ThinLTO linking failure

[alexcrichton]

An extracted test case from https://internals.rust-lang.org/t/help-test-out-thinlto/6017/32 can be minimized down to:

#[inline]          
pub fn wut(a: u8) {
    a.to_string(); 
}                  

and can be reproduced with:

$ rustc +nightly -Z thinlto -C codegen-units=2 src/lib.rs --crate-type dylib
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-L" "/home/alex/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "lib.lib0-8cd878b7c8d78940dfe6697baf5b88ec.rs.rust-cgu.o" "lib.lib1-8cd878b7c8d78940dfe6697baf5b88ec.rs.rust-cgu.o" "-o" "liblib.so" "lib.crate.metadata.rust-cgu.o" "lib.crate.allocator.rust-cgu.o" "-Wl,-z,relro,-z,now" "-nodefaultlibs" "-L" "/home/alex/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-Bstatic" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/libstd-b3337a279b80aa68.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/librand-90ba6ea1a4b00ac9.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/liballoc_system-8b8c81c75416769d.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/libpanic_unwind-835a006d40378e68.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/libunwind-7683a3e743ffc736.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/liblibc-1b0e0cf3656476c6.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/liballoc-86482e4a0f4afc7d.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/libstd_unicode-4c6e67dafb0b1a44.rlib" "-Wl,--no-whole-archive" "-Wl,--whole-archive" "/tmp/rustc.7o4PYrZszXql/libcore-3bc7dbfdcf1abd3d.rlib" "-Wl,--no-whole-archive" "/tmp/rustc.7o4PYrZszXql/libcompiler_builtins-7227b52ccb6f78fe.rlib" "-Wl,-Bdynamic" "-l" "dl" "-l" "rt" "-l" "pthread" "-l" "pthread" "-l" "gcc_s" "-l" "c" "-l" "m" "-l" "rt" "-l" "pthread" "-l" "util" "-shared"
  = note: /usr/bin/ld: error: lib.lib0-8cd878b7c8d78940dfe6697baf5b88ec.rs.rust-cgu.o: requires dynamic R_X86_64_PC32 reloc against '_ZN44_$LT$char$u20$as$u20$core..char..CharExt$GT$11encode_utf817h929e1aa83e3da410E' which may overflow at runtime; recompile with -fPIC
          lib.lib0-8cd878b7c8d78940dfe6697baf5b88ec.rs.rust-cgu.o:lib0-8cd878b7c8d78940dfe6697baf5b88ec.rs:function std_unicode::char::_$LT$impl$u20$char$GT$::encode_utf8::hd034682058f9c2b9: error: undefined reference to '_$LT$char$u20$as$u20$core..char..CharExt$GT$::encode_utf8::h929e1aa83e3da410'
          collect2: error: ld returned 1 exit status
          

error: aborting due to previous error

Oddly enough the error is slightly nondeterministic...

[alexcrichton]

Hm ok that's not actually a great reproduction. ThinLTO is performing as expected for that reproduction, the problem is that we just don't have dead code elimination turned on at O1 in LLVM. Going to try to re-reduce again with optimizations enabled to get closer to the original issue.

[alexcrichton]

Er actually turns out I was testing the wrong thing! I think I've got a fix