[BOLT] Never call fixBranches() on non-simple functions

We should never call fixBranches() on a function with invalid CFG. E.g.,
ValidateInternalCalls modifies CFG for its internal analysis purposes.
At the same time, it marks the function as non-simple with an assumption
that fixBranches() will never run on that function.

However, calculateEmittedSize() by default calls fixBranches() which can
lead to all sorts of issues, including assertions firing in
fixBranches().

The fix is to use the original size for non-simple functions in
calculateEmittedSize() since we are supposed to emit the function
unmodified. Additionally, add an assertion at the start of
fixBranches().

Author: maksfb

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();

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();
 

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