Index: llvm/lib/CodeGen/BasicBlockSections.cpp
===================================================================
--- llvm/lib/CodeGen/BasicBlockSections.cpp
+++ llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -88,6 +88,12 @@
     cl::desc("The text prefix to use for cold basic block clusters"),
     cl::init(".text.split."), cl::Hidden);
 
+cl::opt<bool> DetectSourceDrift(
+    "detect-source-drift",
+    cl::desc("This checks if there is a fdo instr. profile hash "
+             "mismatch for this function"),
+    cl::init(true), cl::Hidden);
+
 namespace {
 
 // This struct represents the cluster information for a machine basic block.
@@ -313,10 +319,31 @@
   return true;
 }
 
+// This checks if the source of this function has drifted since this binary was
+// profiled previously.  For now, we are piggy backing on what PGO does to
+// detect this with instrumented profiles.  PGO emits an hash of the IR and
+// checks if the hash has changed.  Advanced basic block layout is usually done
+// on top of PGO optimized binaries and hence this check works well in practice.
+static bool checkSourceDrift(MachineFunction &MF) {
+  if (!DetectSourceDrift)
+    return false;
+
+  const char MetadataName[] = "instr_prof_hash_mismatch";
+  if (MDTuple *Tuple =
+      cast<MDTuple>(MF.getFunction().getMetadata(LLVMContext::MD_annotation))) {
+    for (auto &N : Tuple->operands())
+      if (cast<MDString>(N.get())->getString() == MetadataName)
+        return true;
+  }
+
+  return false;
+}
+
 bool BasicBlockSections::runOnMachineFunction(MachineFunction &MF) {
   auto BBSectionsType = MF.getTarget().getBBSectionsType();
   assert(BBSectionsType != BasicBlockSection::None &&
          "BB Sections not enabled!");
+
   // Renumber blocks before sorting them for basic block sections.  This is
   // useful during sorting, basic blocks in the same section will retain the
   // default order.  This renumbering should also be done for basic block
@@ -328,6 +355,16 @@
     return true;
   }
 
+  // Check for source drift.  If the source has changed since the profiles
+  // were obtained, optimizing basic blocks might be sub-optimal.
+  // This only applies to BasicBlockSection::List as it creates
+  // clusters of basic blocks using basic block ids. Source drift can
+  // invalidate these groupings leading to sub-optimal code generation with
+  // regards to performance.
+  if (BBSectionsType == BasicBlockSection::List &&
+      checkSourceDrift(MF))
+    return true;
+
   std::vector<Optional<BBClusterInfo>> FuncBBClusterInfo;
   if (BBSectionsType == BasicBlockSection::List &&
       !getBBClusterInfoForFunction(MF, FuncAliasMap, ProgramBBClusterInfo,
Index: llvm/test/CodeGen/X86/bb_sections_hash_mismatch.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/bb_sections_hash_mismatch.ll
@@ -0,0 +1,19 @@
+; RUN: echo "!foo" > %t.order.txt
+; RUN: llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=%t.order.txt  | FileCheck --check-prefix=SOURCE_DRIFT %s
+; RUN: llc < %s -mtriple=x86_64-pc-linux -basic-block-sections=%t.order.txt -detect-source-drift=false | FileCheck --check-prefix=NO_SOURCE_DRIFT %s
+
+define dso_local i32 @foo(i1 zeroext %0, i1 zeroext %1)  !annotation !1 {
+  br i1 %0, label %5, label %3
+
+3:                                                ; preds = %2
+  %4 = select i1 %1, i32 2, i32 0
+  ret i32 %4
+
+5:                                                ; preds = %2
+  ret i32 1
+}
+
+!1 = !{!"instr_prof_hash_mismatch"}
+
+; SOURCE_DRIFT-NOT: .section .text
+; NO_SOURCE_DRIFT: .section .text