[SCEV] Look at backedge dominating conditions.

[sanjoy]

Summary

This change teaches ScalarEvolution::isLoopBackedgeGuardedByCond to look
at edges within the loop body that dominate the latch. We don't do an
exhaustive search for all possible edges, but only a quick walk up the
dom tree.

[sanjoy]

Update: the return true; in this change (line 6725) is executed a total of 1964 times when bootstrapping clang with -O3.

[atrick]

LGTM.

lib/Analysis/ScalarEvolution.cpp

ScalarEvolution::isLoopBackedgeGuardedByCond(const Loop *L,
   BranchInst *LoopContinuePredicate =
     dyn_cast<BranchInst>(Latch->getTerminator());
   if (LoopContinuePredicate && LoopContinuePredicate->isConditional() &&
       isImpliedCond(Pred, LHS, RHS,
                     LoopContinuePredicate->getCondition(),
                     LoopContinuePredicate->getSuccessor(0) != L->getHeader()))
     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;
+
+  for (DomTreeNode *DTN = (*DT)[Latch], *HeaderDTN = (*DT)[L->getHeader()];
+       DTN != HeaderDTN;
+       DTN = DTN->getIDom()) {
+
+    assert(DTN && "should reach the loop header before reaching the root!");
+
+    BasicBlock *BB = DTN->getBlock();
+    BasicBlock *PBB = BB->getSinglePredecessor();
+    if (!PBB)
+      continue;
+
+    BranchInst *ContinuePredicate = dyn_cast<BranchInst>(PBB->getTerminator());
+    if (!ContinuePredicate || !ContinuePredicate->isConditional())
+      continue;
+
+    Value *Condition = ContinuePredicate->getCondition();
+
+    // If we have an edge `E` within the loop body that dominates the only
+    // latch, the condition guarding `E` also guards the backedge.  This
+    // reasoning works only for loops with a single latch.
+
+    BasicBlockEdge DominatingEdge(PBB, BB);
+    if (DominatingEdge.isSingleEdge()) {
+      // We're constructively (and conservatively) enumerating edges within the
+      // loop body that dominate the latch.  The dominator tree better agree
+      // with us on this:
+      assert(DT->dominates(DominatingEdge, Latch) && "should be!");
+
+      if (isImpliedCond(Pred, LHS, RHS, Condition,
+                        BB != ContinuePredicate->getSuccessor(0)))
+        return true;
+    }
+  }
+
   // Check conditions due to any @llvm.assume intrinsics.
   for (auto &AssumeVH : AC->assumptions()) {
     if (!AssumeVH)
       continue;
     auto *CI = cast<CallInst>(AssumeVH);
     if (!DT->dominates(CI, Latch->getTerminator()))
       continue;
 

test/Analysis/ScalarEvolution/latch-dominating-conditions.ll

+; RUN: opt -S -indvars < %s | FileCheck %s
+
+declare void @side_effect(i1)
+
+define void @latch_dominating_0(i8 %start) {
+; CHECK-LABEL: latch_dominating_0
+ entry:
+  %e = icmp slt i8 %start, 42
+  br i1 %e, label %loop, label %exit
+
+ loop:
+; CHECK-LABEL: loop
+  %idx = phi i8 [ %start, %entry ], [ %idx.inc, %be ]
+  %idx.inc = add i8 %idx, 1
+  %folds.to.true = icmp slt i8 %idx, 42
+; CHECK: call void @side_effect(i1 true)
+  call void @side_effect(i1 %folds.to.true)
+  %c0 = icmp slt i8 %idx.inc, 42
+  br i1 %c0, label %be, label %exit
+
+ be:
+; CHECK: call void @side_effect(i1 true)
+  call void @side_effect(i1 %folds.to.true)
+  %c1 = icmp slt i8 %idx.inc, 100
+  br i1 %c1, label %loop, label %exit
+
+ exit:
+  ret void
+}
+
+define void @latch_dominating_1(i8 %start) {
+; CHECK-LABEL: latch_dominating_1
+ entry:
+  %e = icmp slt i8 %start, 42
+  br i1 %e, label %loop, label %exit
+
+ loop:
+; CHECK-LABEL: loop
+  %idx = phi i8 [ %start, %entry ], [ %idx.inc, %be ]
+  %idx.inc = add i8 %idx, 1
+  %does.not.fold.to.true = icmp slt i8 %idx, 42
+; CHECK: call void @side_effect(i1 %does.not.fold.to.true)
+  call void @side_effect(i1 %does.not.fold.to.true)
+  %c0 = icmp slt i8 %idx.inc, 42
+  br i1 %c0, label %be, label %be
+
+ be:
+; CHECK: call void @side_effect(i1 %does.not.fold.to.true)
+  call void @side_effect(i1 %does.not.fold.to.true)
+  %c1 = icmp slt i8 %idx.inc, 100
+  br i1 %c1, label %loop, label %exit
+
+ exit:
+  ret void
+}