diff --git a/llvm/lib/CodeGen/BasicBlockSections.cpp b/llvm/lib/CodeGen/BasicBlockSections.cpp
--- a/llvm/lib/CodeGen/BasicBlockSections.cpp
+++ b/llvm/lib/CodeGen/BasicBlockSections.cpp
@@ -134,6 +134,14 @@
                     "into clusters of basic blocks.",
                     false, false)
 
+// Returns whether the given basic block has an unconditional branch.
+bool hasUnconditionalBranch(const MachineBasicBlock &MBB) {
+  if (MBB.terminators().empty())
+    return false;
+  const MachineInstr &Terminator = *(--MBB.terminators().end());
+  return Terminator.isUnconditionalBranch();
+}
+
 // This function updates and optimizes the branching instructions of every basic
 // block in a given function to account for changes in the layout.
 static void
@@ -145,12 +153,14 @@
     auto NextMBBI = std::next(MBB.getIterator());
     auto *FTMBB = PreLayoutFallThroughs[MBB.getNumber()];
     // If this block had a fallthrough before we need an explicit unconditional
-    // branch to that block if either
+    // branch to that block if either these two conditions hold and the block
+    // doesn't currently have an unconditional branch.
     //     1- the block ends a section, which means its next block may be
     //        reorderd by the linker, or
     //     2- the fallthrough block is not adjacent to the block in the new
     //        order.
-    if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB))
+    if (FTMBB && (MBB.isEndSection() || &*NextMBBI != FTMBB) &&
+        !hasUnconditionalBranch(MBB))
       TII->insertUnconditionalBranch(MBB, FTMBB, MBB.findBranchDebugLoc());
 
     // We do not optimize branches for machine basic blocks ending sections, as
diff --git a/llvm/test/CodeGen/X86/basic-block-sections-avoids-redundant-br.ll b/llvm/test/CodeGen/X86/basic-block-sections-avoids-redundant-br.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/basic-block-sections-avoids-redundant-br.ll
@@ -0,0 +1,56 @@
+; Tests that basic block sections avoids inserting an unconditional branch when a basic block
+; already has an unconditional branch to its fallthrough block.
+; RUN: llc < %s -mtriple=x86_64 -basic-block-sections=all -O0 | FileCheck %s
+; This test case is generated from code:
+; int
+; mapping (int len)
+; {
+;   switch (len)
+;   {
+;     case 7: return 333;
+;     default:
+;       goto unknown;
+;   }
+; unknown:
+;   return 0;
+; }
+; clang -O0 -fbasic-block-sections=all test.c
+
+; Function Attrs: noinline nounwind optnone uwtable
+define dso_local i32 @mapping(i32 noundef %len) {
+entry:
+  %retval = alloca i32, align 4
+  %len.addr = alloca i32, align 4
+  store i32 %len, ptr %len.addr, align 4
+  %0 = load i32, ptr %len.addr, align 4
+  switch i32 %0, label %sw.default [
+    i32 7, label %sw.bb
+  ]
+
+sw.bb:                                            ; preds = %entry
+  store i32 333, ptr %retval, align 4
+  br label %return
+
+sw.default:                                       ; preds = %entry
+  br label %unknown
+
+unknown:                                          ; preds = %sw.default
+  store i32 0, ptr %retval, align 4
+  br label %return
+
+return:                                           ; preds = %unknown, %sw.bb
+  %1 = load i32, ptr %retval, align 4
+  ret i32 %1
+}
+
+; CHECK: mapping:
+; CHECK-NEXT: .cfi_startproc
+; CHECK-NEXT: # %bb.0: # %entry
+; CHECK-NEXT: movl {{.*}}
+; CHECK-NEXT: movl {{.*}}
+; CHECK-NEXT: subl {{.*}}
+; CHECK-NEXT: jne	mapping.__part.2
+; CHECK-NEXT: jmp	mapping.__part.1
+; CHECK-NOT:  jmp
+; CHECK: mapping.__part.1:
+; CHECK: mapping.__part.2: