[llvm] [SandboxIR] Add debug checker to compare IR before/after a revert (PR #115968)

[Jorge Gorbe Moya via llvm-commits]

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@@ -67,6 +67,60 @@ class CmpInst;
 class Module;
 class GlobalVariable;
 
+#ifndef NDEBUG
+
+/// A class that saves hashes and textual IR snapshots of modules in a
+/// SandboxIR Context, and does hash comparison when `expectNoDiff` is called.
+/// If hashes differ, it prints textual IR for both old and new versions to
+/// aid debugging.
+///
+/// This is used as an additional debug check when reverting changes to
+/// SandboxIR, to verify the reverted state matches the initial state.
+class IRSnapshotChecker {
+  Context &Ctx;
+
+  // A snapshot of textual IR for a module, with a hash for quick comparison.
+  struct ModuleSnapshot {
+    llvm::stable_hash Hash;
+    std::string TextualIR;
+  };
+
+  // A snapshot for each llvm::Module found in the SandboxIR Context. In
+  // practice there will always be one module, but sandbox IR save/restore ops
+  // work at the Context level, so we must take the full state into account.
+  using ContextSnapshot = DenseMap<llvm::Module *, ModuleSnapshot>;
+
+  ContextSnapshot OrigContextSnapshot;
+
+  // True if save() was previously called. This helps us distinguish between
+  // "expectNoDiff was called without calling save" and "save was called but
+  // the saved snapshot is empty".
+  bool HasSavedState = false;
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slackito wrote:

I was trying to check we ran `save()` before `expectNoDiff` (that is, an invariant at the IRSnapshotChecker level, not at the Context level). But removing it also works, so I just did that.

https://github.com/llvm/llvm-project/pull/115968