Cannot define `abort()` in bare-metal no-std environment

[techmetx11]

Code

#![no_std]
#![no_main]

#[panic_handler]
fn panic(_: &core::panic::PanicInfo) -> ! {
    loop {}
}

// This should let me define abort(), even if it doesn't exist
#[unsafe(no_mangle)]
pub extern "C" fn abort() -> ! {
    loop {}
}

#[unsafe(no_mangle)]
fn _start() {
    unreachable!("This makes the process abort()");
}

Compiler log

$ RUSTFLAGS="-Zcodegen-backend=/opt/rustc_codegen_gcc/librustc_codegen_gcc.so -Clink-args=-T../../md.ld -Clink-args=-Wl,--oformat=binary" cargo build -Zjson-target-spec --target /opt/rustc_codegen_gcc/m68k-sega-genesis.json --release -Zbuild-std=core,alloc -Zbuild-std-features=compiler-builtins-mem
   Compiling testtest v0.1.0 (/m68k/examples/testtest)
libgccjit.so: error: : gcc_jit_function_new_block: cannot add block to an imported function
gcc_jit_block_get_function: NULL block
gcc_jit_block_get_function: NULL block
libgccjit.so: error: : gcc_jit_function_new_block: cannot add block to an imported function
gcc_jit_block_end_with_jump: NULL block
gcc_jit_block_get_function: NULL block
gcc_jit_block_end_with_jump: NULL block
error: linking with `m68k-unknown-elf-cc` failed: exit status: 1
  |
  = note:  "m68k-unknown-elf-cc" "/m68k/examples/testtest/target/m68k-sega-genesis/release/deps/rustcjxEhMT/symbols.o" "<1 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "/m68k/examples/testtest/target/m68k-sega-genesis/release/deps/{libcore-15668d62f4574fe9,libcompiler_builtins-8254a514a64b84ed}.rlib" "-L" "/m68k/examples/testtest/target/m68k-sega-genesis/release/deps/rustcjxEhMT/raw-dylibs" "-Wl,-Bdynamic" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-o" "/m68k/examples/testtest/target/m68k-sega-genesis/release/deps/testtest-bb2f26fa27bb51e5" "-Wl,--gc-sections" "-no-pie" "-Wl,-O1" "-Wl,--strip-debug" "-nodefaultlibs" "-T../../md.ld" "-Wl,--oformat=binary"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: /gcc/lib/gcc/m68k-unknown-elf/16.0.1/../../../../m68k-unknown-elf/bin/ld: /m68k/examples/testtest/target/m68k-sega-genesis/release/deps/libcompiler_builtins-8254a514a64b84ed.rlib(compiler_builtins-8254a514a64b84ed.compiler_builtins.e29bae6411254326-cgu.1.rcgu.o): in function `compiler_builtins::math::libm_math::cbrt::cbrt':
          fake.c:(.text._RNvNtNtNtCsjsefbAxuTTm_17compiler_builtins4math9libm_math4cbrt4cbrt+0x878): undefined reference to `abort'
          /gcc/lib/gcc/m68k-unknown-elf/16.0.1/../../../../m68k-unknown-elf/bin/ld: /m68k/examples/testtest/target/m68k-sega-genesis/release/deps/libcompiler_builtins-8254a514a64b84ed.rlib(compiler_builtins-8254a514a64b84ed.compiler_builtins.e29bae6411254326-cgu.1.rcgu.o): in function `<i128 as compiler_builtins::math::libm_math::support::int_traits::HInt>::widen_mul':
          fake.c:(.text.unlikely._RNvXs3_NtNtNtNtCsjsefbAxuTTm_17compiler_builtins4math9libm_math7support3bignNtNtB7_10int_traits4HInt9widen_mul+0x2): undefined reference to `abort'
          /gcc/lib/gcc/m68k-unknown-elf/16.0.1/../../../../m68k-unknown-elf/bin/ld: /m68k/examples/testtest/target/m68k-sega-genesis/release/deps/libcompiler_builtins-8254a514a64b84ed.rlib(compiler_builtins-8254a514a64b84ed.compiler_builtins.e29bae6411254326-cgu.1.rcgu.o): in function `<compiler_builtins::math::libm_math::support::big::u256 as core::fmt::Debug>::fmt':
          fake.c:(.text.unlikely._RNvXs6_NtNtNtNtCsjsefbAxuTTm_17compiler_builtins4math9libm_math7support3bigNtB5_4u256NtNtCs6bVXZla6OZW_4core3fmt5Debug3fmt+0x2): undefined reference to `abort'
          /gcc/lib/gcc/m68k-unknown-elf/16.0.1/../../../../m68k-unknown-elf/bin/ld: /m68k/examples/testtest/target/m68k-sega-genesis/release/deps/libcompiler_builtins-8254a514a64b84ed.rlib(compiler_builtins-8254a514a64b84ed.compiler_builtins.e29bae6411254326-cgu.1.rcgu.o): in function `<compiler_builtins::math::libm_math::support::big::i256 as core::fmt::Debug>::fmt':
          fake.c:(.text.unlikely._RNvXs7_NtNtNtNtCsjsefbAxuTTm_17compiler_builtins4math9libm_math7support3bigNtB5_4i256NtNtCs6bVXZla6OZW_4core3fmt5Debug3fmt+0x2): undefined reference to `abort'
          /gcc/lib/gcc/m68k-unknown-elf/16.0.1/../../../../m68k-unknown-elf/bin/ld: /m68k/examples/testtest/target/m68k-sega-genesis/release/deps/libcompiler_builtins-8254a514a64b84ed.rlib(compiler_builtins-8254a514a64b84ed.compiler_builtins.e29bae6411254326-cgu.1.rcgu.o): in function `<compiler_builtins::math::libm_math::support::big::u256 as core::fmt::LowerHex>::fmt':
          fake.c:(.text.unlikely._RNvXs8_NtNtNtNtCsjsefbAxuTTm_17compiler_builtins4math9libm_math7support3bigNtB5_4u256NtNtCs6bVXZla6OZW_4core3fmt8LowerHex3fmt+0x2): undefined reference to `abort'
          /gcc/lib/gcc/m68k-unknown-elf/16.0.1/../../../../m68k-unknown-elf/bin/ld: /m68k/examples/testtest/target/m68k-sega-genesis/release/deps/libcompiler_builtins-8254a514a64b84ed.rlib(compiler_builtins-8254a514a64b84ed.compiler_builtins.e29bae6411254326-cgu.1.rcgu.o):fake.c:(.text.unlikely._RNvXs9_NtNtNtNtCsjsefbAxuTTm_17compiler_builtins4math9libm_math7support3bigNtB5_4i256NtNtCs6bVXZla6OZW_4core3fmt8LowerHex3fmt+0x2): more undefined references to `abort' follow
          collect2: error: ld returned 1 exit status
          
  = note: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib)

error: could not compile `testtest` (bin "testtest") due to 1 previous error

[antoyo]

This is likely due to this line.
We should check if this is still needed.

[techmetx11]

Can I patch that out in my fork in the meantime? (as a workaround)

[antoyo]

Yes, but this might break other things.

[techmetx11]

Thanks.

I tried to patch the compiler, but it didn't really fix stuff and only broke more things (I think it's because the builtin function is used directly in intrinsics.rs?)

The workaround I found that worked was writing a small C library that defined abort and linking that with any Rust programs I make by default:

extern void abort() {
  __asm__ (
    "move.w #0x2700, %sr\n\t"
    "move.w #0x0100, (0xa11100)\n\t"
    ".loop:\n\t"
    "bra.w .loop");
}