#X86_64 #GCC Good day, sorry for a bit trivial question. C code: extern

Question

#X86_64 #GCC Good day, sorry for a bit trivial question. C code: extern

void test();

void my_function()
{
test();
}

Result:
my_function:
xorl %eax, %eax
jmp test

Godbolt link:
https://godbolt.org/z/vhWejE

Why does GCC generate xorl %eax, %eax before tail call to test?
What is exactly the purpose of zeroing %eax in this situation?
Is there an ABI requirement for doing this?

Thanks.

0

18.02.2021

1 ответов

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Ammar Faizi · Answer 1

Ammar Faizi Автор вопроса

Ah okay, I figured it out. It turns out to be an ABI requirement for determining the number of vector registers used. I did realize it when working with printf, but was no idea when random function takes unspecified number of arguments like this.

0

18.02.2021

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#X86_64 #GCC Good day, sorry for a bit trivial question. C code: extern

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