Machine/Assembler Language Control Flow & Compiling Function Calls Noah Mendelsohn Tufts University Web: COMP 40: Machine Structure and Assembly Language Programming – Spring 2014

© 2010 Noah Mendelsohn Goals for today – learn:  Review last week’s intro to machine & assembler lang  More examples  Function calls  Control flow 2

© 2010 Noah Mendelsohn Review & A Few Updates

© 2010 Noah Mendelsohn General Purpose Registers 4 %ah $al %ax%eax %rax 0 63

© 2010 Noah Mendelsohn General Purpose Registers 5 %ah $al %ax%eax %rax 0 63 mov $123,%rax

© 2010 Noah Mendelsohn General Purpose Registers 6 %ah $al %ax%eax %rax 0 63 mov $123,%ax

© 2010 Noah Mendelsohn General Purpose Registers 7 $al %ah %ax%eax %rax 0 63 mov $123,%eax

© 2010 Noah Mendelsohn X86-64 / AMD 64 / IA 64 General Purpose Registers %eax %ecx %edx %ebx %esi %edi %esp %ebp %ah $al %ax %ch $cl %cx %dh $dl %dx %bh $bl %bx %si %di %sp %bp %rax %rcx %rdx %rbx %rsi %rdi %rsp %rbp 63 %r8 %r9 %r10 %r11 %r12 %r13 %r14 %r

© 2010 Noah Mendelsohn Classes of AMD 64 registers  new since last lecture  General purpose registers –16 registers, 64 bits each –Used to compute integer and pointer values –Used for integer call/return values to functions  XMM registers –16 Registers, 128 bits each –Used to compute float/double values, and for parallel integer computation –Used to pass double/float call/return values  X87 Floating Point registers –8 registers, 80 bits each –Used to compute, pass/return long double 9

© 2010 Noah Mendelsohn Machine code (typical)  Simple instructions – each does small unit of work  Stored in memory  Bitpacked into compact binary form  Directly executed by transistor/hardware logic * 10 * We’ll show later that some machines execute user-provided machine code directly, some convert it to an even lower level machine code and execute that.

© 2010 Noah Mendelsohn Here’s the machine code for our function 11 int times16(int i) { return i * 16; } Remember: This is what’s really in memory and what the machine executes! 89 f8 c1 e0 04 c3

© 2010 Noah Mendelsohn Here’s the machine code for our function 12 int times16(int i) { return i * 16; } But what does it mean?? Does it really implement the times16 function? 89 f8 c1 e0 04 c3

© 2010 Noah Mendelsohn Consider a simple function in C 13 int times16(int i) { return i * 16; } 0:89 f8 mov %edi,%eax 2:c1 e0 04 shl $0x4,%eax 5:c3 retq

© 2010 Noah Mendelsohn Consider a simple function in C 14 int times16(int i) { return i * 16; } 0:89 f8 mov %edi,%eax 2:c1 e0 04 shl $0x4,%eax 5:c3 retq Load i into result register %eax

© 2010 Noah Mendelsohn Consider a simple function in C 15 int times16(int i) { return i * 16; } 0:89 f8 mov %edi,%eax 2:c1 e0 04 shl $0x4,%eax 5:c3 retq Shifting left by 4 is quick way to multiply by 16.

© 2010 Noah Mendelsohn Consider a simple function in C 16 int times16(int i) { return i * 16; } 0:89 f8 mov %edi,%eax 2:c1 e0 04 shl $0x4,%eax 5:c3 retq Return to caller, which will look for result in %eax REMEMBER: you can see the assembler code for any C program by running gcc with the –S flag. Do it!!

© 2010 Noah Mendelsohn 17 INTERPRETER Software or hardware that does what the instructions say COMPILER Software that converts a program to another language ASSEMBLER Like a compiler, but the input assembler language is (mostly)1-to-1 with machine instructions

© 2010 Noah Mendelsohn Very simplified view of computer 18 Cache Memory

© 2010 Noah Mendelsohn Very simplified view of computer 19 Cache Memory 89 f8 c1 e0 04 c3

© 2010 Noah Mendelsohn Instructions fetched and decoded 20 Cache Memory 89 f8 c1 e0 04 c3

© 2010 Noah Mendelsohn Instructions fetched and decoded 21 Cache Memory 89 f8 c1 e0 04 c3 ALU Arithmetic and Logic Unit executes instructions like add and shift updating registers.

© 2010 Noah Mendelsohn The MIPS CPU Architecture 22

© 2010 Noah Mendelsohn 23 Remember We will teach some highlights of machine/assembler code here in class but you must take significant time to learn and practice on your own! Suggestions for teaching yourself: Read Bryant and O’Hallaron carefully Remember that 64 bit section was added later Look for online reference material Some linked from HW5 Write examples, compile with –S, figure out resulting.s file!

© 2010 Noah Mendelsohn Moving Data and Calculating Values

© 2010 Noah Mendelsohn Examples 25 %rax // contents of rax is data (%rax) // data pointed to by rax 0x10(%rax) // get *(16 + rax) $0x4089a0(, %rax, 8) // Global array index // of 8-byte things (%ebx, %ecx, 2) // Add base and scaled index 4(%ebx, %ecx, 2) // Add base and scaled // index plus offset 4

© 2010 Noah Mendelsohn Examples 26 %rax // contents of rax is data (%rax) // data pointed to by rax 0x10(%rax) // get *(16 + rax) $0x4089a0(, %rax, 8) // Global array index // of 8-byte things (%ebx, %ecx, 2) // Add base and scaled index 4(%ebx, %ecx, 2) // Add base and scaled // index plus offset 4 movl (%ebx, %ecx, 1), %edx // edx <- *(ebx + (ecx * 1)) leal (%ebx, %ecx, 1), %edx // edx <- (ebx + (ecx * 1))

© 2010 Noah Mendelsohn Examples 27 %rax // contents of rax is data (%rax) // data pointed to by rax 0x10(%rax) // get *(16 + rax) $0x4089a0(, %rax, 8) // Global array index // of 8-byte things (%ebx, %ecx, 2) // Add base and scaled index 4(%ebx, %ecx, 2) // Add base and scaled // index plus offset 4 movl (%ebx, %ecx, 1), %edx // edx <- *(ebx + (ecx * 1)) leal (%ebx, %ecx, 1), %edx // edx <- (ebx + (ecx * 1)) movl Moves the data at the address: similar to: char *ebxp; char edx = ebxp[ecx]

© 2010 Noah Mendelsohn Examples 28 %rax // contents of rax is data (%rax) // data pointed to by rax 0x10(%rax) // get *(16 + rax) $0x4089a0(, %rax, 8) // Global array index // of 8-byte things (%ebx, %ecx, 2) // Add base and scaled index 4(%ebx, %ecx, 2) // Add base and scaled // index plus offset 4 movl (%ebx, %ecx, 1), %edx // edx <- *(ebx + (ecx * 1)) leal (%ebx, %ecx, 1), %edx // edx <- (ebx + (ecx * 1)) leal Moves the address itself: similar to: char *ebxp; char *edxp = &(ebxp[ecx]);

© 2010 Noah Mendelsohn Examples 29 %rax // contents of rax is data (%rax) // data pointed to by rax 0x10(%rax) // get *(16 + rax) $0x4089a0(, %rax, 8) // Global array index // of 8-byte things (%ebx, %ecx, 2) // Add base and scaled index 4(%ebx, %ecx, 2) // Add base and scaled // index plus offset 4 movl (%ebx, %ecx, 1), %edx // edx <- *(ebx + (ecx * 1)) leal (%ebx, %ecx, 1), %edx // edx <- (ebx + (ecx * 1)) movl (%ebx, %ecx, 4), %edx // edx <- *(ebx + (ecx * 4)) leal (%ebx, %ecx, 4), %edx // edx <- (ebx + (ecx * 4)) scale factors support indexing larger types similar to: int *ebxp; int edxp = ebxp[ecx];

© 2010 Noah Mendelsohn Control Flow

© 2010 Noah Mendelsohn Simple jumps 31.L4: …code here… j.L4 // jump back to L4

© 2010 Noah Mendelsohn Conditional jumps 32.L4: movq(%rdi,%rdx), %rcx leaq(%rax,%rcx), %rsi testq%rcx, %rcx cmovg%rsi, %rax addq$8, %rdx cmpq%r8, %rdx jne.L4 // conditional: jump iff %r8 != %rdf This technique is the key to compiling if statements, for loops, while loops, etc. …code here…

© 2010 Noah Mendelsohn Calling Functions

© 2010 Noah Mendelsohn Why have a standard “linkage” for calling functions?  Functions are compiled separately and linked together  We need to standardize enough that function calls will succeed!  Note: optimizing compilers may “cheat” when caller and callee are in the same source file –More on this later 34

© 2010 Noah Mendelsohn An interesting example 35 int fact(int n) { if (n == 0) return 1; else return n * fact(n - 1); } See course notes on “Calls and Returns”“Calls and Returns”

© 2010 Noah Mendelsohn The process memory illusion  Process thinks it's running in a private space  Separated into segments, from address 0  Stack: memory for executing subroutines  Heap: memory for malloc/new  Global static variables  Text segment: where program lives 36 Now we’ll learn how your program uses these!

© 2010 Noah Mendelsohn The process memory illusion  Process thinks it's running in a private space  Separated into segments, from address 0  Stack: memory for executing subroutines  Heap: memory for malloc/new  Global static variables  Text segment: where program lives 37 Stack Text (code) Static initialized Static uninitialized Heap (malloc’d) argv, environ Loaded with your program 0

© 2010 Noah Mendelsohn The process memory illusion  Process thinks it's running in a private space  Separated into segments, from address 0  Stack: memory for executing subroutines  Heap: memory for malloc/new  Global static variables  Text segment: where program lives 38 Stack Text (code) Static initialized Static uninitialized Heap (malloc’d) argv, environ Loaded with your program 0 We’re about to study in depth how function calls use the stack.

© 2010 Noah Mendelsohn Function calls on Linux/AMD 64  Caller “pushes” return address on stack  Where practical, arguments passed in registers  Exceptions: –Structs, etc. –Too many –What can’t be passed in registers is at known offsets from stack pointer!  Return values –In register, typically %rax for integers and pointers –Exception: structures  Each function gets a stack frame –Leaf functions that make no calls may skip setting one up 39

© 2010 Noah Mendelsohn The stack – general case 40 Before call ???? %rsp Arguments Return address After callq %rsp Arguments %rsp If callee needs frame ???? Return address Args to next call? Callee vars sub $0x{framesize},%rsp Arguments framesize

© 2010 Noah Mendelsohn Arguments/return values in registers 41 Arguments and return values passed in registers when types are suitable and when there aren’t too many Return values usually in %rax, %eax, etc. Callee may change these and some other registers! MMX and FP 87 registers used for floating point Read the specifications for full details! Operand Size Argument Number %rdi%rsi%rdx%rcx%r8%r9 32%edi%esi%edx%ecx%r8d%r9d 16%di%si%dx%cx%r8w%r9w 8%dil%sil%dl%cl%r8b%r9b

© 2010 Noah Mendelsohn Factorial Revisited 42 int fact(int n) { if (n == 0) return 1; else return n * fact(n - 1); } fact:.LFB2: pushq %rbx.LCFI0: movq %rdi, %rbx movl $1, %eax testq %rdi, %rdi je.L4 leaq -1(%rdi), %rdi call fact imulq %rbx, %rax.L4: popq %rbx ret

© 2010 Noah Mendelsohn Function calls on Linux/AMD 64 (cont.)  Much of what you’ve seen can be skipped for “leaf” functions in the call tree  Inlining: –If small procedure is in same source file (or included header): just merge the code –Unless function is static (I.e. private to source file, the compiler still needs to create the normal version, in case anyone outside calls it!  Optimizing compilers “cheat” –Don’t build full stack –Leave return address for called function (last thing A calls B; last think B does is call C  B leaves return address on stack and branches to C instead of calling it…when C does normal return, it goes straigt back to A!) –Many other wild optimizations, always done so other functions can’t tell anything unusual is happening! 43

© 2010 Noah Mendelsohn Optimized version 44 int fact(int n) { if (n == 0) return 1; else return n * fact(n - 1); } fact:.LFB2: pushq %rbx.LCFI0: movq %rdi, %rbx movl $1, %eax testq %rdi, %rdi je.L4 leaq -1(%rdi), %rdi call fact imulq %rbx, %rax.L4: popq %rbx ret Lightly optimized = O1 (what we saw before) fact:.LFB2: testq %rdi, %rdi movl $1, %eax je.L6.p2align 4,,7.L5: imulq %rdi, %rax subq $1, %rdi jne.L5.L6: rep ; ret Heavily optimized = O2 What happened to the recursion?!?!? This version doesn’t need to create a stack frame either!

© 2010 Noah Mendelsohn Getting the details on function call “linkages”  Bryant and O’Halloran has excellent introduction –Watch for differences between 32 and 64 bit  The official specification: –System V Application Binary Interface: AMD64 Architecture Processor Supplement –Find it at: –See especially sections 3.1 and

© 2010 Noah Mendelsohn Summary  C code compiled to assembler  Data moved to registers for manipulation  Conditional and jump instructions for control flow  Stack used for function calls  Compilers play all sorts of tricks when compiling code 46