If the condition flags have to be preserved, you are right. But otherwise, read the linked article (https://randomascii.wordpress.com/2012/12/29/the-surprising-...):
"On Sandybridge this gets even better. The register renamer detects certain instructions (xor reg, reg and sub reg, reg and various others) that always zero a register. In addition to realizing that these instructions do not really have data dependencies, the register renamer also knows how to execute these instructions – it can zero the registers itself. It doesn’t even bother sending the instructions to the execution engine, meaning that these instructions use zero execution resources, and have zero latency! See section 2.1.3.1 of Intel’s optimization manual where it talks about dependency breaking idioms. It turns out that the only thing faster than executing an instruction is not executing it."
> -O1 is `mov eax, 0`
Simply because it is shorter: On x86-64 (and x86-32)
encodes as (depending on the assembler; typically the first one is used) - 2 bytes, while encodes as - 5 bytes.Having privately analyzed some 256b demos I cannot even imagine how one could even come to the idea to use `mov r32, imm32` for zeroing a register (except for the reason that people don't want to understand how the assembly code is internally encoded) - the canonical way to use is `xor` (`sub` also works in principle, but `xor` is the way that is recommended by Intel).
EDIT: Here is an article about that topic: https://randomascii.wordpress.com/2012/12/29/the-surprising-...