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Asked: 2026-05-29T11:24:00+00:00

The classical Multiply-Accumulate operation is a = a + b*c . But I currently

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The classical Multiply-Accumulate operation is a = a + b*c. But I currently wonder if there exist an instruction that allows to do the following operations on integer in 1 clock cycle: (a and b are unsigned 64-bit integers: unsigned long long int)

a = a*2-1
a = a*2+b

Currently, I use:

a *= 2
--a

for the first one and

a *= 2
a += b

for the second one. And I think that each one is translated to 2 instructions in ASM. But is there a way to use 1 ASM instruction instead (and with which instruction set extension on Intel CPU)?

(I search that because I do this operation billions times)

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  1. Editorial Team

    1. For Intel CPU, see the LEA instruction. It can do both of your tasks in one instruction (not sure about cycles though) each. (eg. LEA EAX, [EAX*2+EBX]). Note that this wasn’t really meant as a multiply-add, hence its funny name (load effective address).

    2. In C and C++, you shouldn’t bother. The compiler will do what it thinks is best and you can probably just hinder its effort. I’d stay with good old a = a*2-1.

    PS: If you think something’s translated as two instructions, there is nothing easier than looking in the assembly. Then you would know.

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