stage2: function params get clobbered

[g-w1]

Zig Version

0.9.0-dev.1686+696e51be4

Steps to Reproduce

compile and run this with the latest master stage2:

const plan9 = @import("std").os.plan9;
pub fn main() void {
    const file = "a_file.f";
    const str = "you got created!\n";
    const fd = create(file, .ORDWR, 0o777);
    _ = pwrite(fd, str, str.len, 0);
    _ = close(fd);
}
pub fn pwrite(fd: usize, buf: [*]const u8, count: usize, offset: usize) usize {
    return syscall4(.PWRITE, fd, @ptrToInt(buf), count, offset);
}
pub fn syscall4(sys: plan9.SYS, arg0: usize, arg1: usize, arg2: usize, arg3: usize) usize {
    _ = sys;
    _ = arg0;
    _ = arg1;
    _ = arg2;
    _ = arg3;
    if (arg1 == arg2) unreachable;

    return arg1; // len
}

pub fn create(path: [*:0]const u8, omode: enum(usize) { ORDWR }, perms: usize) usize {
    _ = path;
    _ = omode;
    _ = perms;
    return 42;
}
pub fn close(fd: usize) usize {
    if (fd != 42) unreachable;
    return 0;
}

Expected Behavior

buf (arg1) and count (arg2) not to be the same in syscall4

Actual Behavior

buf (arg1) and count (arg2) are the same in syscall4:

Really interestingly, this issue goes away if we replace sys: plan9.SYS with sys: enum(usize) { PWRITE }, which is basically what std.os.plan9.SYS is:

The two assembly versions and the diff between them:

❯ cat zig/build-release/oldd
push   rcx
push   rsi
push   rdi
push   rbp
mov    rbp,rsp
sub    rsp,0x20
mov    DWORD PTR [rbp-0x10],edi
mov    DWORD PTR [rbp-0x18],esi
mov    DWORD PTR [rbp-0x20],ecx
call   QWORD PTR ds:0x4000068
mov    rcx,rax
mov    DWORD PTR [rbp-0x8],ecx
mov    rax,QWORD PTR [rbp-0x8]
add    rsp,0x20
pop    rbp
pop    rdi
pop    rsi
pop    rcx
ret
~/dev
❯ cat zig/build-release/newd
push   rcx
push   rsi
push   rdi
push   rbp
mov    rbp,rsp
sub    rsp,0x20
mov    DWORD PTR [rbp-0x10],edi
mov    rdi,0x33
mov    DWORD PTR [rbp-0x18],esi
mov    rsi,QWORD PTR [rbp-0x10]
mov    rdx,QWORD PTR [rbp-0x18]
mov    DWORD PTR [rbp-0x20],ecx
mov    rcx,rdx
mov    r8,QWORD PTR [rbp-0x20]
call   QWORD PTR ds:0x4000068
mov    rcx,rax
mov    DWORD PTR [rbp-0x8],ecx
mov    rax,QWORD PTR [rbp-0x8]
add    rsp,0x20
pop    rbp
pop    rdi
pop    rsi
pop    rcx
ret
~/dev
❯ diff zig/build-release/oldd zig/build-release/newd
7a8
> mov    rdi,0x33
8a10,11
> mov    rsi,QWORD PTR [rbp-0x10]
> mov    rdx,QWORD PTR [rbp-0x18]
9a13,14
> mov    rcx,rdx
> mov    r8,QWORD PTR [rbp-0x20]
~/dev
❯

new is using plan9.SYS, old is enum(usize) { PWRITE }