[LoopPassManager] Assert that MemorySSA is preserved if used

[nikic]

Summary

Currently it's possible to silently use a loop pass that does not preserve MemorySSA in a loop-mssa pass manager, as we don't statically know which loop passes preserve MemorySSA (as was the case with the legacy pass manager).

However, we can at least add a check after the fact that if MemorySSA is used, then it should also have been preserved. Hopefully this will reduce confusion as seen in https://bugs.llvm.org/show_bug.cgi?id=51020.

[asbirlea]

Thank you!

[aeubanks]

could you add a test?

[nikic]

Add test

[aeubanks]

perhaps this could go under NDEBUG? since that's when we're going to see issues anyway. but looks good as is

[nikic]

In D108399#2955724, @aeubanks wrote:

perhaps this could go under NDEBUG? since that's when we're going to see issues anyway. but looks good as is

I think it makes sense to have this for non-assertion builds as well, as it can be triggered by user error.

llvm/lib/Transforms/Scalar/LoopPassManager.cpp

#endif
     }
 
     // Do not pass deleted Loop into the instrumentation.
     if (Updater.skipCurrentLoop())
       PI.runAfterPassInvalidated<Loop>(*Pass, PassPA);
     else
       PI.runAfterPass<Loop>(*Pass, *L, PassPA);
 
+    if (LAR.MSSA && !PassPA.getChecker<MemorySSAAnalysis>().preserved())
+      report_fatal_error("Loop pass manager using MemorySSA contains a pass "
+                         "that does not preserve MemorySSA");
+
 #ifndef NDEBUG
     // LoopAnalysisResults should always be valid.
     // Note that we don't LAR.SE.verify() because that can change observed SE
     // queries. See PR44815.
     if (VerifyDomInfo)
       LAR.DT.verify();
     if (VerifyLoopInfo)
       LAR.LI.verify(LAR.DT);

llvm/test/Other/loop-mssa-not-preserved.ll

+; RUN: not --crash opt -passes='loop-mssa(loop-unroll-full)' 2>&1 < %s | FileCheck %s
+
+; CHECK: LLVM ERROR: Loop pass manager using MemorySSA contains a pass that does not preserve MemorySSA
+
+define void @test() {
+entry:
+  br label %loop
+
+loop:
+  %i = phi i32 [ 0, %entry ], [ %i.inc, %loop ]
+  %i.inc = add i32 %i, 1
+  %c = icmp ult i32 %i, 8
+  br i1 %c, label %loop, label %exit
+
+exit:
+  ret void
+}