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#include <stdio.h>
long sum(long *start, long count)
{
long sum = 0;
while (count) {
sum += *start;
start++;
count--;
}
return sum;
}
int main() {
long start[] = { 42, 11, 12 };
return printf("%d\n", sum(start, (long)1));
}
/*
long sum(long *start, long count)
start in %rdi, count in %rsi
sum:
movl $0, %eax sum = 0
jmp .12 Goto test
.L3: loop:
addq (%rdi), %rax Add *start
addq $8, %rdi start++
subq $1, %rsi count--
.L2: test:
testq %rsi, %rsi Test sum
jne .L3 If !=O, goto loop
rep; ret Return
*/
/*
long sum(long *start, long count)
start in %rdi, count in %rsi
sum:
irmovq $8,%r8 Constant 8
irmovq $1,%r9 Constant 1
xorq %rax,%rax sum = 0
andq %rsi,%rsi Set cc
jmp test Goto test
loop:
mrmovq (%rdi), %r10 Get *start
addq %r10,%rax Add to sum
addq %r8,%rdi start++
subq %r9,%rsi count--. Set CC
test:
jne loop Stop when 0
ret
*/
/* Using iaddq
First byte
- C, 12
- 0, first variant of iadd
Second byte
- F, 15, absence of first argument
- rB, register number 0-15 to add V to
Other 0-8 bytes V
long sum(long *start, long count)
start in %rdi, count in %rsi
sum:
xorq %rax,%rax sum = 0
andq %rsi,%rsi Set cc (nop), ZF=0 if count != 0 else 1, SF=0 if count > 0, OF=0
jmp test Goto test
loop:
mrmovq (%rdi), %r10 Get *start
addq %r10,%rax Add to sum
iaddq $8,%rdi start++
iaddq $-1,%rsi count--. Set CC
test:
jne loop Stop when 0, (~ZF)
ret
*/
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