How to implement shr instructions using lea instructions?

Question

If I write code like lea eax, [2 * eax]; eax * 2lea eax, [2 * eax]; eax * 2 is the same as shl eax, 1; eax * 2 shl eax, 1; eax * 2

My question is:

1. How to implement the shr instruction? like lea eax, [2... eax] ?
2. If possible, are there certain tricks?

Answer 1

You can't emulate a right shift in terms of a left shift, sorry. (The scale factor in x86 addressing modes is encoded in machine code as a 2-bit shift count from 0 to 3.)

Also note that LEA doesn't set FLAGS, SHL does.

AFAIK there are no tricks, not even inefficient ones. This problem has come up before for toy ISAs without a right shift; you typically need something like a lookup table or a way to shift bits from one register into another, like add eax,eax / adc edx,edx to move a bit from the top of EAX to the bottom of EDX. Starting with EDX=0 and repeating that in a loop for the right number of iterations (32-n or something) will give the same result as a right shift by n .

SHR / SAR are pretty necessary primitive operations that can't easily be emulated in terms of anything else. (Except for right-rotate + zeroing high bits by generating an AND mask.)

Right shift is division by a power of 2. Division is hard; unlike left-shift / multiplication, you can't build it out of other things easily. eg add eax,eax shifts left by 1 because carry propagates from low to high in addition.