26.5 Integer multiplication (all processors)
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26.5 Integer multiplication (all processors)
An integer multiplication takes approximately 9 clock cycles on PPlain and PMMX and 4 on PPro, PII and PIII. It is therefore often advantageous to replace a multiplication by a constant with a combination of other instructions such as SHL, ADD, SUB, and LEA. Example:
IMUL EAX,10
can be replaced with
MOV EBX,EAX / ADD EAX,EAX / SHL EBX,3 / ADD EAX,EBX
or
LEA EAX,[EAX+4*EAX] / ADD EAX,EAX
Floating point multiplication is faster than integer multiplication on PPlain and PMMX, but the time spent on converting integers to float and converting the product back again is usually more than the time saved by using floating point multiplication, except when the number of conversions is low compared with the number of multiplications. MMX multiplication is fast, but is only available with 16-bit operands.