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From: Stefan Ring <stefanrin@gmail.com>
Date: Sun, 14 Feb 2021 12:38:26 +0100
Message-ID: <CAAxjCEzkE=LNsQhJvM9f7QUh-LfPLrcxNO0dbX8cfRA1BeMC0g@mail.gmail.com>
Subject: Interesting regression in parameter passing (x86_64)
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I recently noticed that gcc 9 introduced a strange push/pop pair in a
function that does nothing other than shift all arguments by one
position and transfer control to another function:

int func(int, int, int, int, int, int);
int caller(int a, int b, int c, int d, int e) { return func(0, a, b, c, d, e); }

pushq %r12
movl %r8d, %r9d
popq %r12
movl %ecx, %r8d
movl %edx, %ecx
movl %esi, %edx
movl %edi, %esi
xorl %edi, %edi
jmp func

Obviously, pushing and popping r12 serves no useful purpose, and gcc 8
does not produce it. It also disappears when a is used instead of the
constant 0 as the first argument. Where does this come from?