Page 117 - ARM 64 Bit Assembly Language

Data processing and other instructions 103

Name Effect Description
csetm if < cond > then Fill Rn with ones or zeroes.
Rd ← 0xffffffffffffffff
else
Rd ← 0x0000000000000000
end if
ccmp if < cond > then Test and set PSTATE flags to a com-
PSTATE ← flags(Rn − Rm|imm5) parison or immediate.
else
PSTATE ← nzcv
end if
ccmn if < cond > then Test and set PSTATE flags to a negated
PSTATE ← flags(Rn + Rm|imm5) comparison or immediate.
else
PSTATE ← nzcv
end if

4.2.10.3 Examples

1 csel x0, x1, x2, ge // x0 = (ge ? x1 : x2)
2 csneg x2, x3, x4, ne // x2 = (ne ? x3: -x4)
3 cinv x9, x7, hi // x9 = (hi ? ~x7 : x7)
4 cset x1, eq // x1 = (eq ? 1 : 0)
5 ccmp x4, #31, 0b0100, mi // nzcv = (mi ? flags(x4 - 32) : 4)

The following C function will flip the parity of a number and alternate between two values if
its output is re-used as the input:

1 int flipFlop(int num) {
2 if (num % 2 == 0) { /* is even */
3 return num + 1;
4 } else {
5 return num - 1;
6 }
7 }

The AArch64 translation uses conditional selection instead of branching to implement the
if-else statement. The input num is in w0 and is also returned in w0 as the output:

1 .text
2 .type flipFlop, %function
3 flipFlop:
4 // Tests for parity by setting the PSTATE flags to w0 AND 1