Index: include/llvm/CodeGen/AsmPrinter.h
===================================================================
--- include/llvm/CodeGen/AsmPrinter.h
+++ include/llvm/CodeGen/AsmPrinter.h
@@ -71,6 +71,7 @@
 class MDNode;
 class Module;
 class raw_ostream;
+class StackMaps;
 class TargetLoweringObjectFile;
 class TargetMachine;
 
@@ -365,6 +366,9 @@
   /// emit the proxies we previously omitted in EmitGlobalVariable.
   void emitGlobalGOTEquivs();
 
+  /// Emit the stack maps.
+  void emitStackMaps(StackMaps &SM);
+
   //===------------------------------------------------------------------===//
   // Overridable Hooks
   //===------------------------------------------------------------------===//
Index: include/llvm/CodeGen/GCMetadataPrinter.h
===================================================================
--- include/llvm/CodeGen/GCMetadataPrinter.h
+++ include/llvm/CodeGen/GCMetadataPrinter.h
@@ -29,6 +29,7 @@
 class GCModuleInfo;
 class GCStrategy;
 class Module;
+class StackMaps;
 
 /// GCMetadataPrinterRegistry - The GC assembly printer registry uses all the
 /// defaults from Registry.
@@ -60,6 +61,11 @@
   /// Called after the assembly for the module is generated by
   /// the AsmPrinter (but before target specific hooks)
   virtual void finishAssembly(Module &M, GCModuleInfo &Info, AsmPrinter &AP) {}
+
+  /// Called when the stack maps are generated. Return true if
+  /// stack maps with a custom format are generated. Otherwise
+  /// returns false and the default format will be used.
+  virtual bool emitStackMaps(StackMaps &SM, AsmPrinter &AP) { return false; }
 };
 
 } // end namespace llvm
Index: lib/CodeGen/AsmPrinter/AsmPrinter.cpp
===================================================================
--- lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -53,6 +53,7 @@
 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
 #include "llvm/CodeGen/MachineOperand.h"
 #include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
+#include "llvm/CodeGen/StackMaps.h"
 #include "llvm/CodeGen/TargetFrameLowering.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
@@ -2976,11 +2977,6 @@
   if (!S.usesMetadata())
     return nullptr;
 
-  assert(!S.useStatepoints() && "statepoints do not currently support custom"
-         " stackmap formats, please see the documentation for a description of"
-         " the default format.  If you really need a custom serialized format,"
-         " please file a bug");
-
   gcp_map_type &GCMap = getGCMap(GCMetadataPrinters);
   gcp_map_type::iterator GCPI = GCMap.find(&S);
   if (GCPI != GCMap.end())
@@ -3001,6 +2997,19 @@
   report_fatal_error("no GCMetadataPrinter registered for GC: " + Twine(Name));
 }
 
+void AsmPrinter::emitStackMaps(StackMaps &SM) {
+  GCModuleInfo *MI = getAnalysisIfAvailable<GCModuleInfo>();
+  assert(MI && "AsmPrinter didn't require GCModuleInfo?");
+  for (auto &I : *MI)
+    if (GCMetadataPrinter *MP = GetOrCreateGCPrinter(*I))
+      if (MP->emitStackMaps(SM, *this))
+        // We've emitted a custom format, done.
+        return;
+
+  // Use the default serialization if no hooks.
+  SM.serializeToStackMapSection();
+}
+
 /// Pin vtable to this file.
 AsmPrinterHandler::~AsmPrinterHandler() = default;
 
Index: lib/Target/AArch64/AArch64AsmPrinter.cpp
===================================================================
--- lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -208,7 +208,7 @@
     // linker can safely perform dead code stripping.  Since LLVM never
     // generates code that does this, it is always safe to set.
     OutStreamer->EmitAssemblerFlag(MCAF_SubsectionsViaSymbols);
-    SM.serializeToStackMapSection();
+    emitStackMaps(SM);
   }
 }
 
Index: lib/Target/PowerPC/PPCAsmPrinter.cpp
===================================================================
--- lib/Target/PowerPC/PPCAsmPrinter.cpp
+++ lib/Target/PowerPC/PPCAsmPrinter.cpp
@@ -327,7 +327,7 @@
 }
 
 void PPCAsmPrinter::EmitEndOfAsmFile(Module &M) {
-  SM.serializeToStackMapSection();
+  emitStackMaps(SM);
 }
 
 void PPCAsmPrinter::LowerSTACKMAP(StackMaps &SM, const MachineInstr &MI) {
Index: lib/Target/SystemZ/SystemZAsmPrinter.cpp
===================================================================
--- lib/Target/SystemZ/SystemZAsmPrinter.cpp
+++ lib/Target/SystemZ/SystemZAsmPrinter.cpp
@@ -647,7 +647,7 @@
 }
 
 void SystemZAsmPrinter::EmitEndOfAsmFile(Module &M) {
-  SM.serializeToStackMapSection();
+  emitStackMaps(SM);
 }
 
 // Force static initialization.
Index: lib/Target/X86/X86AsmPrinter.cpp
===================================================================
--- lib/Target/X86/X86AsmPrinter.cpp
+++ lib/Target/X86/X86AsmPrinter.cpp
@@ -674,7 +674,7 @@
     emitNonLazyStubs(MMI, *OutStreamer);
 
     // Emit stack and fault map information.
-    SM.serializeToStackMapSection();
+    emitStackMaps(SM);
     FM.serializeToFaultMapSection();
 
     // This flag tells the linker that no global symbols contain code that fall
@@ -695,12 +695,12 @@
   }
 
   if (TT.isOSBinFormatCOFF()) {
-    SM.serializeToStackMapSection();
+    emitStackMaps(SM);
     return;
   }
 
   if (TT.isOSBinFormatELF()) {
-    SM.serializeToStackMapSection();
+    emitStackMaps(SM);
     FM.serializeToFaultMapSection();
     return;
   }