[lldb][swift] Use frame formation as a guide for async unwinding

This patch changes how the swift language runtime obtains the async
context of the currently executing function.

The old strategy consists of querying unwind plans for what they believe
the location of the async register is. This proved problematic in cases
where the register is not saved directly, e.g. in arm64e, where the
register is signed and moved to a scratch register prior to being saved.

The new strategy relies on ABI guarantees: the async context is _always_
saved at the stack slot immediately beneath where FP is saved. As long
as a frame has been formed, we can always access the async context by
reading memory @ [CFA-24] (FP is saved in [CFA-16]). Before the frame is
formed, the async register can be accessed directly.

In other words, the main problem this patch has to solve is detecting
the point at which a frame has been fully formed. This is accomplished
by scanning the first few Rows of the assembly unwind plan, until a Row
is found where FP's location is "AtCFAPlusOffset". The Row immediately
after that is used. This is reliable because async prologues follow this
pattern in all tested architectures (x86-64, arm64, arm64e):

// 0. adjust sp
// 1. save lr        @ CFA-8
// 2. save fp        @ CFA-16
// 3. save async_reg @ CFA-24  << "row immediately after"

For frameless functions, we can always use the async register directly.

While this patch adds no tests, all of the existing stepping tests and
tests inspecting variables exercise the new behavior.

Author: felipepiovezan

lldb/source/Plugins/LanguageRuntime/Swift/SwiftLanguageRuntime.cpp

@@ -2474,14 +2474,21 @@ static llvm::Expected<addr_t> ReadPtrFromAddr(Process &process, addr_t addr,
 /// simplified version of the methods in RegisterContextUnwind, since plumbing
 /// access to those here would be challenging.
 static llvm::Expected<addr_t> GetCFA(Process &process, RegisterContext &regctx,
-                                     RegisterKind regkind,
-                                     UnwindPlan::Row::FAValue cfa_loc) {
+                                     addr_t pc_offset,
+                                     UnwindPlan &unwind_plan) {
+  UnwindPlan::RowSP row = unwind_plan.GetRowForFunctionOffset(pc_offset);
+  if (!row)
+    return llvm::createStringError(
+        "SwiftLanguageRuntime: Invalid Unwind Row when computing CFA");
+
+  UnwindPlan::Row::FAValue cfa_loc = row->GetCFAValue();
+
   using ValueType = UnwindPlan::Row::FAValue::ValueType;
   switch (cfa_loc.GetValueType()) {
   case ValueType::isRegisterPlusOffset: {
     unsigned regnum = cfa_loc.GetRegisterNumber();
-    if (llvm::Expected<addr_t> regvalue =
-            ReadRegisterAsAddress(regctx, regkind, regnum))
+    if (llvm::Expected<addr_t> regvalue = ReadRegisterAsAddress(
+            regctx, unwind_plan.GetRegisterKind(), regnum))
       return *regvalue + cfa_loc.GetOffset();
     else
       return regvalue;
@@ -2513,13 +2520,8 @@ static UnwindPlanSP GetUnwindPlanForAsyncRegister(FuncUnwinders &unwinders,
   return unwinders.GetUnwindPlanAtNonCallSite(target, thread);
 }
 
-/// Attempts to use UnwindPlans that inspect assembly to recover the entry value
-/// of the async context register. This is a simplified version of the methods
-/// in RegisterContextUnwind, since plumbing access to those here would be
-/// challenging.
-static llvm::Expected<addr_t> ReadAsyncContextRegisterFromUnwind(
-    SymbolContext &sc, Process &process, Address pc, Address func_start_addr,
-    RegisterContext &regctx, AsyncUnwindRegisterNumbers regnums) {
+static llvm::Expected<UnwindPlanSP>
+GetAsmUnwindPlan(Address pc, SymbolContext &sc, Thread &thread) {
   FuncUnwindersSP unwinders =
       pc.GetModule()->GetUnwindTable().GetFuncUnwindersContainingAddress(pc,
                                                                          sc);
@@ -2528,77 +2530,132 @@ static llvm::Expected<addr_t> ReadAsyncContextRegisterFromUnwind(
                                    "function unwinder at address 0x%8.8" PRIx64,
                                    pc.GetFileAddress());
 
-  Target &target = process.GetTarget();
-  UnwindPlanSP unwind_plan =
-      GetUnwindPlanForAsyncRegister(*unwinders, target, regctx.GetThread());
+  UnwindPlanSP unwind_plan = GetUnwindPlanForAsyncRegister(
+      *unwinders, thread.GetProcess()->GetTarget(), thread);
   if (!unwind_plan)
     return llvm::createStringError(
         "SwiftLanguageRuntime: Failed to find non call site unwind plan at "
         "address 0x%8.8" PRIx64,
         pc.GetFileAddress());
+  return unwind_plan;
+}
 
-  const RegisterKind unwind_regkind = unwind_plan->GetRegisterKind();
-  UnwindPlan::RowSP row = unwind_plan->GetRowForFunctionOffset(
-      pc.GetFileAddress() - func_start_addr.GetFileAddress());
-
-  // To request info about a register from the unwind plan, the register must
-  // be in the same domain as the unwind plan's registers.
-  uint32_t async_reg_unwind_regdomain;
+static llvm::Expected<uint32_t> GetFpRegisterNumber(UnwindPlan &unwind_plan,
+                                                    RegisterContext &regctx) {
+  uint32_t fp_unwind_regdomain;
   if (!regctx.ConvertBetweenRegisterKinds(
-          regnums.GetRegisterKind(), regnums.async_ctx_regnum, unwind_regkind,
-          async_reg_unwind_regdomain)) {
+          lldb::eRegisterKindGeneric, LLDB_REGNUM_GENERIC_FP,
+          unwind_plan.GetRegisterKind(), fp_unwind_regdomain)) {
     // This should never happen.
     // If asserts are disabled, return an error to avoid creating an invalid
     // unwind plan.
-    auto error_msg = "SwiftLanguageRuntime: Failed to convert register domains";
+    const auto *error_msg =
+        "SwiftLanguageRuntime: Failed to convert register domains";
     llvm_unreachable(error_msg);
     return llvm::createStringError(error_msg);
   }
+  return fp_unwind_regdomain;
+}
 
-  // If the plan doesn't have information about the async register, we can use
-  // its current value, as this is a callee saved register.
-  UnwindPlan::Row::AbstractRegisterLocation regloc;
-  if (!row->GetRegisterInfo(async_reg_unwind_regdomain, regloc))
-    return ReadRegisterAsAddress(regctx, regnums.GetRegisterKind(),
-                                 regnums.async_ctx_regnum);
+struct FrameSetupInfo {
+  addr_t frame_setup_func_offset;
+  int fp_cfa_offset;
+};
 
-  // Handle the few abstract locations we are likely to encounter.
-  using RestoreType = UnwindPlan::Row::AbstractRegisterLocation::RestoreType;
-  RestoreType loctype = regloc.GetLocationType();
-  switch (loctype) {
-  case RestoreType::same:
+/// Detect the point in the function where the prologue created a frame,
+/// returning:
+/// 1. The offset of the first instruction after that point. For a frameless
+/// function, this offset is UINT_MAX.
+/// 2. The CFA offset at which FP is stored.
+static llvm::Expected<FrameSetupInfo>
+GetFrameSetupInfo(UnwindPlan &unwind_plan, RegisterContext &regctx) {
+  using RowSP = UnwindPlan::RowSP;
+  using AbstractRegisterLocation = UnwindPlan::Row::AbstractRegisterLocation;
+
+  llvm::Expected<uint32_t> fp_unwind_regdomain =
+      GetFpRegisterNumber(unwind_plan, regctx);
+  if (!fp_unwind_regdomain)
+    return fp_unwind_regdomain.takeError();
+
+  // Look at the first few (4) rows of the plan and store FP's location.
+  const int upper_bound = std::min(4, unwind_plan.GetRowCount());
+  llvm::SmallVector<AbstractRegisterLocation, 4> fp_locs;
+  for (int row_idx = 0; row_idx < upper_bound; row_idx++) {
+    RowSP row = unwind_plan.GetRowAtIndex(row_idx);
+    AbstractRegisterLocation regloc;
+    if (!row->GetRegisterInfo(*fp_unwind_regdomain, regloc))
+      regloc.SetSame();
+    fp_locs.push_back(regloc);
+  }
+
+  // Find first location where FP is stored *at* some CFA offset.
+  auto *it = llvm::find_if(
+      fp_locs, [](auto fp_loc) { return fp_loc.IsAtCFAPlusOffset(); });
+
+  // This is a frameless function, use an infinite offset to represent this.
+  if (it == fp_locs.end())
+    return FrameSetupInfo{std::numeric_limits<addr_t>::max(), 0};
+
+  // This is an async function with a frame. The prologue is typically:
+  // 0. adjust sp
+  // 1. save lr        @ CFA-8
+  // 2. save fp        @ CFA-16  << `it` points to this row.
+  // 3. save async_reg @ CFA-24  << subsequent row.
+  // Use subsequent row, if available.
+  int row_idx = fp_locs.end() - it;
+  int next_row_idx = row_idx + 1;
+
+  // If subsequent row is invalid, approximate through current row.
+  if (next_row_idx == unwind_plan.GetRowCount() ||
+      next_row_idx == upper_bound ||
+      !fp_locs[next_row_idx].IsAtCFAPlusOffset()) {
+    LLDB_LOG(GetLog(LLDBLog::Unwind), "SwiftLanguageRuntime:: UnwindPlan did "
+                                      "not contain a valid row after FP setup");
+    UnwindPlan::RowSP row = unwind_plan.GetRowAtIndex(row_idx);
+    return FrameSetupInfo{row->GetOffset(), fp_locs[row_idx].GetOffset()};
+  }
+
+  UnwindPlan::RowSP subsequent_row = unwind_plan.GetRowAtIndex(next_row_idx);
+  return FrameSetupInfo{subsequent_row->GetOffset(),
+                        fp_locs[next_row_idx].GetOffset()};
+}
+
+/// Reads the async register from its ABI-guaranteed stack-slot, or directly
+/// from the register depending on where pc is relative to the start of the
+/// function.
+static llvm::Expected<addr_t> ReadAsyncContextRegisterFromUnwind(
+    SymbolContext &sc, Process &process, Address pc, Address func_start_addr,
+    RegisterContext &regctx, AsyncUnwindRegisterNumbers regnums) {
+  llvm::Expected<UnwindPlanSP> unwind_plan =
+      GetAsmUnwindPlan(pc, sc, regctx.GetThread());
+  if (!unwind_plan)
+    return unwind_plan.takeError();
+  llvm::Expected<FrameSetupInfo> frame_setup =
+      GetFrameSetupInfo(**unwind_plan, regctx);
+  if (!frame_setup)
+    return frame_setup.takeError();
+
+  // Is PC before the frame formation? If so, use async register directly.
+  // This handles frameless functions, as frame_setup_func_offset is INT_MAX.
+  addr_t pc_offset = pc.GetFileAddress() - func_start_addr.GetFileAddress();
+  if (pc_offset < frame_setup->frame_setup_func_offset)
     return ReadRegisterAsAddress(regctx, regnums.GetRegisterKind(),
                                  regnums.async_ctx_regnum);
-  case RestoreType::inOtherRegister: {
-    unsigned regnum = regloc.GetRegisterNumber();
-    return ReadRegisterAsAddress(regctx, unwind_regkind, regnum);
-  }
-  case RestoreType::atCFAPlusOffset: {
-    llvm::Expected<addr_t> cfa =
-        GetCFA(process, regctx, unwind_regkind, row->GetCFAValue());
-    if (!cfa)
-      return cfa.takeError();
-    return ReadPtrFromAddr(process, *cfa, regloc.GetOffset());
-  }
-  case RestoreType::isCFAPlusOffset: {
-    if (llvm::Expected<addr_t> cfa =
-            GetCFA(process, regctx, unwind_regkind, row->GetCFAValue()))
-      return *cfa + regloc.GetOffset();
-    else
-      return cfa;
-  }
-  case RestoreType::isConstant:
-    return regloc.GetConstant();
-  case RestoreType::unspecified:
-  case RestoreType::undefined:
-  case RestoreType::atAFAPlusOffset:
-  case RestoreType::isAFAPlusOffset:
-  case RestoreType::isDWARFExpression:
-  case RestoreType::atDWARFExpression:
-    break;
-  }
-  return llvm::createStringError(
-      "SwiftLanguageRuntime: Unsupported register location type = %d", loctype);
+
+  // A frame was formed, and FP was saved at a CFA offset. Compute CFA and read
+  // the location beneath where FP was saved.
+  llvm::Expected<addr_t> cfa =
+      GetCFA(process, regctx, pc_offset, **unwind_plan);
+  if (!cfa)
+    return cfa.takeError();
+
+  addr_t async_reg_addr = process.FixDataAddress(
+      *cfa + frame_setup->fp_cfa_offset - process.GetAddressByteSize());
+  Status error;
+  addr_t async_reg = process.ReadPointerFromMemory(async_reg_addr, error);
+  if (error.Fail())
+    return error.ToError();
+  return async_reg;
 }
 
 /// Returns true if the async register should be dereferenced once to obtain the