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lea 0x0(%esi),%esi
I believe it has no result and is simply filling space. Is this the case?
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lea — Load effective address. The lea instruction places the address specified by its first operand into the register specified by its second operand. Note, the contents of the memory location are not loaded, only the effective address is computed and placed into the register.
lea is an abbreviation of "load effective address". It loads the address of the location reference by the source operand to the destination operand.
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LEA does not affect any flag. This instruction loads new values into the specified register and into the DS register from four successive memory locations. The word from two memory locations is copied into the specified register and the word from the next two memory locations is copied into the DS registers.
Its a NOP. It adds the contents of %esi and 0x0, and puts the result in %esi. Somebody either has a clumsy code generator or needs to fill N bytes, where this instruction is the right size.
LEA instructions execute quite fast (typically 1 clock), so this is a lot better than N nops.
The x86 being as quirky as it is, has a variety of instructions that effectively don't do anything but fill differing numbers of bytes. You may find other useless instructions of different lengths. You tend to find instructions that are long but execute in 1 clock or less.
The AMD x86-64 manual has some suggestions as to what should be used for NOPs; they suggest one of the prefix opcodes repeated a number of times before an actual NOP, IIRC. Such prefix opcodes are consumed very quickly by the instruction fetch engine; mostly their cost is hidden in instruction pre-fetch, and not in instruction execution time.
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