[Pass] Add RequiredPassMixin for non skippable passes

[paperchalice]

Follow the idea in #115471, add a new kind of mix-in. This PR demonstrates how it works:

// A required pass
struct SomePass : PassInfoMixin<SomePass>, RequiredPassMixin {
    PreservedAnalysis run(...);
};

Rather than provide isRequired for each pass, it seems that checking the sub-class relationship is enough.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: None (paperchalice)

Changes

Follow the idea in #115471, add a new kind of mix-in. This PR demonstrates how it works:

// A required pass
struct SomePass : PassInfoMixin<SomePass>, RequiredPassMixin {
    PreservedAnalysis run(...);
};

Rather than provide isRequired for each pass, it seems that checking the sub-class relationship is enough.

---

Full diff: https://github.com/llvm/llvm-project/pull/115692.diff

2 Files Affected:

• (modified) llvm/include/llvm/IR/PassManager.h (+11)
• (modified) llvm/include/llvm/IR/PassManagerInternal.h (+5-1)

diff --git a/llvm/include/llvm/IR/PassManager.h b/llvm/include/llvm/IR/PassManager.h
index d269221fac0701..dd4bbb7da165ce 100644
--- a/llvm/include/llvm/IR/PassManager.h
+++ b/llvm/include/llvm/IR/PassManager.h
@@ -112,6 +112,17 @@ struct AnalysisInfoMixin : PassInfoMixin<DerivedT> {
   }
 };
 
+/// A mix-in indicates that the pass cannot be skipped
+/// in pass instrumentation.
+struct RequiredPassMixin {};
+
+/// A convenient mix-in to indicate the pass cannot be skipped
+/// in pass instrumentation.
+///
+/// It automatically mixes in \c PassInfoMixin and \c RequiredPassMixin.
+template <typename DerivedT>
+struct RequiredPassInfoMixin : PassInfoMixin<DerivedT>, RequiredPassMixin {};
+
 namespace detail {
 
 /// Actual unpacker of extra arguments in getAnalysisResult,
diff --git a/llvm/include/llvm/IR/PassManagerInternal.h b/llvm/include/llvm/IR/PassManagerInternal.h
index 4ada6ee5dd6831..3fdc584c091afe 100644
--- a/llvm/include/llvm/IR/PassManagerInternal.h
+++ b/llvm/include/llvm/IR/PassManagerInternal.h
@@ -29,6 +29,7 @@ namespace llvm {
 template <typename IRUnitT> class AllAnalysesOn;
 template <typename IRUnitT, typename... ExtraArgTs> class AnalysisManager;
 class PreservedAnalyses;
+struct RequiredPassMixin;
 
 // Implementation details of the pass manager interfaces.
 namespace detail {
@@ -112,7 +113,10 @@ struct PassModel : PassConcept<IRUnitT, AnalysisManagerT, ExtraArgTs...> {
     return false;
   }
 
-  bool isRequired() const override { return passIsRequiredImpl<PassT>(); }
+  bool isRequired() const override {
+    return passIsRequiredImpl<PassT>() ||
+           std::is_base_of_v<RequiredPassMixin, PassT>;
+  }
 
   PassT Pass;
 };

[aeubanks]

I actually meant that we'd have

struct RequiredPassInfoMixin : PassInfoMixin {
static constexpr bool isRequired = true;
};
struct OptionalPassInfoMixin: PassInfoMixin {
static constexpr bool isRequired = false;
};

and we'd get rid of the default isRequired = false here. passes must inherit from one of them or get a compilation error. this forces people to make the decision on whether or not the pass is a required or optional one (maybe we should rename to unconditional/conditional, or mandatory, or something else), as opposed to getting a default "optional" pass which is often times incorrect

if we do this, we should probably have an RFC since it affects a lot of people. there are also things we can do to make the transition from static bool isRequired() to this new method easier for people