[cherrypick][SCEV] Pick fixes for recursive loop guard collection. (#9797)

* [SCEV] Fix exit condition for recursive loop guard collection (llvm#120442)

When assumptions are present `Terms.size()` does not actually count the
number of conditions collected from dominating branches; introduce a
separate counter.

Fixes llvm#120237

(cherry picked from commit acfd26a)

* [SCEV] Make sure starting block is marked as visited when recursively collecting loop guards. (llvm#120749)

When `collectFromBlock` is called without a predecessor (in particular
for loops that don't have a unique predecessor outside the loop) we
never start climbing the predecessor chain, and thus don't mark the
starting block as visited.

Fixes llvm#120615.

(cherry picked from commit f035351)

Author: juliannagele

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -15532,6 +15532,8 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
   // predecessors that can be found that have unique successors leading to the
   // original header.
   // TODO: share this logic with isLoopEntryGuardedByCond.
+  unsigned NumCollectedConditions = 0;
+  VisitedBlocks.insert(Block);
   std::pair<const BasicBlock *, const BasicBlock *> Pair(Pred, Block);
   for (; Pair.first;
        Pair = SE.getPredecessorWithUniqueSuccessorForBB(Pair.first)) {
@@ -15543,10 +15545,11 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
 
     Terms.emplace_back(LoopEntryPredicate->getCondition(),
                        LoopEntryPredicate->getSuccessor(0) == Pair.second);
+    NumCollectedConditions++;
 
     // If we are recursively collecting guards stop after 2
-    // predecessors to limit compile-time impact for now.
-    if (Depth > 0 && Terms.size() == 2)
+    // conditions to limit compile-time impact for now.
+    if (Depth > 0 && NumCollectedConditions == 2)
       break;
   }
   // Finally, if we stopped climbing the predecessor chain because

llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info-with-multiple-predecessors.ll

@@ -277,3 +277,62 @@ epilogue:
 exit:
   ret void
 }
+
+declare void @llvm.assume(i1)
+
+; Checks that the presence of assumptions does not interfere with
+; exiting loop guard collection via following loop predecessors.
+define void @pr120442(i1 %c.1, i1 %c.2) {
+; CHECK-LABEL: 'pr120442'
+; CHECK-NEXT:  Determining loop execution counts for: @pr120442
+; CHECK-NEXT:  Loop %inner.header: backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %inner.header: constant max backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %inner.header: symbolic max backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %inner.header: Trip multiple is 1
+entry:
+  call void @llvm.assume(i1 %c.1)
+  call void @llvm.assume(i1 %c.2)
+  br label %outer.header
+
+outer.header:
+  %phi7 = phi i32 [ 0, %bb ], [ 0, %entry ]
+  br label %inner.header
+
+bb:
+  br i1 false, label %outer.header, label %bb
+
+inner.header:
+  %phi = phi i32 [ %add, %inner.header ], [ 0, %outer.header ]
+  %add = add i32 %phi, 1
+  %icmp = icmp ugt i32 %add, 0
+  br i1 %icmp, label %exit, label %inner.header
+
+exit:
+  ret void
+}
+
+; Checks correct traversal for loops without a unique predecessor
+; outside the loop.
+define void @pr120615() {
+; CHECK-LABEL: pr120615
+; CHECK-NEXT:  Determining loop execution counts for: @pr120615
+; CHECK-NEXT:  Loop %header: backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %header: constant max backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %header: symbolic max backedge-taken count is i32 0
+; CHECK-NEXT:  Loop %header: Trip multiple is 1
+entry:
+  br label %header
+
+bb:
+  br label %header
+
+header:
+  %0 = phi i32 [ %1, %header ], [ 0, %bb ], [ 0, %entry ]
+  %1 = add i32 %0, 1
+  %icmp = icmp slt i32 %0, 0
+  br i1 %icmp, label %header, label %exit
+
+exit:
+  ret void
+
+}