[PATCH] D129878: [SCEV][NFC][CT] Do not waste time proving contextual facts for unreached loops and blocks

[Max Kazantsev via Phabricator via llvm-commits]

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In fact, in unreached code we can say that every fact is true. So do not waste time trying to
do something smarter.

Formally it's not an NFC because it may change query results in unreached code, but they
won't have any impact on execution.


https://reviews.llvm.org/D129878

Files:
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11012,8 +11012,9 @@
                                              ICmpInst::Predicate Pred,
                                              const SCEV *LHS, const SCEV *RHS) {
   // Interpret a null as meaning no loop, where there is obviously no guard
-  // (interprocedural conditions notwithstanding).
-  if (!L) return true;
+  // (interprocedural conditions notwithstanding). Do not bother about unreached
+  // loops.
+  if (!L || !DT.isReachableFromEntry(L->getHeader())) return true;
 
   if (VerifyIR)
     assert(!verifyFunction(*L->getHeader()->getParent(), &dbgs()) &&
@@ -11070,12 +11071,6 @@
       return true;
   }
 
-  // If the loop is not reachable from the entry block, we risk running into an
-  // infinite loop as we walk up into the dom tree.  These loops do not matter
-  // anyway, so we just return a conservative answer when we see them.
-  if (!DT.isReachableFromEntry(L->getHeader()))
-    return false;
-
   if (isImpliedViaGuard(Latch, Pred, LHS, RHS))
     return true;
 
@@ -11121,6 +11116,9 @@
                                                      ICmpInst::Predicate Pred,
                                                      const SCEV *LHS,
                                                      const SCEV *RHS) {
+  // Do not bother proving facts for unreached code.
+  if (!DT.isReachableFromEntry(BB))
+    return true;
   if (VerifyIR)
     assert(!verifyFunction(*BB->getParent(), &dbgs()) &&
            "This cannot be done on broken IR!");


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