X86-64 Architecture Guide

For the code-generation project, we expect your compiler to produce simple assembly code. We shall expose you to a subset of the x86-64 platform.

Example

Consider the following Decaf program:

class Program {

int foo(int x) {

return x + 3;

}

void main() {

int y;

y = foo(callout("get_int_035"));

if (y == 15) {

callout("printf", "Indeed! \'tis 15!\n");

} else {

callout("printf", "What! %d\n", y);

}

}

}

For the code generation phase of the compiler project, you are encouraged to output simple and inefficient (but correct!) assembly code. This assembly code can assign every variable and temporary to a location on the current stack frame. Every expression value will be loaded from the stack, manipulated using the registers %r10 and %r11, and then the result will be written back to the stack, ready to be used in another expression. Compiling the above decaf code using this simple scheme might look like this:

foo:

enter $(8*2), $0

mov %rdi, -8(%rbp)

mov -8(%rbp), %r10

add $3, %r10

mov %r10, -16(%rbp)

mov -16(%rbp), %rax

leave

ret .globl main

main:

enter $(8 * 6), $0

call get_int_035

mov %rax, -8(%rbp)

mov -8(%rbp), %rdi

call foo

mov %rax, -16(%rbp)

mov -16(%rbp), %r10

mov %r10, -24(%rbp)

mov -24(%rbp), %r10

mov $15, %r11

cmp %r10, %r11

mov $0, %r11

mov $1, %r10

cmove %r10, %r11

mov %r11, -32(%rbp)

mov -32(%rbp), %r10

mov $1, %r11

cmp %r10, %r11

je .fifteen

mov $.what, %rdi

mov -24(%rbp), %rsi

mov $0, %rax

call printf

mov %rax, -40(%rbp)

jmp .fifteen_done

.fifteen:

mov $.indeed, %rdi

mov $0, %rax

call printf

mov %rax, -48(%rbp)

.fifteen_done:

mov $0, %rax

leave

ret

.indeed:

.string "Indeed, \'tis 15!\n"

.what:

.string "What! %d\n"

We shall dissect this assembly listing carefully and relate it to the Decaf code. Note that this is not the only possible assembly of the program; it only serves as an illustration of some techniques you can use in this project phase.

foo:

enter $(8 * 2), $0

mov %rdi, -8(%rbp)

...

leave

ret

• This is the standard boilerplate code for a function definition. The first line creates a label which names the entry point of the function. The following enter instruction sets up the stack frame. After the function is done with its actual work, the leave instruction restores the stack frame for the caller, and ret passes control back to the caller.
• Notice that one of the operands to enter is a static arithmetic expression. Such expressions are evaluated by the assembler and converted into constants in the final output.
• Enter first saves the callers frame (base) pointer (%rbp) unto the stack. Then it sets the frame pointer to the stack pointer (%rsp) to set the current frame pointer. Enter next allocates N bytes (where N is the left operand) of stack space to be used for locals and temporaries of the stack frame. It does this by subtracting N from %rsp (remember that the stack grows downward toward address 0). This space is allocated after the caller's frame (base) pointer is saved to the stack.
• The mov instruction moves the 1st argument (passed in %rdi) to its place on the stack. The argument occupies the first stack position (stack entries are 8 bytes) after the base pointer (%rbp). 0(%rbp) stores the previous frame's base pointer.

  mov -8(%rbp), %r10

  add $3, %r10

  mov %r10, -16(%rbp)

• The purpose of foo (int) is to add 3 to its argument, and return the result. The first mov instruction fetches the argument from the stack and places it in the temporary register %r10. The next instruction increments the value in %r10 by the literal or immediate value 3. Note that immediate values are always prefixed by a '$'.
• The second mov instruction stores the value of the addition back onto the stack at the second position of the frame (after the saved %rbp).

  mov -16(%rbp), %rax

• According to the calling convention, a function must place its return value in the %rax register, so foo has succeeded in returning x + 3 by moving the value of the x + 3 expression into %rax.

.globl main

main:

enter $(8 * 6), $0

...

• The .globl main directive makes the symbol main accessible to modules other than this one. This is important, because the C run-time library, which we link against, expects to find a main procedure to call at program startup.
• The enter instruction allocates space for 6 quadwords on the stack: one for a local variable and 5 for temporaries.

call get_int_035

mov %rax, -8(%rbp)

• We call the get_int_035 function, which reads an integer from standard input and returns it. The function takes no arguments.
• The integer is returned in %rax, and we store the value of the method call expression onto the stack.

  mov -8(%rbp), %rdi

  call foo

  mov %rax, -16(%rbp)

  mov -16(%rbp), %r10

  mov %r10, -24(%rbp)

• Now we are ready to call foo. We start by loading the temporary that stored the return value of get_int_035 into %rdi. According to the calling convention defined in the Linux ABI (see below), %rbi is used to pass the first argument. Then we call foo.
• Once foo returns, we store the return value, stored in %rax, onto the stack at location -16(%rbp).
• Next, we perform the assignment to y of the return value of foo by loading the temporary into %r10 and storing %r10 into the stack location designated for y, -24(%rbp).

  mov -24(%rbp), %r10

  mov $15, %r11

  cmp %r10, %r11

  mov $0, %r11

  mov $1, %r10

  cmove %r10, %r11

  mov %r11, -32(%rbp)

• This sequence demonstrates how a comparison operation might be implemented using only two registers and temporary storage. We begin by loading the values to compare, i.e., y and the literal 15, into registers. This is necessary because the comparison instructions only work on register operands.
• Then, we perform the actual comparison using the cmp instruction. The result of the comparison is to change the internal flags register.
• Our aim is to store a boolean value—1 or 0—in a temporary variable as the result of this operation. To set this up, we place the two possible values, 1 and 0, in registers %r10 and %r11.
• Then we use the cmove instruction (read c-mov-e, or conditional move if equal) to decide whether our output value should be 0 or 1, based on the flags set by our previous comparison. The instruction puts the result in %r11.
• Finally, we store the boolean value from %r11 to a temporary variable at -32(%rbp).

  mov -32(%rbp), %r10

  mov $1, %r11

  cmp %r10, %r11

  je .fifteen

  ...

  jmp .fifteen_done

  .fifteen:

  ....fifteen_done:

• This is the standard linearized structure of a conditional statement. We compare a boolean variable to 1, and perform a je (jump if equal) instruction which jumps to its target block if the comparison succeeded. If the comparison failed, je acts as a no-op.
• We mark the end of the target block with a label, and jump to it at the end of the fall-through block. Conventionally, such local labels, which do not define functions, are named starting with a period.

  mov $.what, %rdi

  mov -24(%rbp), %rsi

  mov $0, %rax

  call printf

  mov %rax, -40(%rbp)

• The block of instructions performs the false (else) block of the if statement.
• We first load the value of the .what string (see below) into %rdi. Next we load the value of y into %rsi. %rsi is the register designated for the second argument of a function.
• The third mov instruction is necessary for printf because printf uses a variable list of arguments. We must assign 0 to %rax to let printf know that we are not using SSE registers to pass any of the arguments.
• After the call, the final move instruction stores the return value of printf onto the stack. Note that the return value is never referenced.

  mov $0, %rax

  leave

  ret

• At the end of the procedure, we set %rax to 0 to indicate that the program has terminated successfully (used even though the main method is declared to be of type void).

  .indeed:

  .string "Indeed, \'tis 15!\n"

  .what:

  .string "What! %d\n"

• These labels point to static strings defined in the program. They are used as arguments to callout functions.

Reference

This handout only mentions a small subset of the rich possibilities provided by the x86-64 instruction set and architecture. For a more complete (but still readable) introduction, consult The AMD64 Architecture Programmer's Manual, Volume 1: Application Programming.

Registers

In the assembly syntax accepted by gcc, register names are always prefixed with %. All of these registers are 64 bits wide.

The register file is as follows:

For the code generation phase of the project you will not be performing register allocation. You should use %r10 and %r11 for temporary values that you load from the stack.

Instruction Set

Each mnemonic opcode presented here represents a family of instructions. Within each family, there are variants which take different argument types (registers, immediate values, or memory addresses) and/or argument sizes (byte, word, double-word, or quad-word). The former can be distinguished from the prefixes of the arguments, and the latter by an optional one-letter suffix on the mnemonic.

For example, a mov instruction which sets the value of the 64-bit %rax register to the immediate value 3 can be written as

movq $3, %rax

Immediate operands are always prefixed by $. Un-prefixed operands are treated as memory addresses, and should be avoided since they are confusing.

For instructions which modify one of their operands, the operand which is modified appears second. This differs from the convention used by Microsoft's and Borland's assemblers, which are commonly used on DOS and Windows.

Stack Organization

Global and local variables are stored on the stack, a region of memory that is typically addressed by offsets from the registers %rbp and %rsp. Each procedure call results in the creation of a stack frame where the procedure can store local variables and temporary intermediate values for that invocation. The stack is organized as follows:

Calling Convention

We will use the standard Linux function calling convention. The calling convention is defined in detail in System V Application Binary Interface—AMD64 Architecture Processor Supplement. We will summarize the calling convention as it applies to decaf. (PDF)

The caller uses registers to pass the first 6 arguments to the callee. Given the arguments in left-to-right order, the order of registers used is: %rdi, %rsi, %rdx, %rcx, %r8, and %r9. Any remaining arguments are passed on the stack in reverse order so that they can be popped off the stack in order.

The callee is responsible for preserving the value of registers %rbp %rbx, and %r12-r15, as these registers are owned by the caller. The remaining registers are owned by the callee.

The callee places its return value in %rax and is responsible for cleaning up its local variables as well as for removing the return address from the stack.

The call, enter, leave and ret instructions make it easy to follow this calling convention.

Since we follow the standard linux ABI, we can call C functions and library functions using our callout structure. For the purposes of the project we are only going to call printf and get_int_035. When calling printf, we must set the value of register %rax to 0 before issuing the call instruction. This is because printf uses a variable number of arguments and %rax specifies how many SSE registers are used for the arguments. For our purposes the value will always be 0. Since callouts can only return an single integer value, we have provided a function get_int_035(), which will read a single integer input from the terminal and return its integer value. This function is included in the 6035 static library. We cannot use scanf because it returns the number of items read.