Mastodawn

Day 1 of Advent of Compiler Optimisations!

Why do compilers love `xor eax, eax` for zeroing registers? It's brilliant: saves bytes compared to `mov eax, 0`, AND x86 CPUs recognise this "zeroing idiom" early in the pipeline—breaking register dependencies and removing it from execution entirely. Even better: writing to `eax` zeroes the top 32 bits of `rax` for free, handling 64-bit longs in one instruction.

#AoCO2025

Why xor eax, eax? — Matt Godbolt’s blog

Why do compilers love xor-ing registers so much?

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HP van Braam Dec 1

@mattgodbolt wait xor eax, eax clears the top of rax?

I never knew that, and also that seems pretty bad. Does xor ax, ax also clear ALL of eax and rax?

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azul

@hp @mattgodbolt XOR AX, AX does not modify the top 48 bits of RAX. See Section 3.4.1.1 of Volume 1 of the Intel SDM.