Any advantage of XOR AL,AL + MOVZX EAX, AL over XOR EAX,EAX?

Question

I have some unknown C++ code that was compiled in Release build, so it's optimized. The point I'm struggling with is:

xor     al, al
add     esp, 8
cmp     byte ptr [ebp+userinput], 31h
movzx   eax, al

This is my understanding:

xor     al, al    ; set eax to 0x??????00 (clear last byte)
add     esp, 8    ; for some unclear reason, set the stack pointer higher
cmp     byte ptr [ebp+userinput], 31h ; set zero flag if user input was "1"
movzx   eax, al   ; set eax to AL and extend with zeros, so eax = 0x000000??

I don't care about line 2 and 3. They might be there in this order for pipelining reasons and IMHO have nothing to do with EAX.

However, I don't understand why I would clear AL first, just to clear the rest of EAX later. The result will IMHO always be EAX = 0, so this could also be

xor eax, eax

instead. What is the advantage or "optimization" of that piece of code?

Some background info:

I will get the source code later. It's a short C++ console demo program, maybe 20 lines of code only, so nothing that I would call "complex" code. IDA shows a single loop in that program, but not around this piece. The Stud_PE signature scan didn't find anything, but likely it's Visual Studio 2013 or 2015 compiler.

user_224132 · Answer

xor al,al is already slower than xor eax,eax on most CPUs. e.g. on Haswell/Skylake it needs an ALU uop and doesn't break the dependency on the old value of eax/rax. It's equally bad on AMD CPUs, or Atom/Silvermont. (Well, maybe not equally because AMD doesn't eliminate xor eax,eax at issue/rename, but it still has a false dependency which could serialize the new dependency chain with whatever used eax last).

On CPUs that do rename al separately from the rest of the register (Intel pre-IvyBridge), the xor al,al may still be recognized as a zeroing idiom, but unless you actively want to preserve the upper bytes of the register, the best way to zero al is xor eax,eax.

Doing movzx on top of that just makes it even worse.

I'm guessing your compiler somehow got confused and decided it needed a 1-byte zero, but then realized it needed to promote it to 32 bits. xor sets flags, so it couldn't xor-zero after the cmp, and it failed to notice that it could have just xor-zeroed eax before the cmp.

Either that or it's something like Jester's suggestion, where the movzx is a branch target. Even if that's the case, xor eax,eax would still have been better because zero-extending into eax follows unconditionally on this code path.

I'm curious what compiler produced this from what source.

Any advantage of XOR AL,AL + MOVZX EAX, AL over XOR EAX,EAX?

1 Answers