[SCEV] Add PHI SCEV verification to ensure availability at loop entry

[aleks-tmb]

No description provided.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Author: Aleksandr Popov (aleks-tmb)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/127550.diff

1 Files Affected:

• (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+10)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 8f74c1c398ced..2d1a57a72721d 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -14482,6 +14482,16 @@ void ScalarEvolution::verify() const {
                << "Delta: " << *Delta << "\n";
         std::abort();
       }
+
+      if (isa<PHINode>(I))
+        for (const Loop *L : ValidLoops)
+          if (SE.isAvailableAtLoopEntry(OldSCEV, L) !=
+              SE2.isAvailableAtLoopEntry(NewSCEV, L)) {
+            dbgs() << "Incorrect SCEV availability at loop entry\n"
+                   << "Old: " << *OldSCEV << "\n"
+                   << "New: " << *NewSCEV << "\n";
+            std::abort();
+          }
     }
   }
 

[nikic]

Why doesn't the existing loop and block disposition verification catch this? isAvailableAtEntry is a combination of LoopInvariant and ProperlyDominatesBlock dispositions.

[aleks-tmb]

Why doesn't the existing loop and block disposition verification catch this? isAvailableAtEntry is a combination of LoopInvariant and ProperlyDominatesBlock dispositions.

The motivating example can be found here: #127534.
In this case, both LoopDispositions and BlockDispositions are empty

[aleks-tmb]

@nikic @fhahn Hi,

Here’s another case of missing SCEV invalidation, which leads to incorrect availability at the loop entry: #130930

The reason the existing loop and block disposition verification doesn't catch this issue is that disposition caches are cleared inside the loop-simplifycfg pass.
However, when isAvailableAtLoopEntry is called, it recomputes the disposition for the cached SCEV and exposes the issue.