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A comment on The smallest Hello World program. Parent comment

Re:The smallest Hello World program by bd01 on Sunday January 05, 2025 @08:46AM (#42601714)
That's because you're using `mov rdi, rax` again. You keep changing `edi, eax` to `rdi, rax`. Why?

The default operand size in 64-bit mode is, for most instructions, still 32 bits. So `mov edi, eax` encodes the same in 32- and 64-bit mode.

For `mov rdi, rax` you need an extra REX prefix byte [1], that's the 48 you're seeing above, but you don't need it here.

[1] https://wiki.osdev.org/X86-64_Instruction_Encoding#REX_prefi...

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Re:The smallest Hello World program by michidk on Sunday January 05, 2025 @08:56AM (#42601758)
okay, I didn't know that, thanks for the background. I wonder why the assembler would not optimize this though.

I noticed that I then could also shave of one byte more by using lea esi, [rel msg] instead of lea rsi, [rel msg].

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