[LVI] Fix a bug with a guard being the very first instruction in a BB not taken into account

While looking for guards use reverse iterator and scan up to rend() not to begin()

llvm-svn: 284827

llvm/lib/Analysis/LazyValueInfo.cpp

@@ -978,12 +978,11 @@ void LazyValueInfoImpl::intersectAssumeOrGuardBlockValueConstantRange(
   if (!GuardDecl || GuardDecl->use_empty())
     return;
 
-  for (BasicBlock::iterator I = BBI->getIterator(),
-                            E = BBI->getParent()->begin(); I != E; I--) {
+  for (Instruction &I : make_range(BBI->getIterator().getReverse(),
+                                   BBI->getParent()->rend())) {
     Value *Cond = nullptr;
-    if (!match(&*I, m_Intrinsic<Intrinsic::experimental_guard>(m_Value(Cond))))
-      continue;
-    BBLV = intersect(BBLV, getValueFromCondition(Val, Cond));
+    if (match(&I, m_Intrinsic<Intrinsic::experimental_guard>(m_Value(Cond))))
+      BBLV = intersect(BBLV, getValueFromCondition(Val, Cond));
   }
 }
 

llvm/test/Transforms/CorrelatedValuePropagation/guards.ll

@@ -93,3 +93,20 @@ continue:
   %result = or i1 %dead, %alive
   ret i1 %result
 }
+
+; Check that we handle the case when the guard is the very first instruction in
+; a basic block.
+define i1 @test6(i32 %a) {
+; CHECK-LABEL: @test6(
+; CHECK: %alive = icmp eq i32 %a, 8
+; CHECK-NEXT: %result = or i1 false, %alive
+  %cmp = icmp ult i32 %a, 16
+  br label %continue
+
+continue:
+  call void(i1,...) @llvm.experimental.guard(i1 %cmp) [ "deopt"() ]
+  %dead = icmp eq i32 %a, 16
+  %alive = icmp eq i32 %a, 8
+  %result = or i1 %dead, %alive
+  ret i1 %result
+}