diff --git a/llvm/include/llvm/CodeGen/IndirectThunks.h b/llvm/include/llvm/CodeGen/IndirectThunks.h
--- a/llvm/include/llvm/CodeGen/IndirectThunks.h
+++ b/llvm/include/llvm/CodeGen/IndirectThunks.h
@@ -69,10 +69,11 @@
   // IR-level constructs we already made. Create them and insert them into the
   // module.
   MachineFunction &MF = MMI.getOrCreateMachineFunction(*F);
-  MachineBasicBlock *EntryMBB = MF.CreateMachineBasicBlock(Entry);
+  // A MachineBasicBlock must not be created for the Entry block; code
+  // generation from an empty naked function in C source code also does not
+  // generate one.  At least GlobalISel asserts if this invariant isn't
+  // respected.
 
-  // Insert EntryMBB into MF. It's not in the module until we do this.
-  MF.insert(MF.end(), EntryMBB);
   // Set MF properties. We never use vregs...
   MF.getProperties().set(MachineFunctionProperties::Property::NoVRegs);
 }
diff --git a/llvm/lib/Target/AArch64/AArch64SLSHardening.cpp b/llvm/lib/Target/AArch64/AArch64SLSHardening.cpp
--- a/llvm/lib/Target/AArch64/AArch64SLSHardening.cpp
+++ b/llvm/lib/Target/AArch64/AArch64SLSHardening.cpp
@@ -214,12 +214,9 @@
 
   const TargetInstrInfo *TII =
       MF.getSubtarget<AArch64Subtarget>().getInstrInfo();
-  // Grab the entry MBB and erase any other blocks. O0 codegen appears to
-  // generate two bbs for the entry block.
+  assert (MF.size() == 1);
   MachineBasicBlock *Entry = &MF.front();
   Entry->clear();
-  while (MF.size() > 1)
-    MF.erase(std::next(MF.begin()));
 
   //  These thunks need to consist of the following instructions:
   //  __llvm_slsblr_thunk_xN:
diff --git a/llvm/lib/Target/X86/X86IndirectThunks.cpp b/llvm/lib/Target/X86/X86IndirectThunks.cpp
--- a/llvm/lib/Target/X86/X86IndirectThunks.cpp
+++ b/llvm/lib/Target/X86/X86IndirectThunks.cpp
@@ -79,12 +79,9 @@
     createThunkFunction(MMI, R11LVIThunkName);
   }
   void populateThunk(MachineFunction &MF) {
-    // Grab the entry MBB and erase any other blocks. O0 codegen appears to
-    // generate two bbs for the entry block.
+    assert (MF.size() == 1);
     MachineBasicBlock *Entry = &MF.front();
     Entry->clear();
-    while (MF.size() > 1)
-      MF.erase(std::next(MF.begin()));
 
     // This code mitigates LVI by replacing each indirect call/jump with a
     // direct call/jump to a thunk that looks like:
@@ -209,12 +206,9 @@
   }
 
   const TargetInstrInfo *TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
-  // Grab the entry MBB and erase any other blocks. O0 codegen appears to
-  // generate two bbs for the entry block.
+  assert (MF.size() == 1);
   MachineBasicBlock *Entry = &MF.front();
   Entry->clear();
-  while (MF.size() > 1)
-    MF.erase(std::next(MF.begin()));
 
   MachineBasicBlock *CaptureSpec =
       MF.CreateMachineBasicBlock(Entry->getBasicBlock());
diff --git a/llvm/test/CodeGen/AArch64/speculation-hardening-sls.ll b/llvm/test/CodeGen/AArch64/speculation-hardening-sls.ll
--- a/llvm/test/CodeGen/AArch64/speculation-hardening-sls.ll
+++ b/llvm/test/CodeGen/AArch64/speculation-hardening-sls.ll
@@ -1,7 +1,8 @@
 ; RUN: llc -mattr=harden-sls-retbr,harden-sls-blr -verify-machineinstrs -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,HARDEN,ISBDSB,ISBDSBDAGISEL
 ; RUN: llc -mattr=harden-sls-retbr,harden-sls-blr -mattr=+sb -verify-machineinstrs -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,HARDEN,SB,SBDAGISEL
 ; RUN: llc -verify-machineinstrs -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,NOHARDEN
-
+; RUN: llc -global-isel -global-isel-abort=0 -mattr=harden-sls-retbr,harden-sls-blr -verify-machineinstrs -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,HARDEN,ISBDSB
+; RUN: llc -global-isel -global-isel-abort=0 -mattr=harden-sls-retbr,harden-sls-blr -mattr=+sb -verify-machineinstrs -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s --check-prefixes=CHECK,HARDEN,SB
 
 ; Function Attrs: norecurse nounwind readnone
 define dso_local i32 @double_return(i32 %a, i32 %b) local_unnamed_addr {