Chapter 3, Problem 3.58HW

For a function with prototype

long decoda2(long x, long y, long z);

GCC generates the following assembly code:

Parameters x, y, and z are passed in registers %rdi, %rsi, and %rdx. The code stores the return value in register %rax.

  Write C code for decode2 that will have an effect equivalent to the assembly code shown.

Explanation of Solution

Given assembly code:

x in %rdi, y in %rsi and z in %rdx

decode2:

subq %rdx, %rsi

imulq %rsi, %rdi

movq %rsi, %rax

salq $63, %rax

sarq $63, %rax

xorq %rdi, %rax

ret

Load Effective Address:

• The load effective address instruction “leaq” is a variant of “movq” instruction.
• The instruction form reads memory to a register, but memory is not been referenced at all.
• The first operand of instruction is a memory reference; the effective address is been copied to destination.
• The pointers could be generated for later references of memory.
• The common arithmetic operations could be described compactly using this instruction.
• The operand in destination should be a register.

Data movement instructions:

• The different instructions are been grouped as “instruction classes”.
• The instructions in a class performs same operation but with different sizes of operand.
• The “Mov” class denotes data movement instructions that copy data from a source location to a destination.
• The class has 4 instructions that includes:
  • movb:
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 1 byte data size.
  • movw: 
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 2 bytes data size.
  • movl:
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 4 bytes data size.
  • movq:
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 8 bytes data size.

Comparison Instruction:

• The “CMP” instruction sets condition code according to differences of their two operands.
• The working pattern is same as “SUB” instruction but it sets condition code without updating destinations.
• The zero flag is been set if two operands are equal.
• The ordering relations between operands could be determined using other flags.
• The “cmpl” instruction compares values that are double word.

Unary and Binary Operations:

• The details of unary operations includes:
  • The single operand functions as both source as well as destination.
  • It can either be a memory location or a register.
  • The instruction “incq” causes 8 byte element on stack top to be incremented.
  • The instruction “decq” causes 8 byte element on stack top to be decremented.
• The details of binary operations includes:
  • The first operand denotes the source.
  • The second operand works as both source as well as destination.
  • The first operand can either be an immediate value, memory location or register.
  • The second operand can either be a register or a memory location.

Corresponding C code:

// Define method decode

long decode(long x, long y, long z)

{

// Declare variable

long tmp = y - z;

//Return

return (tmp * x)^(tmp << 63 >> 63);

}

Explanation:

• The register “%rdi” has value for “x”, register “%rsi” has value for “y” and register “%rdx” has value for “z”.
• The details of assembly code is shown below:
  • The instruction “subq %rdx, %rsi” performs operation “y - z” and stores result in register “%rsi”.
    • The statement “long tmp = y - z” corresponds to C code.
  •  The instruction “imulq %rsi, %rdi” multiplies result of operation with “x” and stores result in register “%rdi”.
    • The statement “(tmp * x)” corresponds to C code.
  • The instruction “movq %rsi, %rax” moves value in register “%rsi” to register “%rax”.
  • The instruction “salq $63, %rax” performs left shift on value in register “%rax”.
    • The statement “tmp << 63” corresponds to C code.
  • The instruction “sarq $63, %rax” performs right shift on value in register “%rax”.
    • The statement “tmp << 63 >> 63” corresponds to C code.
  • The instruction “xorq %rdi, %rax” performs “XOR” operation on values in registers “%rax” and “%rdi”.
    • The statement “return (tmp * x)^(tmp << 63 >> 63)” corresponds to C statement.