Panics do not get fully eliminated from shared libraries compiled for Solaris

[newpavlov]

Let's use the following crate as an example:

[package]
name = "foo"
version = "0.1.0"
edition = "2021"


[lib]
name = "foo"
crate-type = ["cdylib"]

[profile.release]
panic = "abort"

#![no_std]

#[cfg(not(test))]
#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    extern "C" {
        fn panic_nonexistent() -> !;
    }
    unsafe { panic_nonexistent() }
}

#[no_mangle]
pub extern "C" fn add(a: u32, b: u32) -> u32 {
    a.wrapping_add(b)
}

Compiler successfully eliminates all panics from generated shared library while compiling for a Linux and many other targets (i.e. the panic_nonexistent symbol is not present). But compiling it for x86_64-pc-solaris (e.g. using cross build --release --target=x86_64-pc-solaris) we get a shared library which contains rust_begin_unwind and panic_nonexistent symbols. Here how output of readelf -Ws looks like:

     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 00000000000000e8     0 SECTION LOCAL  DEFAULT    1 
     2: 0000000000000140     0 SECTION LOCAL  DEFAULT    2 
     3: 00000000000002d8     0 SECTION LOCAL  DEFAULT    3 
     4: 00000000000003c8     0 SECTION LOCAL  DEFAULT    4 
     5: 0000000000000428     0 SECTION LOCAL  DEFAULT    5 
     6: 0000000000000440     0 SECTION LOCAL  DEFAULT    6 
     7: 0000000000000450     0 SECTION LOCAL  DEFAULT    7 
     8: 0000000000000470     0 SECTION LOCAL  DEFAULT    8 
     9: 0000000000000480     0 SECTION LOCAL  DEFAULT    9 
    10: 0000000000000630     0 SECTION LOCAL  DEFAULT   10 
    11: 0000000000000640     0 SECTION LOCAL  DEFAULT   11 
    12: 0000000000200648     0 SECTION LOCAL  DEFAULT   12 
    13: 0000000000200658     0 SECTION LOCAL  DEFAULT   13 
    14: 0000000000200668     0 SECTION LOCAL  DEFAULT   14 
    15: 00000000002007b8     0 SECTION LOCAL  DEFAULT   15 
    16: 00000000002007d8     0 SECTION LOCAL  DEFAULT   16 
    17: 00000000002007f8     0 SECTION LOCAL  DEFAULT   17 
    18: 0000000000200800     0 SECTION LOCAL  DEFAULT   18 
    19: 0000000000000000     0 SECTION LOCAL  DEFAULT   19 
    20: 0000000000000000     0 SECTION LOCAL  DEFAULT   20 
    21: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crti.s
    22: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    23: 0000000000200648     0 OBJECT  LOCAL  DEFAULT   12 __CTOR_LIST__
    24: 0000000000200658     0 OBJECT  LOCAL  DEFAULT   13 __DTOR_LIST__
    25: 0000000000000640     0 OBJECT  LOCAL  DEFAULT   11 __EH_FRAME_BEGIN__
    26: 0000000000000480     0 FUNC    LOCAL  DEFAULT    9 deregister_tm_clones
    27: 00000000000004b0     0 FUNC    LOCAL  DEFAULT    9 register_tm_clones
    28: 00000000000004f0     0 FUNC    LOCAL  DEFAULT    9 __do_global_dtors_aux
    29: 0000000000200800     1 OBJECT  LOCAL  DEFAULT   18 completed.6467
    30: 0000000000200808     8 OBJECT  LOCAL  DEFAULT   18 dtor_idx.6469
    31: 0000000000000580     0 FUNC    LOCAL  DEFAULT    9 frame_dummy
    32: 0000000000200820    48 OBJECT  LOCAL  DEFAULT   18 object.6479
    33: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtstuff.c
    34: 0000000000200650     0 OBJECT  LOCAL  DEFAULT   12 __CTOR_END__
    35: 0000000000000640     0 OBJECT  LOCAL  DEFAULT   11 __FRAME_END__
    36: 00000000000005e0     0 FUNC    LOCAL  DEFAULT    9 __do_global_ctors_aux
    37: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS crtn.s
    38: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS foo.f48edae78843035f-cgu.0
    39: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS 
    40: 0000000000200800     0 OBJECT  LOCAL  DEFAULT   17 __TMC_END__
    41: 0000000000200660     0 OBJECT  LOCAL  DEFAULT   13 __DTOR_END__
    42: 00000000002007f8     0 OBJECT  LOCAL  DEFAULT   17 __dso_handle
    43: 0000000000200850     0 OBJECT  LOCAL  DEFAULT   18 _END_
    44: 00000000000005c0     6 FUNC    LOCAL  DEFAULT    9 rust_begin_unwind
    45: 0000000000000480     0 OBJECT  LOCAL  DEFAULT    9 _START_
    46: 0000000000200668     0 OBJECT  GLOBAL DEFAULT   14 _DYNAMIC
    47: 0000000000000450     0 OBJECT  GLOBAL DEFAULT    7 _PROCEDURE_LINKAGE_TABLE_
    48: 0000000000000440     0 FUNC    GLOBAL DEFAULT    6 _init
    49: 00000000000005d0     4 FUNC    GLOBAL DEFAULT    9 add
    50: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __deregister_frame_info
    51: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
    52: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable
    53: 0000000000200800     0 NOTYPE  GLOBAL DEFAULT   18 __bss_start
    54: 0000000000000630     0 FUNC    GLOBAL DEFAULT   10 _fini
    55: 0000000000200800     0 NOTYPE  GLOBAL DEFAULT   17 _edata
    56: 00000000002007d8     0 OBJECT  GLOBAL DEFAULT   16 _GLOBAL_OFFSET_TABLE_
    57: 0000000000200850     0 NOTYPE  GLOBAL DEFAULT   18 _end
    58: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND panic_nonexistent
    59: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __register_frame_info

Looking at disassembled library, `rust_begin_unwind` is not used anywhere, so I think it should be eliminated from compiled libraries.

[bjorn3]

The linker is the one responsible for omitting unused functions. Maybe the Solaris linker is more conservative in it's --gc-sections implementation than other linkers?