What does the test instruction do and when is it used?

test computes a bitwise AND of its operands and sets the flags, but throws the result away — it's and without the side effect of changing the destination.

test is the idiomatic way to ask yes/no questions about a value without disturbing it. Three classic uses:

1. Is a register zero? ANDing a value with itself is zero only if the value is zero:

test %rax, %rax
jz   is_zero

2. Is a particular bit set? AND with a mask:

test $1, %al      # bit 0 — is the number odd?
jnz  is_odd
test $0x80, %al   # bit 7 — the sign bit
jnz  is_negative

3. Is a pointer NULL?

test %rdi, %rdi
jz   handle_null

Why test %rax,%rax instead of cmp $0,%rax? It produces a shorter encoding (no immediate to store) and is the conventional zero-check, so compilers emit it constantly — recognizing it is essential when reading disassembly.

Go deeper:

• Felix Cloutier — TEST — the bitwise-AND-that-sets-flags-and-discards-result semantics.