Question

I am looking at some assembly that was generated by disassembling some C programs and I am confused by a single optimization that I see repeated frequently.

When I have no optimizations on the GCC compiler uses the subl instruction for subtraction, but when I do have optimizations turned on (-O3 to be precise) the compiler uses a leal instruction instead of subtraction, example below:

without optimizations:

83 e8 01     subl $0x1, %eax 

with optimizations

8d 6f ff     leal -0x1(%edi), %ebp 

Both of these instructions are 3 bytes long, so I am not seeing an optimization here. Could someone help me out and try to explain the compiler’s choice ?

Any help would be appreciated.

Answer 1

It’s hard to tell without seeing the original C code that produces this.

But if I had to guess, it’s because the leal allows the subtraction to be done out-of-place without destroying the source register.

This can save an extra register move.

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The first example:

83 e8 01     subl $0x1, %eax 

overwrites %eax thereby destroying the original value.

The second example :

8d 6f ff     leal -0x1(%edi), %ebp 

stores %edi - 1 into %ebp. %edi is preserved for future use.