parallel compiler: `threads::spawn`ning loop not reproducible

[matthiaskrgr]

I tried this code:

use std::thread;

fn _main() {
    let _t1 = thread::spawn(|| {
        for _ in 0..100 {
            println!("test");
        }
    });
}

compiled with RUSTFLAGS="" rustc a.rs -Zthreads=3 --crate-type lib -Clink-dead-code=true -Copt-level=0 -Ccodegen-units=1 ; md5sum liba.rlib

I expected to see this happen: explanation
same checksums all the time

Instead, this happened: explanation
different checksums:

ddeb820dec9e3451d3353aa1041e4ef3  liba.rlib
0604d96b3340f2b6e3bed389738515c9  liba.rlib
0604d96b3340f2b6e3bed389738515c9  liba.rlib
ddeb820dec9e3451d3353aa1041e4ef3  liba.rlib
0604d96b3340f2b6e3bed389738515c9  liba.rlib
0604d96b3340f2b6e3bed389738515c9  liba.rlib
3c3905784182c1b6958da68edba17c5d  liba.rlib
ddeb820dec9e3451d3353aa1041e4ef3  liba.rlib

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rustc --version --verbose:

rustc 1.94.0-nightly (1107bbac4 2025-12-26)
binary: rustc
commit-hash: 1107bbac4b303d49c3e67a2ec62710902bf4b341
commit-date: 2025-12-26
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

[bjorn3]

I can reproduce this with rustc +nightly a.rs -Zthreads=3 --crate-type lib -Clink-dead-code=true; md5sum liba.rlib. It is the object file contained inside the rlib that changes. In particular the bitcode stored inside the object file.

diff -u <(objdump -D a.o) <(objdump -D b.o)
--- /proc/self/fd/11    2025-12-28 22:29:58.172799347 +0100
+++ /proc/self/fd/18    2025-12-28 22:29:58.172799347 +0100
@@ -1,5 +1,5 @@
 
-a.o:     file format elf64-x86-64
+b.o:     file format elf64-x86-64
 
 
 Disassembly of section .llvmbc:
@@ -3330,12 +3330,12 @@
     2169:      11 4c 90 8c             adc    %ecx,-0x74(%rax,%rdx,4)
     216d:      09 26                   or     %esp,(%rsi)
     216f:      47 c6 04 43 9a          rex.RXB movb $0x9a,(%r11,%r8,2)
-    2174:      94                      xchg   %eax,%esp
-    2175:      e2 2e                   loop   21a5 <.llvmbc+0x21a5>
-    2177:      2a 21                   sub    (%rcx),%ah
-    2179:      a0 ee 61 11 05 0b a2    movabs 0x4a08a20b051161ee,%al
-    2180:      08 4a 
-    2182:      80 00 23                addb   $0x23,(%rax)
+    2174:      14 a0                   adc    $0xa0,%al
+    2176:      b1 ed                   mov    $0xed,%cl
+    2178:      20 38                   and    %bh,(%rax)
+    217a:      3a ba 0d 05 0b a2       cmp    -0x5df4faf3(%rdx),%bh
+    2180:      08 4a 80                or     %cl,-0x80(%rdx)
+    2183:      00 23                   add    %ah,(%rbx)
     2185:      00 24 48                add    %ah,(%rax,%rcx,2)
     2188:      08 02                   or     %al,(%rdx)
     218a:      00 00                   add    %al,(%rax)
@@ -12669,19 +12669,23 @@
     88ad:      83 d9 01                sbb    $0x1,%ecx
     88b0:      ec                      in     (%dx),%al
     88b1:      3e 00 00                ds add %al,(%rax)
-    88b4:      8b 8a b7 e7 67 35       mov    0x3567e7b7(%rdx),%ecx
-    88ba:      0f 6a 5a 6f             punpckhdq 0x6f(%rdx),%mm3
-    88be:      5d                      pop    %rbp
-    88bf:      4f ec                   rex.WRXB in (%dx),%al
-    88c1:      c5 dd 4d                (bad)
-    88c4:      44 b4 7e                rex.R mov $0x7e,%spl
-    88c7:      e7 38                   out    %eax,$0x38
-    88c9:      50                      push   %rax
-    88ca:      41 19 57 41             sbb    %edx,0x41(%r15)
-    88ce:      01 00                   add    %eax,(%rax)
-    88d0:      65 0c 00                gs or  $0x0,%al
-    88d3:      00 d1                   add    %dl,%cl
-    88d5:      05 00 00 12 03          add    $0x3120000,%eax
+    88b4:      8b 0a                   mov    (%rdx),%ecx
+    88b6:      5a                      pop    %rdx
+    88b7:      7e e1                   jle    889a <.llvmbc+0x889a>
+    88b9:      7e 1d                   jle    88d8 <.llvmbc+0x88d8>
+    88bb:      d8 b9 77 d3 54 f8       fdivrs -0x7ab2c89(%rcx)
+    88c1:      d1 13                   rcll   $1,(%rbx)
+    88c3:      8c eb                   mov    %gs,%ebx
+    88c5:      60                      (bad)
+    88c6:      4f d9 70 ee             rex.WRXB fnstenv -0x12(%r8)
+    88ca:      54                      push   %rsp
+    88cb:      d6                      (bad)
+    88cc:      5d                      pop    %rbp
+    88cd:      d7                      xlat   %ds:(%rbx)
+    88ce:      06                      (bad)
+    88cf:      00 65 0c                add    %ah,0xc(%rbp)
+    88d2:      00 00                   add    %al,(%rax)
+    88d4:      d1 05 00 00 12 03       roll   $1,0x3120000(%rip)        # 31288da <_RINvNtNtCshSK8eIQvpwe_3std6thread9lifecycle15spawn_uncheckedNCNvCs75Gc0XuCLnc_3foo5__main0uEB10_+0x31288da>
     88da:      94                      xchg   %eax,%esp
     88db:      78 5e                   js     893b <.llvmbc+0x893b>
     88dd:      00 00                   add    %al,(%rax)