diff --git a/bolt/lib/Core/BinaryContext.cpp b/bolt/lib/Core/BinaryContext.cpp
index 73059ef23775a..36b789d429363 100644
--- a/bolt/lib/Core/BinaryContext.cpp
+++ b/bolt/lib/Core/BinaryContext.cpp
@@ -2425,6 +2425,10 @@ BinaryContext::createInstructionPatch(uint64_t Address,
 
 std::pair<size_t, size_t>
 BinaryContext::calculateEmittedSize(BinaryFunction &BF, bool FixBranches) {
+  // Use the original size for non-simple functions.
+  if (!BF.isSimple() || BF.isIgnored())
+    return std::make_pair(BF.getSize(), 0);
+
   // Adjust branch instruction to match the current layout.
   if (FixBranches)
     BF.fixBranches();
diff --git a/bolt/lib/Core/BinaryFunction.cpp b/bolt/lib/Core/BinaryFunction.cpp
index 22bd8f8ddb110..ceb9390b546e1 100644
--- a/bolt/lib/Core/BinaryFunction.cpp
+++ b/bolt/lib/Core/BinaryFunction.cpp
@@ -3579,6 +3579,8 @@ bool BinaryFunction::validateCFG() const {
 }
 
 void BinaryFunction::fixBranches() {
+  assert(isSimple() && "Expected function with valid CFG.");
+
   auto &MIB = BC.MIB;
   MCContext *Ctx = BC.Ctx.get();
 
diff --git a/bolt/test/X86/fix-branches-broken-cfg.s b/bolt/test/X86/fix-branches-broken-cfg.s
new file mode 100644
index 0000000000000..0de1d7ea92f9b
--- /dev/null
+++ b/bolt/test/X86/fix-branches-broken-cfg.s
@@ -0,0 +1,42 @@
+## Check that fixBranches() is not invoked on a broken CFG which could lead to
+## unintended consequences including a firing assertion.
+
+# RUN: llvm-mc --filetype=obj --triple x86_64-unknown-unknown %s -o %t.o
+# RUN: link_fdata %s %t.o %t.fdata
+# RUN: llvm-strip --strip-unneeded %t.o
+# RUN: %clang %cflags %t.o -o %t.exe -Wl,-q
+# RUN: llvm-bolt %t.exe -o %t.bolt --split-functions --split-strategy=cdsplit \
+# RUN:         --data=%t.fdata --reorder-blocks=ext-tsp 2>&1 | FileCheck %s
+
+# CHECK: internal call detected
+
+        .text
+
+        .globl  foo
+        .type   foo, @function
+foo:
+        ret
+        .size foo, .-foo
+
+## main contains an internal call. ValidateInternalCalls pass will modify CFG
+## (making it invalid) and mark the function as non-simple. After that, we
+## cannot make any assumption about the CFG.
+
+        .globl  main
+        .type   main, @function
+main:
+        call .L1
+        ret
+.L1:
+        pushq   %rbp
+        movq    %rsp, %rbp
+        movl    $1, %edi
+LLmain_foo1:
+        call    foo
+# FDATA: 1 main #LLmain_foo1# 1 foo 0 0 600
+        movl    $4, %edi
+        xorl    %eax, %eax
+        popq    %rbp
+        retq
+.Lmain_end:
+        .size   main, .Lmain_end-main