[PATCH] D149323: [SCEV] Don't invalidate past dependency-breaking instructions

[Nikita Popov via Phabricator via llvm-commits]

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When invalidating a value, we walk all users of that value and invalidate them as well. This can be very expensive for large use graphs.

However, we only need to invalidate a user U of instruction I if SCEV(U) can depend on SCEV(I). This is not the case if U is an instruction that always produces a SCEVUnknown, such as a load. If the load pointer operand is invalidated, there is no need to invalidate the load result, which is completely unrelated from a SCEV perspective.

This is a decent compile-time improvement in some cases: http://llvm-compile-time-tracker.com/compare.php?from=da4fcb0c0b281746067f92d8804c18dbce4269bd&to=3d7d37d4cc0118af994c70037753358868f25147&stat=instructions:u


https://reviews.llvm.org/D149323

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -4791,6 +4791,18 @@
   }
 }
 
+/// Determine whether a SCEV for this instruction will not depend on the SCEVs
+/// for its operands. We do not have to walk past such instructions when
+/// invalidating.
+static bool isDependencyBreaking(const Instruction *I) {
+  switch (I->getOpcode()) {
+  case Instruction::Load:
+    return true;
+  default:
+    return false;
+  }
+}
+
 /// Push users of the given Instruction onto the given Worklist.
 static void PushDefUseChildren(Instruction *I,
                                SmallVectorImpl<Instruction *> &Worklist,
@@ -4798,6 +4810,8 @@
   // Push the def-use children onto the Worklist stack.
   for (User *U : I->users()) {
     auto *UserInsn = cast<Instruction>(U);
+    if (isDependencyBreaking(UserInsn))
+      continue;
     if (Visited.insert(UserInsn).second)
       Worklist.push_back(UserInsn);
   }


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