Re: Question about AMD64 ABI

On Sun, Nov 13, 2022 at 7:41 AM Daniel Cervus <DanieltheDeer@outlook.com>
wrote:

>   Sorry if I didn’t make myself clear. in other words, I mean if a
> function prototype requires a parameter less than 64 bits (such as int,
> short...), since the ABI requires it to be passed by a 64-bit register,
> will the function completely ignore the unused bits? Can I safely leave the
> garbage in upper bits? Or I have to expand everything to 64 bits?
>

>> 64 bits (such as int, short...), since the ABI requires it to be passed
by a 64-bit register, will the function completely ignore the unused bits?
Can I safely leave the garbage in upper bits? Or I have to expand
everything to 64 bits?

This depends on the function and who wrote it.

What I mean specifically is, if you're a fledgling asm programmer and you
wrote a function with the intention of addressing a 32-bit register, but
instead you address it as a 64-bit register, then that's a problem because
whatever results you get are erroneous.

If instead you're leaning on a compiler to build the function body, then
the compiler will undoubtedly do the right thing.

The above is a long winded way for me to say:  For most purposes and in my
experience has always been that it's safe to ignore higher bits of passed
32-bit registers.

Thanks,
Paul

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