What does the TEST instruction do and when is it useful?

TEST computes Src1 & Src2 (bitwise AND), sets ZF and SF, and discards the result — it's the AND counterpart to CMP's subtract.

test shines when you want to inspect bits rather than compare magnitudes, especially with a mask.

testq Src2, Src1   # compute Src1 & Src2, set ZF/SF, store nothing

• ZF set when Src1 & Src2 == 0
• SF set when the AND's top bit is 1

Common patterns:

testq %rax, %rax   # is %rax zero?  (a value AND itself is 0 only if 0)
jz    .is_zero

testq $0x1, %rax   # is bit 0 set?
jnz   .bit_set

testq %rax, %rax   # is %rax negative?
js    .is_negative

This is exactly the tool for C tests like if (a & 0xffff) where one operand is a bit mask.

Why testq %rax,%rax over cmpq $0,%rax? It's a shorter encoding with no immediate and no subtraction — the standard, compact zero-check that compilers emit everywhere.

Go deeper:

• Felix Cloutier — TEST — TEST as the AND-based flag setter.