[lldb] Handle backwards branches in UnwindAssemblyInstEmulation (llvm#169633)

This allows the unwinder to handle code with mid-function epilogues
where the subsequent code is reachable through a backwards branch.

Two changes are required to accomplish this:

1. Do not enqueue the subsequent instruction if the current instruction
   is a barrier(*).
2. When processing an instruction, stop ignoring branches with negative
   offsets.

(*) As per the definition in LLVM's MC layer, a barrier is any
instruction that "stops control flow from executing the instruction
immediately following it". See `MCInstrDesc::isBarrier` in MCInstrDesc.h

Part of a sequence of PRs:
[lldb][NFCI] Rewrite UnwindAssemblyInstEmulation in terms of a CFG visit
llvm#169630
[lldb][NFC] Rename forward_branch_offset to branch_offset in
UnwindAssemblyInstEmulation llvm#169631
[lldb] Add DisassemblerLLVMC::IsBarrier API llvm#169632
[lldb] Handle backwards branches in UnwindAssemblyInstEmulation llvm#169633

commit-id:fd266c13
(cherry picked from commit 4e4763a)

Author: felipepiovezan

lldb/source/Plugins/UnwindAssembly/InstEmulation/UnwindAssemblyInstEmulation.cpp

@@ -227,6 +227,10 @@ bool UnwindAssemblyInstEmulation::GetNonCallSiteUnwindPlanFromAssembly(
       }
     }
 
+    // If inst is a barrier, do not propagate state to the next instruction.
+    if (inst.IsBarrier())
+      continue;
+
     // Were there any changes to the CFI while evaluating this instruction?
     if (m_curr_row_modified) {
       // Save the modified row if we don't already have a CFI row in the
@@ -530,19 +534,19 @@ bool UnwindAssemblyInstEmulation::WriteRegister(
   case EmulateInstruction::eContextAbsoluteBranchRegister:
   case EmulateInstruction::eContextRelativeBranchImmediate: {
     if (context.GetInfoType() == EmulateInstruction::eInfoTypeISAAndImmediate &&
-        context.info.ISAAndImmediate.unsigned_data32 > 0) {
+        context.info.ISAAndImmediate.unsigned_data32 != 0) {
       m_branch_offset = context.info.ISAAndImmediate.unsigned_data32;
     } else if (context.GetInfoType() ==
                    EmulateInstruction::eInfoTypeISAAndImmediateSigned &&
-               context.info.ISAAndImmediateSigned.signed_data32 > 0) {
+               context.info.ISAAndImmediateSigned.signed_data32 != 0) {
       m_branch_offset = context.info.ISAAndImmediateSigned.signed_data32;
     } else if (context.GetInfoType() ==
                    EmulateInstruction::eInfoTypeImmediate &&
-               context.info.unsigned_immediate > 0) {
+               context.info.unsigned_immediate != 0) {
       m_branch_offset = context.info.unsigned_immediate;
     } else if (context.GetInfoType() ==
                    EmulateInstruction::eInfoTypeImmediateSigned &&
-               context.info.signed_immediate > 0) {
+               context.info.signed_immediate != 0) {
       m_branch_offset = context.info.signed_immediate;
     }
   } break;

lldb/source/Plugins/UnwindAssembly/InstEmulation/UnwindAssemblyInstEmulation.h

@@ -152,7 +152,7 @@ class UnwindAssemblyInstEmulation : public lldb_private::UnwindAssembly {
   bool m_curr_row_modified;
   // The instruction is branching forward with the given offset. 0 value means
   // no branching.
-  uint32_t m_branch_offset = 0;
+  int64_t m_branch_offset = 0;
 };
 
 #endif // LLDB_SOURCE_PLUGINS_UNWINDASSEMBLY_INSTEMULATION_UNWINDASSEMBLYINSTEMULATION_H

lldb/unittests/UnwindAssembly/ARM64/TestArm64InstEmulation.cpp

@@ -879,7 +879,7 @@ TEST_F(TestArm64InstEmulation, TestMidFunctionEpilogueAndBackwardsJump) {
       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.
+      // AFTER_EPILOGUE
       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]
@@ -946,12 +946,19 @@ TEST_F(TestArm64InstEmulation, TestMidFunctionEpilogueAndBackwardsJump) {
   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`.
+  // Row for offset +32 should not inherit the state of the `ret` instruction
+  // in +28. Instead, it should inherit the state of the branch in +64.
+  // Check for register x22, which is available in row +64.
   // <+28>: ret
   // <+32>: mov    x23, #0x1
   row = unwind_plan.GetRowForFunctionOffset(32);
-  // FIXME: EXPECT_NE(28, row->GetOffset());
+  EXPECT_EQ(32, row->GetOffset());
+  {
+    UnwindPlan::Row::AbstractRegisterLocation loc;
+    EXPECT_TRUE(row->GetRegisterInfo(gpr_x22_arm64, loc));
+    EXPECT_TRUE(loc.IsAtCFAPlusOffset());
+    EXPECT_EQ(loc.GetOffset(), -32);
+  }
 
   // Check that the state of this branch
   // <+16>: b.ne   ; <+52> DO_SOMETHING_AND_GOTO_AFTER_EPILOGUE
@@ -962,4 +969,12 @@ TEST_F(TestArm64InstEmulation, TestMidFunctionEpilogueAndBackwardsJump) {
   EXPECT_TRUE(row->GetCFAValue().IsRegisterPlusOffset());
   EXPECT_EQ(row->GetCFAValue().GetRegisterNumber(), gpr_fp_arm64);
   EXPECT_EQ(row->GetCFAValue().GetOffset(), 16);
+
+  row = unwind_plan.GetRowForFunctionOffset(64);
+  {
+    UnwindPlan::Row::AbstractRegisterLocation loc;
+    EXPECT_TRUE(row->GetRegisterInfo(gpr_x22_arm64, loc));
+    EXPECT_TRUE(loc.IsAtCFAPlusOffset());
+    EXPECT_EQ(loc.GetOffset(), -32);
+  }
 }