Index: llvm/include/llvm/Analysis/ProfileSummaryInfo.h
===================================================================
--- llvm/include/llvm/Analysis/ProfileSummaryInfo.h
+++ llvm/include/llvm/Analysis/ProfileSummaryInfo.h
@@ -118,6 +118,8 @@
   bool isFunctionEntryCold(const Function *F);
   /// Returns true if \p F contains only cold code.
   bool isFunctionColdInCallGraph(const Function *F, BlockFrequencyInfo &BFI);
+  /// Returns true if the hotness of \p F is unknown.
+  bool isFunctionHotnessUnknown(const Function &F);
   /// Returns true if \p F contains hot code with regard to a given hot
   /// percentile cutoff value.
   bool isFunctionHotInCallGraphNthPercentile(int PercentileCutoff,
Index: llvm/lib/Analysis/ProfileSummaryInfo.cpp
===================================================================
--- llvm/lib/Analysis/ProfileSummaryInfo.cpp
+++ llvm/lib/Analysis/ProfileSummaryInfo.cpp
@@ -192,6 +192,11 @@
   return true;
 }
 
+bool ProfileSummaryInfo::isFunctionHotnessUnknown(const Function &F) {
+  assert(hasSampleProfile() && "It should only be called in sampleFDO mode");
+  return !F.getEntryCount().hasValue();
+}
+
 template<bool isHot>
 bool ProfileSummaryInfo::isFunctionHotOrColdInCallGraphNthPercentile(
     int PercentileCutoff, const Function *F, BlockFrequencyInfo &BFI) {
Index: llvm/lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -177,6 +177,17 @@
     "profile-guided-section-prefix", cl::Hidden, cl::init(true), cl::ZeroOrMore,
     cl::desc("Use profile info to add section prefix for hot/cold functions"));
 
+static cl::opt<bool> ProfileUnknownInSpecialSection(
+    "profile-unknown-in-special-section", cl::Hidden, cl::init(false),
+    cl::ZeroOrMore,
+    cl::desc("In profiling mode like sampleFDO, if a function doesn't have "
+             "profile, we cannot tell the function is cold for sure because "
+             "it may be a function newly added without ever being sampled. "
+             "With the flag enabled, compiler can put such profile unknown "
+             "functions into a special section, so runtime system can choose "
+             "to handle it in a different way than .text section, to save "
+             "RAM for example. "));
+
 static cl::opt<unsigned> FreqRatioToSkipMerge(
     "cgp-freq-ratio-to-skip-merge", cl::Hidden, cl::init(2),
     cl::desc("Skip merging empty blocks if (frequency of empty block) / "
@@ -446,6 +457,9 @@
       F.setSectionPrefix(".hot");
     else if (PSI->isFunctionColdInCallGraph(&F, *BFI))
       F.setSectionPrefix(".unlikely");
+    else if (ProfileUnknownInSpecialSection && PSI->isPartialProfile() &&
+             PSI->hasSampleProfile() && PSI->isFunctionHotnessUnknown(F))
+      F.setSectionPrefix(".unknown");
   }
 
   /// This optimization identifies DIV instructions that can be
Index: llvm/lib/Transforms/IPO/SampleProfile.cpp
===================================================================
--- llvm/lib/Transforms/IPO/SampleProfile.cpp
+++ llvm/lib/Transforms/IPO/SampleProfile.cpp
@@ -166,6 +166,10 @@
     "sample-profile-cold-inline-threshold", cl::Hidden, cl::init(45),
     cl::desc("Threshold for inlining cold callsites"));
 
+static cl::opt<bool> PartialProfile(
+    "partial-profile", cl::Hidden, cl::init(false),
+    cl::desc("Specify the current profile is used as a partial profile."));
+
 namespace {
 
 using BlockWeightMap = DenseMap<const BasicBlock *, uint64_t>;
@@ -1851,6 +1855,8 @@
   if (M.getProfileSummary(/* IsCS */ false) == nullptr)
     M.setProfileSummary(Reader->getSummary().getMD(M.getContext()),
                         ProfileSummary::PSK_Sample);
+  if (PartialProfile)
+    PSI->setPartialProfile(true);
 
   // Compute the total number of samples collected in this profile.
   for (const auto &I : Reader->getProfiles())
Index: llvm/test/Transforms/SampleProfile/section-accurate-samplepgo.ll
===================================================================
--- llvm/test/Transforms/SampleProfile/section-accurate-samplepgo.ll
+++ llvm/test/Transforms/SampleProfile/section-accurate-samplepgo.ll
@@ -1,5 +1,6 @@
 ; REQUIRES: x86-registered-target
 ; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/inline.prof -codegenprepare -S | FileCheck %s
+; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/inline.prof -codegenprepare -profile-unknown-in-special-section -S | FileCheck %s --check-prefix UNKNOWN
 ; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/inline.prof -codegenprepare -profile-sample-accurate -S | FileCheck %s --check-prefix ACCURATE
 
 target triple = "x86_64-pc-linux-gnu"
@@ -11,7 +12,8 @@
 declare void @hot_func()
 
 ; CHECK-NOT: foo_not_in_profile{{.*}}!section_prefix
-; CHECK: foo_not_in_profile{{.*}}!prof ![[UNKNOWN_ID:[0-9]+]]
+; CHECK: foo_not_in_profile{{.*}}!prof ![[NOPROFILE_ID:[0-9]+]]
+; UNKNOWN: foo_not_in_profile{{.*}}!prof ![[NOPROFILE_ID:[0-9]+]] !section_prefix ![[UNKNOWN_ID:[0-9]+]]
 ; ACCURATE: foo_not_in_profile{{.*}}!prof ![[ZERO_ID:[0-9]+]] !section_prefix ![[COLD_ID:[0-9]+]]
 ; The function not appearing in profile is cold when -profile-sample-accurate
 ; is on.
@@ -31,9 +33,11 @@
 
 attributes #0 = { "profile-sample-accurate" }
 
-; CHECK: ![[UNKNOWN_ID]] = !{!"function_entry_count", i64 -1}
+; CHECK: ![[NOPROFILE_ID]] = !{!"function_entry_count", i64 -1}
 ; CHECK: ![[ZERO_ID]] = !{!"function_entry_count", i64 0}
 ; CHECK: ![[COLD_ID]] = !{!"function_section_prefix", !".unlikely"}
+; UNKNOWN: ![[NOPROFILE_ID]] = !{!"function_entry_count", i64 -1}
+; UNKNOWN: ![[UNKNOWN_ID]] = !{!"function_section_prefix", !".unknown"}
 ; ACCURATE: ![[ZERO_ID]] = !{!"function_entry_count", i64 0}
 ; ACCURATE: ![[COLD_ID]] = !{!"function_section_prefix", !".unlikely"}
 !llvm.module.flags = !{!1}