[lldb] Add test showing UnwindAssemblyInstEmulation can't handle backwards branches

If we have a conditional branch, followed by an epilogue, followed by
more code, LLDB will incorrectly compute unwind information through
instruction emulation. Consider this:

```
// ...
<+16>: b.ne   ; <+52> DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE

// epilogue start
<+20>: ldp    x29, x30, [sp, #0x20]
<+24>: add    sp, sp, #0x30
<+28>: ret
// epilogue end

AFTER_EPILOGUE:
<+32>: do something
// ...
<+48>: ret

DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE:
<+52>: stp    x22, x23, [sp, #0x10]
<+56>: mov    x22, #0x1
<+64>: b      ; <+32> AFTER_EPILOGUE
```

LLDB will think that the unwind state of +32 is the same as +28. This is
false, as +32 _never_ executes after +28.

The root cause of the problem is the order in which instructions are
visited; they are visited in the order they appear in the text, with
unwind state always being forwarded to positive branch offsets, but
never to negative offsets.

In the example above, `AFTER_EPILOGUE` should inherit the state of the
branch in +64, but it doesn't because `AFTER_EPILOGUE` is visited right
after the `ret` in +28.

Fixing this should be simple: maintain a stack of instructions to
visit. While the stack is not empty, take the next instruction on stack
and visit it.
* After visiting a non-branching instruction, push the next instruction
  and forward unwind state to it.
* After visiting a branch with one or more known targets, push the known
  branch targets and forward state to them.
* In all other cases (ret, or branch to register), don't push nor forward
anything.

Never push an instruction already on the stack. Like the algorithm
today, this new algorithm also assume that, if two instructions branch
to the same target, the unwind state in both better be the same.

(Note: yes, branch to register is also handled incorrectly today, and
will still be incorrect).

Author: felipepiovezan

lldb/unittests/UnwindAssembly/ARM64/TestArm64InstEmulation.cpp

@@ -856,3 +856,110 @@ TEST_F(TestArm64InstEmulation, TestCFAResetToSP) {
   EXPECT_TRUE(row->GetCFAValue().GetRegisterNumber() == gpr_sp_arm64);
   EXPECT_TRUE(row->GetCFAValue().IsRegisterPlusOffset() == true);
 }
+
+TEST_F(TestArm64InstEmulation, TestMidFunctionEpilogueAndBackwardsJump) {
+  ArchSpec arch("arm64-apple-ios15");
+  std::unique_ptr<UnwindAssemblyInstEmulation> engine(
+      static_cast<UnwindAssemblyInstEmulation *>(
+          UnwindAssemblyInstEmulation::CreateInstance(arch)));
+  ASSERT_NE(nullptr, engine);
+
+  const UnwindPlan::Row *row;
+  AddressRange sample_range;
+  UnwindPlan unwind_plan(eRegisterKindLLDB);
+  UnwindPlan::Row::AbstractRegisterLocation regloc;
+
+  // clang-format off
+  uint8_t data[] = {
+      0xff, 0xc3, 0x00, 0xd1, // <+0>:  sub    sp, sp, #0x30
+      0xfd, 0x7b, 0x02, 0xa9, // <+4>:  stp    x29, x30, [sp, #0x20]
+      0xfd, 0x83, 0x00, 0x91, // <+8>:  add    x29, sp, #0x20
+      0x1f, 0x04, 0x00, 0xf1, // <+12>: cmp    x0, #0x1
+      0x21, 0x01, 0x00, 0x54, // <+16>: b.ne   ; <+52> DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE
+      0xfd, 0x7b, 0x42, 0xa9, // <+20>: ldp    x29, x30, [sp, #0x20]
+      0xff, 0xc3, 0x00, 0x91, // <+24>: add    sp, sp, #0x30
+      0xc0, 0x03, 0x5f, 0xd6, // <+28>: ret
+      // AFTER_EPILOGUE:  LLDB computes the next 5 unwind states incorrectly.
+      0x37, 0x00, 0x80, 0xd2, // <+32>: mov    x23, #0x1
+      0xf6, 0x5f, 0x41, 0xa9, // <+36>: ldp    x22, x23, [sp, #0x10]
+      0xfd, 0x7b, 0x42, 0xa9, // <+40>: ldp    x29, x30, [sp, #0x20]
+      0xff, 0xc3, 0x00, 0x91, // <+44>: add    sp, sp, #0x30
+      0xc0, 0x03, 0x5f, 0xd6, // <+48>: ret
+      // DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE
+      0xf6, 0x5f, 0x01, 0xa9, // <+52>: stp    x22, x23, [sp, #0x10]
+      0x36, 0x00, 0x80, 0xd2, // <+56>: mov    x22, #0x1
+      0x37, 0x00, 0x80, 0xd2, // <+60>: mov    x23, #0x1
+      0xf8, 0xff, 0xff, 0x17, // <+64>: b      ; <+32> AFTER_EPILOGUE
+  };
+
+  // UnwindPlan we expect:
+  // row[0]:    0: CFA=sp +0 =>
+  // row[1]:    4: CFA=sp+48 =>
+  // row[2]:    8: CFA=sp+16 => fp=[CFA-16] lr=[CFA-8]
+  // row[3]:   12: CFA=fp+16 => fp=[CFA-16] lr=[CFA-8]
+  // row[4]:   24: CFA=sp+48 => fp=<same>   lr=<same>
+  //
+  // This must come from +56
+  // row[5]:   32: CFA=fp+16 => fp=[CFA-16] lr=[CFA-8] x22=[CFA-24], x23=[CFA-32]
+  // row[6]:   40: CFA=fp+16 => fp=[CFA-16] lr=[CFA-8] x22=same,     x23 = same
+  // row[6]:   44: CFA=sp+48 => fp=same     lr=same    x22=same,     x23 = same
+  // row[6]:   48: CFA=sp0   => fp=same     lr=same    x22=same,     x23 = same
+  //
+  // row[x]:   52: CFA=fp+16 => fp=[CFA-16] lr=[CFA-8]
+  // row[x]:   56: CFA=fp+16 => fp=[CFA-16] lr=[CFA-8] x22=[CFA-24], x23=[CFA-32]
+  // clang-format on
+
+  sample_range = AddressRange(0x1000, sizeof(data));
+
+  EXPECT_TRUE(engine->GetNonCallSiteUnwindPlanFromAssembly(
+      sample_range, data, sizeof(data), unwind_plan));
+
+  // At the end of prologue (+12), CFA = fp + 16.
+  // <+0>:  sub    sp, sp, #0x30
+  // <+4>:  stp    x29, x30, [sp, #0x20]
+  // <+8>:  add    x29, sp, #0x20
+  row = unwind_plan.GetRowForFunctionOffset(12);
+  EXPECT_EQ(12, row->GetOffset());
+  EXPECT_TRUE(row->GetCFAValue().IsRegisterPlusOffset());
+  EXPECT_EQ(row->GetCFAValue().GetRegisterNumber(), gpr_fp_arm64);
+  EXPECT_EQ(row->GetCFAValue().GetOffset(), 16);
+
+  // +16 and +20 are the same as +12.
+  // <+12>: cmp    x0, #0x1
+  // <+16>: b.ne   ; <+52> DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE
+  EXPECT_EQ(12, unwind_plan.GetRowForFunctionOffset(16)->GetOffset());
+  EXPECT_EQ(12, unwind_plan.GetRowForFunctionOffset(20)->GetOffset());
+
+  // After restoring $fp to caller's value, CFA = $sp + 48
+  // <+20>: ldp    x29, x30, [sp, #0x20]
+  row = unwind_plan.GetRowForFunctionOffset(24);
+  EXPECT_EQ(24, row->GetOffset());
+  EXPECT_TRUE(row->GetCFAValue().IsRegisterPlusOffset());
+  EXPECT_TRUE(row->GetCFAValue().GetRegisterNumber() == gpr_sp_arm64);
+  EXPECT_EQ(row->GetCFAValue().GetOffset(), 48);
+
+  // $sp has been restored
+  // <+24>: add    sp, sp, #0x30
+  row = unwind_plan.GetRowForFunctionOffset(28);
+  EXPECT_EQ(28, row->GetOffset());
+  EXPECT_TRUE(row->GetCFAValue().IsRegisterPlusOffset());
+  EXPECT_TRUE(row->GetCFAValue().GetRegisterNumber() == gpr_sp_arm64);
+  EXPECT_EQ(row->GetCFAValue().GetOffset(), 0);
+
+  // FIXME: Row for offset +32 incorrectly inherits the state of the `ret`
+  // instruction, but +32 _never_ executes after the `ret`.
+  // <+28>: ret
+  // <+32>: mov    x23, #0x1
+  row = unwind_plan.GetRowForFunctionOffset(32);
+  // FIXME: EXPECT_NE(32, row->GetOffset());
+
+  // Check that the state of this branch
+  // <+16>: b.ne   ; <+52> DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE
+  // was forwarded to the branch target:
+  // <+52>: stp    x22, x23, [sp, #0x10]
+  row = unwind_plan.GetRowForFunctionOffset(52);
+  EXPECT_EQ(52, row->GetOffset());
+  EXPECT_TRUE(row->GetCFAValue().IsRegisterPlusOffset());
+  EXPECT_EQ(row->GetCFAValue().GetRegisterNumber(), gpr_fp_arm64);
+  EXPECT_EQ(row->GetCFAValue().GetOffset(), 16);
+}