Index: lib/CodeGen/XRayInstrumentation.cpp
===================================================================
--- lib/CodeGen/XRayInstrumentation.cpp
+++ lib/CodeGen/XRayInstrumentation.cpp
@@ -43,7 +43,7 @@
   // This is the approach to go on CPUs which have a single RET instruction,
   //   like x86/x86_64.
   void replaceRetWithPatchableRet(MachineFunction &MF,
-    const TargetInstrInfo *TII);
+                                  const TargetInstrInfo *TII);
 
   // Prepend the original return instruction with the exit sled code ("patchable
   //   function exit" pseudo-instruction), preserving the original return
@@ -54,13 +54,12 @@
   //   have to call the trampoline and return from it to the original return
   //   instruction of the function being instrumented.
   void prependRetWithPatchableExit(MachineFunction &MF,
-    const TargetInstrInfo *TII);
+                                   const TargetInstrInfo *TII);
 };
 } // anonymous namespace
 
-void XRayInstrumentation::replaceRetWithPatchableRet(MachineFunction &MF,
-  const TargetInstrInfo *TII)
-{
+void XRayInstrumentation::replaceRetWithPatchableRet(
+    MachineFunction &MF, const TargetInstrInfo *TII) {
   // We look for *all* terminators and returns, then replace those with
   // PATCHABLE_RET instructions.
   SmallVector<MachineInstr *, 4> Terminators;
@@ -91,9 +90,8 @@
     I->eraseFromParent();
 }
 
-void XRayInstrumentation::prependRetWithPatchableExit(MachineFunction &MF,
-  const TargetInstrInfo *TII)
-{
+void XRayInstrumentation::prependRetWithPatchableExit(
+    MachineFunction &MF, const TargetInstrInfo *TII) {
   for (auto &MBB : MF) {
     for (auto &T : MBB.terminators()) {
       unsigned Opc = 0;
@@ -106,13 +104,16 @@
       if (Opc != 0) {
         // Prepend the return instruction with PATCHABLE_FUNCTION_EXIT or
         //   PATCHABLE_TAIL_CALL .
-        BuildMI(MBB, T, T.getDebugLoc(),TII->get(Opc));
+        BuildMI(MBB, T, T.getDebugLoc(), TII->get(Opc));
       }
     }
   }
 }
 
 bool XRayInstrumentation::runOnMachineFunction(MachineFunction &MF) {
+  if (MF.empty())
+    return false; // The function is empty.
+
   auto &F = *MF.getFunction();
   auto InstrAttr = F.getFnAttribute("function-instrument");
   bool AlwaysInstrument = !InstrAttr.hasAttribute(Attribute::None) &&
@@ -129,23 +130,24 @@
       return false; // Function is too small.
   }
 
-  auto &FirstMBB = *MF.begin();
-  auto &FirstMI = *FirstMBB.begin();
-
-  if (!MF.getSubtarget().isXRaySupported()) {
-    FirstMI.emitError("An attempt to perform XRay instrumentation for an"
-      " unsupported target.");
-    return false;
+  auto *TII = MF.getSubtarget().getInstrInfo();
+  auto &FirstMBB = MF.front();
+  if (FirstMBB.empty()) {
+    // It could be that the first MachineBasicBlock is empty, despite the
+    // function not being empty. So we insert the instruction in this basic
+    // block always.
+    BuildMI(FirstMBB, FirstMBB.begin(), DebugLoc(),
+            TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
+  } else {
+    // Since the first basic block is not empty, we just add the entry
+    // instruction there.
+    auto &FirstMI = FirstMBB.instr_front();
+    BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
+            TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
   }
 
   // FIXME: Do the loop triviality analysis here or in an earlier pass.
 
-  // First, insert an PATCHABLE_FUNCTION_ENTER as the first instruction of the
-  // MachineFunction.
-  auto *TII = MF.getSubtarget().getInstrInfo();
-  BuildMI(FirstMBB, FirstMI, FirstMI.getDebugLoc(),
-          TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
-
   switch (MF.getTarget().getTargetTriple().getArch()) {
   case Triple::ArchType::arm:
   case Triple::ArchType::thumb:
Index: test/CodeGen/X86/xray-empty-firstmbb.mir
===================================================================
--- /dev/null
+++ test/CodeGen/X86/xray-empty-firstmbb.mir
@@ -0,0 +1,23 @@
+# RUN: llc -run-pass xray-instrumentation -mtriple=x86_64-unknown-linux-gnu -o - %s | FileCheck %s
+#
+# Make sure we can handle empty first basic blocks.
+
+--- |
+
+  define i32 @foo() noinline uwtable "xray-instruction-threshold"="1" {
+  entry:
+    unreachable
+  }
+
+...
+
+---
+name: foo
+tracksRegLiveness: true
+liveins:
+  - { reg: '%edi'}
+body:            |
+  bb.0.entry:
+    liveins: %edi
+    ; CHECK: PATCHABLE_FUNCTION_ENTER
+...