Multiversioning: support for aliases (from @ccallable)

[maleadt]

Fixes #34064, cc @KristofferC, verified with https://s3.amazonaws.com/julialangnightlies/assert_pretesting/linux/x64/1.6/julia-c2db4248a9-linux64.tar.gz (from #37510, which includes the change) where the MWE from #34064 works:

julia> app_content = """
       Base.@ccallable function julia_main()::Cint
           return 0
       end
       """
"Base.@ccallable function julia_main()::Cint\n    return 0\nend\n"

julia> # Create the sysimage
       cmd = `$(get_julia_cmd()) --sysimage=$tmp_sys_ji --cpu-target='generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)'
                                 --output-o=sys.o -e $app_content`
`/tmp/julia-c2db4248a9/bin/julia --color=yes --startup-file=no --sysimage=/tmp/julia-c2db4248a9/sys.ji '--cpu-target=generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)' --output-o=sys.o -e 'Base.@ccallable function julia_main()::Cint
    return 0
end
'`

julia> run(cmd)
Process(`/tmp/julia-c2db4248a9/bin/julia --color=yes --startup-file=no --sysimage=/tmp/julia-c2db4248a9/sys.ji '--cpu-target=generic;sandybridge,-xsaveopt,clone_all;haswell,-rdrnd,base(1)' --output-o=sys.o -e 'Base.@ccallable function julia_main()::Cint
    return 0
end
'`, ProcessExited(0))

[yuyichao]

I don't think this is correct. If the function is cloned then all uses of it, including global aliases must be updated.

[maleadt]

I don't think this is correct. If the function is cloned then all uses of it, including global aliases must be updated.

Yeah, this was a quick fix that happened to work 🙂 Don't we keep the original functions for aliases to point to? From my reading of fix_gv_uses, we save global variable addresses to relocate at run time, but with aliases we can't do that?

[yuyichao]

Correct, it was not supported and doesn't have a trivial fix #21849 (review) .

There are basically two options,

1. Do not clone a function if there's a global alias pointing to it. (will need to update the clone_all check). This will also need to add assertions to make sure the calling convention isn't broken. (no vector type allowed in the argument/return value)

2. Emit a wrapper function for global alias. Two suboptions for this one.

   1. This can be either done via a normal function, which suffers from the same no vector register constraint.
   2. Or done with an module level assembly to emit an architecture depending tail call.

[yuyichao]

Also ref #21849 (comment) which is essentially about the second option above.

[maleadt]

I went with the custom trampoline + slot option, which seems to work nicely for the @ccallable stuff I'm introducing in #37510:

julia> Float16(1.)

Thread 1 "julia" hit Breakpoint 1, 0x00007fffecd6e250 in __truncdfhf2 () from usr/lib/julia/sys.so
(gdb) disassemble
Dump of assembler code for function __truncdfhf2:
=> 0x00007fffecd6e250 <+0>:     push   %rax
   0x00007fffecd6e251 <+1>:     mov    0x507bba0(%rip),%rax        # 0x7ffff1de9df8 <__truncdfhf2.reloc_slot>
   0x00007fffecd6e258 <+8>:     pop    %rcx
   0x00007fffecd6e259 <+9>:     jmpq   *%rax
End of assembler dump.
(gdb) ni 4
0x00007fffec565950 in jlcapi___truncdfhf2_17404 () from usr/lib/julia/sys.so

[yuyichao]

The way the information is collected is more or less OK I guess. There are still a few other issues though. The module merger doesn't seem to handle use of @ccallable functions very well, the use of the alias should also be fixed, and there are still a few old changes here that should be removed. I'm testing locally.

[maleadt]

Any update, @yuyichao? I had hoped to get this in 1.6.

[yuyichao]

Not before the build on master is unbroken...

[maleadt]

Rebased.

@yuyichao, could you give this another look?

@KristofferC Verified that this still fixes the issue (the MWE from #34064 (comment)). I did run into an LLVM assertion failure in some of the code added here, but can't reproduce that anymore; could you give this a quick spin to see if you can?

[KristofferC]

I do in fact get an assertion with this PR:

❯ ./bin/MyApp
Assertion failed: (success), function jl_reinit_item, file /Users/kristoffercarlsson/julia/src/staticdata.c, line 1455.
[1]    63440 abort      ./bin/MyApp

Example repro (in PackageCompiler repo):

using PackageCompiler
create_app("examples/MyApp/", "/tmp/MyAppCompiled";force=true, incremental=true, cpu_target="native")
run(`/tmp/MyAppCompiled/bin/MyApp`)

Outdated.

[maleadt]

Verified to fix the linked PackageCompiler issue, so let's go ahead and merge this.