Autodiff flags

[ZuseZ4]

Interestingly, it seems that some other projects have conflicts with exactly the same LLVM optimization passes as autodiff.
At least LLVMRustOptimize has exactly the flags that we need to disable problematic opt passes.

This PR enables us to compile code where users differentiate two identical functions in the same module. This has been especially common in test cases, but it's not impossible to encounter in the wild.

It also enables two new flags for testing/debugging. I consider writing an MCP to upgrade PrintPasses to be a standalone -Z flag, since it is not the same as -Z print-llvm-passes, which IMHO gives less useful output. A discussion can be found here: #t-compiler/llvm > Print llvm passes. @ 💬

Finally, it improves PrintModBefore and PrintModAfter. They used to work reliable, but now we just schedule enzyme as part of an existing ModulePassManager (MPM). Since Enzyme is last in the MPM scheduling, PrintModBefore became very inaccurate. It used to print the input module, which we gave to the Enzyme and was great to create llvm-ir reproducer. However, lately the MPM would run the whole default<O3> pipeline, which heavily modifies the llvm module, before we pass it to Enzyme. That made it impossible to use the flag to create llvm-ir reproducers for Enzyme bugs. We now schedule a PrintModule pass just before Enzyme, solving this problem.

Based on the PrintPass output, it also seems like changing registerEnzymeAndPassPipeline(PB, true); to registerEnzymeAndPassPipeline(PB, false); has no effect. In theory, the bool should tell Enzyme to schedule some helpful passes in the PassBuilder. However, since it doesn't do anything and I'm not 100% sure anymore on whether we really need it, I'll just disable it for now and postpone investigations.

r? @oli-obk

closes #139471

Tracking:

• Tracking Issue for autodiff #124509

[oli-obk]

@bors r+

[bors]

📌 Commit f79a992 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit f79a992 with merge f0309ce...

[rust-log-analyzer]

The job x86_64-mingw-1 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [run-make] tests\run-make\amdgpu-kd stdout ----

error: rmake recipe failed to complete
status: exit code: 1
command: "D:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\test\\run-make\\amdgpu-kd\\rmake.exe"
stdout: none
--- stderr -------------------------------
command failed at line 18
Command { cmd: "D:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\stage2\\bin\\rustc.exe" "-L" "D:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\test\\run-make\\amdgpu-kd\\rmake_out" "--crate-name" "foo" "--target=amdgcn-amd-amdhsa" "-Ctarget-cpu=gfx900" "--crate-type" "cdylib" "foo.rs", stdin_buf: None, stdin: None, stdout: None, stderr: None, drop_bomb: DropBomb { command: "D:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\stage2\\bin\\rustc.exe", defused: true, armed_location: Location { file: "D:\\a\\rust\\rust\\tests\\run-make\\amdgpu-kd\\rmake.rs", line: 12, col: 5 } }, already_executed: true }
output status: `exit code: 1`
=== STDOUT ===



=== STDERR ===
error: linking with `rust-lld` failed: exit code: 0xc0000374
  |
  = note: "rust-lld" "-flavor" "gnu" "--version-script=C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\rustcMu3Dj2\\list" "--no-undefined-version" "<1 object files omitted>" "--as-needed" "-L" "C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\rustcMu3Dj2\\raw-dylibs" "-Bdynamic" "--eh-frame-hdr" "-z" "noexecstack" "-L" "D:\\a\\rust\\rust\\build\\x86_64-pc-windows-gnu\\test\\run-make\\amdgpu-kd\\rmake_out" "-o" "foo.elf" "--gc-sections" "-shared"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: 

error: aborting due to 1 previous error
------------------------------------------
---
test result: FAILED. 279 passed; 1 failed; 110 ignored; 0 measured; 6 filtered out; finished in 210.08s

Some tests failed in compiletest suite=run-make mode=run-make host=x86_64-pc-windows-gnu target=x86_64-pc-windows-gnu
Build completed unsuccessfully in 2:34:05
make: *** [Makefile:124: ci-mingw-x] Error 1
  local time: Tue Apr 22 18:02:12 CUT 2025
  network time: Tue, 22 Apr 2025 18:02:13 GMT
##[error]Process completed with exit code 2.
Post job cleanup.
[command]"C:\Program Files\Git\bin\git.exe" version

[bors]

💔 Test failed - checks-actions

[oli-obk]

@bors retry