Chapter 4.1, Problem 4.4PP

Explanation of Solution

Given C code:

long rsum(long *start, long count)

{

if(count <= 0)

return 0;

return *start + rsum(start+1, count-1);

}

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.

Corresponding x86-64 code:

long rsum(long *start, long count)

start in %rdi, count in %rsi

rsum:

movl $0, %eax     

testq %rsi, %rsi

jle .L9

pushq %rbx

movq (%rdi),%rbx

subq $1, %rsi

addq $8, %rdi

call rsum

addq %rbx, %rax

popq %rbx

.L9:

rep; ret

Explanation:

• The instruction “movl $0, %eax” initializes value in register “%eax” to 0.
• The instruction “testq %rsi, %rsi” checks for count to be zero.
• The instruction “jle .L9” jumps to label “.L9” if first value is less than or equal to second.
• The instruction “pushq %rbx” saves callee-saved register.
• The instruction “movq (%rdi),%rbx” moves value at location of register “%rdi” to register “%rbx”.
• The instruction “subq $1, %rsi” decrements value of “count” stored in register “%rsi”...