What's the purpose of the LEA instruction?

 

For me, it just seems like a funky MOV. What's its purpose and when should I use it?

As others have pointed out, LEA (load effective address) is often used as a "trick" to do certain computations, but that's not its primary purpose. The x86 instruction set was designed to support high-level languages like Pascal and C, where arrays—especially arrays of ints or small structs—are common. Consider, for example, a struct representing (x, y) coordinates: 

struct Point
{
     int xcoord;
     int ycoord;
};

Now imagine a statement like: 

int y = points[i].ycoord;

where points[] is an array of Point . Assuming the base of the array is already in EBX , and variable i is in EAX , and xcoord and ycoord are each 32 bits (so ycoord is at offset 4 bytes in the struct), this statement can be compiled to: 

MOV EDX, [EBX + 8*EAX + 4]    ; right side is "effective address"

which will land y in EDX . The scale factor of 8 is because each Point is 8 bytes in size. Now consider the same expression used with the "address of" operator &: 

int *p = &points[i].ycoord;

In this case, you don't want the value of ycoord , but its address. That's where LEA (load effective address) comes in. Instead of a MOV , the compiler can generate 

LEA ESI, [EBX + 8*EAX + 4]

which will load the address in ESI .

From the "Zen of Assembly" by Abrash:

LEA , the only instruction that performs memory addressing calculations but doesn't actually address memory. LEA accepts a standard memory addressing operand, but does nothing more than store the calculated memory offset in the specified register, which may be any general purpose register.

What does that give us? Two things that ADD doesn't provide:

  • the ability to perform addition with either two or three operands, and
  • the ability to store the result in any register; not just one of the source operands.

    • And LEA does not alter the flags.

    Examples

  • LEA EAX, [ EAX + EBX + 1234567 ] calculates EAX + EBX + 1234567 (that's three operands)
  • LEA EAX, [ EBX + ECX ] calculates EBX + ECX without overriding either with the result.
  • multiplication by constant (by two, three, five or nine), if you use it like LEA EAX, [ EBX + N * EBX ] (N can be 1,2,4,8).

    • Other usecase is handy in loops: the difference between LEA EAX, [ EAX + 1 ] and INC EAX is that the latter changes EFLAGS but the former does not; this preserves CMP state.

    Another important feature of the LEA instruction is that it does not alter the condition codes such as CF and ZF , while computing the address by arithmetic instructions like ADD or MUL does. This feature decreases the level of dependency among instructions and thus makes room for further optimization by the compiler or hardware scheduler.

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