PureBoard

3.5.1.7 Compares
Use TEST when comparing a value in a register with zero. TEST essentially ANDs
operands together without writing to a destination register. TEST is preferred over
AND because AND produces an extra result register. TEST is better than CMP ..., 0
because the instruction size is smaller.
Use TEST when comparing the result of a logical AND with an immediate constant for
equality or inequality if the register is EAX for cases such as:
IF (AVAR &8) { }
The TEST instruction can also be used to detect rollover of modulo of a power of 2.
For example, the C code:
IF ( (AVAR % 16) == 0 ) { }
can be implemented using:
TEST EAX, 0x0F
JNZ AfterIf
Using the TEST instruction between the instruction that may modify part of the flag
register and the instruction that uses the flag register can also help prevent partial
flag register stall.
Assembly/Compiler Coding Rule 39. (ML impact, M generality) Use the TEST
instruction instead of AND when the result of the logical AND is not used. This saves
μops in execution. Use a TEST if a register with itself instead of a CMP of the register
to zero, this saves the need to encode the zero and saves encoding space. Avoid
comparing a constant to a memory operand. It is preferable to load the memory
operand and compare the constant to a register.
Often a produced value must be compared with zero, and then used in a branch.
Because most Intel architecture instructions set the condition codes as part of their
execution, the compare instruction may be eliminated. Thus the operation can be
tested directly by a JCC instruction. The notable exceptions are MOV and LEA. In
these cases, use TEST.

Assembly/Compiler Coding Rule 40. (ML impact, M generality) Eliminate
unnecessary compare with zero instructions by using the appropriate conditional
jump instruction when the flags are already set by a preceding arithmetic
instruction. If necessary, use a TEST instruction instead of a compare. Be certain
that any code transformations made do not introduce problems with overflow.