rax cleared before fnc call on x86_64

[Johannes Bauer]

Hello group,

I'm using gcc to hack some low-level stuff - until recently I was
developing on x86_32 (i686), but now I switched  to x86-64. As the
calling convention is different obviously I had to change large portions
of my code. Something was weird, however:

When calling functions, gcc always emits code to clear %rax before the
call is made. Is this some kind of calling convention I'm not aware of?
Can I specify a certain function attribute to stop gcc from emitting
that clearing beforehand?

Kind regards,
Johannes

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