple\nestingsimple\nesting-simple.cpp(16): error C2601: 'localfunct_e_in_a' : local function definitions are illegal 2010\projects\nesting-simple\nesting-simple\nesting-simple.cpp(43): error C3861: 'localfunct_e_in_a': identifier not found 1> 1>Build FAILED. Due to nesting, the scope of the nested function is inside the enclosing function. This means that it can access local variables and other local functions in the enclosing function, while all of this is invisible outside the enclosing function. Sam Siewert 10

11 E.g. MIPS, ARM ABI Calling Conventions Maintenance of stack is responsibility of calling sequence and subroutine prolog and epilog space is saved by putting as much in the prolog and epilog as possible time may be saved by putting stuff in the caller instead, where more information may be known Common strategy is to divide registers into caller-saves and callee-saves sets caller uses the "callee-saves" registers first "caller-saves" registers if necessary Sam Siewert 11

12 MIPS Calling Convention (reminder) Caller saves into the temporaries and locals area any callersaves registers whose values will be needed after the call puts up to 4 small arguments into registers $4-$7 (a0-a3) it depends on the types of the parameters and the order in which they appear in the argument list puts the rest of the arguments into the arg build area at the top of the stack frame does jal, which puts return address into register ra and branches note that jal, like all branches, has a delay slot Sam Siewert 12

13 MIPS Convention From PLP In prolog, Callee subtracts framesize from sp saves callee-saves registers used anywhere inside callee copies sp to fp In epilog, Callee puts return value into registers (mem if large) copies fp into sp (see below for rationale) restores saved registers using sp as base adds to sp to deallocate frame does jra After call, Caller moves return value from register to wherever it's needed (if appropriate) restores caller-saves registers lazily over time, as their values are needed All arguments have space in the stack, whether passed in registers or not Sam Siewert 13

14 MIPS Convention - PLP Many parts of the calling sequence, prologue, and/or epilogue can be omitted in common cases particularly LEAF routines (those that don't call other routines) leaving things out saves time simple leaf routines don't use the stack - don't even use memory and are exceptionally fast This is Also True for ARM E.g. ode/subroutines/ Sam Siewert 14

15 Exploration Questions Why Does ANSI C not support Nested Functions? Why does GCC support anyway? Why does GCC also not support in C++ too? Does Java Support Nested Functions? C#? Python? Ada? Others? Why are Nested Functions Useful? Sam Siewert 15

16 Discussion - Need for New PLs? Reflective programming (Java library) Observing and Modifying Programs While Running RTI (Real-Time Innovations Stethoscope - ) Kernel Shark for Linux System Code Systemtap Approximate Computing (No Specific Language) Reconfigurable Computing (Verilog and VHDL) Co-Processors (CUDA extension to C, OpenCL) Thomas Standish on Extensible Programming Languages Paraphrase Extension Orthophrase Extension Metaphrase Extension Libraries (not one of the 3, but commonly used) Sam Siewert 16

17 Parameter Passing Part B Mapping of Actual to Formal Arguments By Value C Default By Value/Result (Copying), Scripting Languages References (Aliases) Using Pointers or Access/Ref Operators in Strongly Typed PLs Closure/name Sam Siewert 17