Rollup merge of #139700 - EnzymeAD:autodiff-flags, r=oli-obk

Autodiff flags

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. @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/187780-t-compiler.2Fllvm/topic/Print.20llvm.20passes.2E/near/511533038)

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:

- https://github.com/rust-lang/rust/issues/124509

compiler/rustc_session/src/config.rs

@@ -246,6 +246,10 @@ pub enum AutoDiff {
     /// Print the module after running autodiff and optimizations.
     PrintModFinal,
 
+    /// Print all passes scheduled by LLVM
+    PrintPasses,
+    /// Disable extra opt run after running autodiff
+    NoPostopt,
     /// Enzyme's loose type debug helper (can cause incorrect gradients!!)
     /// Usable in cases where Enzyme errors with `can not deduce type of X`.
     LooseTypes,

compiler/rustc_session/src/options.rs

@@ -711,7 +711,7 @@ mod desc {
     pub(crate) const parse_list: &str = "a space-separated list of strings";
     pub(crate) const parse_list_with_polarity: &str =
         "a comma-separated list of strings, with elements beginning with + or -";
-    pub(crate) const parse_autodiff: &str = "a comma separated list of settings: `Enable`, `PrintSteps`, `PrintTA`, `PrintAA`, `PrintPerf`, `PrintModBefore`, `PrintModAfter`, `PrintModFinal`, `LooseTypes`, `Inline`";
+    pub(crate) const parse_autodiff: &str = "a comma separated list of settings: `Enable`, `PrintSteps`, `PrintTA`, `PrintAA`, `PrintPerf`, `PrintModBefore`, `PrintModAfter`, `PrintModFinal`, `PrintPasses`, `NoPostopt`, `LooseTypes`, `Inline`";
     pub(crate) const parse_comma_list: &str = "a comma-separated list of strings";
     pub(crate) const parse_opt_comma_list: &str = parse_comma_list;
     pub(crate) const parse_number: &str = "a number";
@@ -1360,6 +1360,8 @@ pub mod parse {
                 "PrintModBefore" => AutoDiff::PrintModBefore,
                 "PrintModAfter" => AutoDiff::PrintModAfter,
                 "PrintModFinal" => AutoDiff::PrintModFinal,
+                "NoPostopt" => AutoDiff::NoPostopt,
+                "PrintPasses" => AutoDiff::PrintPasses,
                 "LooseTypes" => AutoDiff::LooseTypes,
                 "Inline" => AutoDiff::Inline,
                 _ => {
@@ -2098,6 +2100,8 @@ options! {
         `=PrintModBefore`
         `=PrintModAfter`
         `=PrintModFinal`
+        `=PrintPasses`,
+        `=NoPostopt`
         `=LooseTypes`
         `=Inline`
         Multiple options can be combined with commas."),