'undefined reference to __morestack' when compiling with -O

[ebfe]

fn main() {}

$ rustc t.rs
$ rustc -O t.rs
error: linking with `cc` failed: exit code: 1
note: cc '-m64' '-L' '/usr/lib64/rustlib/x86_64-unknown-linux-gnu/lib' '-o' 't' 't.o' '-lmorestack' '-nodefaultlibs' '-Wl,--gc-sections' '-Wl,--as-needed' '-Wl,-O1' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-59beb4f7-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libsync-305341d2-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustrt-d8560cb2-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/librand-2ea8f361-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcollections-d412c0c4-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-1085c790-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-c5ed6fb4-0.11.0-pre.rlib' '/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-4f9a876d-0.11.0-pre.rlib' '-L' '/home/mg/.rust' '-L' '/home/mg' '-Wl,-Bdynamic' '-ldl' '-lpthread' '-lgcc_s' '-lpthread' '-lc' '-lm' '-Wl,-rpath,$ORIGIN/../../usr/lib/rustlib/x86_64-unknown-linux-gnu/lib' '-Wl,-rpath,/usr/lib64/rustlib/x86_64-unknown-linux-gnu/lib' '-lcompiler-rt'
note: /usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib(native.o): In function `io::helper_thread::imp::new::h69049da469566067Aga::v0.11.0.pre':
(.text._ZN2io13helper_thread3imp3new20h69049da469566067Aga11v0.11.0.preE+0x20): undefined reference to `__morestack'
/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib(native.o): In function `io::helper_thread::imp::signal::h13cdde7c153bbf21Vga::v0.11.0.pre':
(.text._ZN2io13helper_thread3imp6signal20h13cdde7c153bbf21Vga11v0.11.0.preE+0x20): undefined reference to `__morestack'
/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib(native.o): In function `io::file::FileDesc::inner_write::h2c415f6b9c18fc02zuc::v0.11.0.pre':
(.text._ZN2io4file8FileDesc11inner_write20h2c415f6b9c18fc02zuc11v0.11.0.preE+0x20): undefined reference to `__morestack'
/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib(native.o): In function `io::file::FileDesc::new::h56aff56801c246e39sc::v0.11.0.pre':
(.text._ZN2io4file8FileDesc3new20h56aff56801c246e39sc11v0.11.0.preE+0x20): undefined reference to `__morestack'
/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib(native.o): In function `io..file..FileDesc::glue_drop.4662::h71b0d96b8d3b9466':
(.text._ZN18io..file..FileDesc14glue_drop.466217h71b0d96b8d3b9466E+0x20): undefined reference to `__morestack'
/usr/lib/rustlib/x86_64-unknown-linux-gnu/lib/libnative-1fb5e2c0-0.11.0-pre.rlib(native.o):(.text._ZN2io8addrinfo18GetAddrInfoRequest3run20h8447a365fad4be72Kha11v0.11.0.preE+0x28): more undefined references to `__morestack' follow
collect2: error: ld returned 1 exit status

error: aborting due to previous error

[alexcrichton]

Interesting! It turns out that LLVM got real smart recently (or it always has been!)

$ rustc foo.rs -O --emit=ir -o - 
; ModuleID = '-.rs'
target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

; Function Attrs: nounwind readnone uwtable
define internal void @_ZN4main20hbfcbec594e62aee2eaa4v0.0E() unnamed_addr #0 {
entry-block:
  ret void
}

define i64 @main(i64, i8**) unnamed_addr #1 {
top:
  %2 = tail call i64 @_ZN10lang_start20h53a1e3fe8c23b585vme11v0.11.0.preE(i8* bitcast (void ()* @_ZN4main20hbfcbec594e62aee2eaa4v0.0E to i8*), i64 %0, i8** %1)
  ret i64 %2
}

declare i64 @_ZN10lang_start20h53a1e3fe8c23b585vme11v0.11.0.preE(i8*, i64, i8**) unnamed_addr #1

attributes #0 = { nounwind readnone uwtable "split-stack" }
attributes #1 = { "split-stack" }

$ objdump -S foo.o

foo.o:     file format elf64-x86-64


Disassembly of section .text._ZN4main20h22fc21a23bd4f614eaa4v0.0E:

0000000000000000 <_ZN4main20h22fc21a23bd4f614eaa4v0.0E>:
   0:   c3                      retq   

Disassembly of section .text.main:

0000000000000000 <main>:
  10:   48 89 f0                mov    %rax,%rdx
  13:   48 89 f9 48 8d          jmpq   18 <main+0x8>
  18:   Address 0x0000000000000018 is out of bounds.

Basically, this comment is not quite accurate.