[compiler] Rework calculation to start of return slots

- Changes GetOffsetToReturns to take into account return slot padding
  and argument padding.
- Changes GetStackParameterDelta to use GetOffsetToReturns for the SP
  delta calculation.
- Removes GetFirstUnusedStackSlot.

Change-Id: I13df72e86750c62798bae262f0560cf1d7f981db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593306
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72078}

src/compiler/backend/instruction-selector.cc

@@ -3076,7 +3076,7 @@ void InstructionSelector::VisitTailCall(Node* node) {
   // instruction. This is used by backends that need to pad arguments for stack
   // alignment, in order to store an optional slot of padding above the
   // arguments.
-  const int optional_padding_slot = callee->GetFirstUnusedStackSlot();
+  const int optional_padding_slot = callee->GetOffsetToReturns();
   buffer.instruction_args.push_back(g.TempImmediate(optional_padding_slot));
 
   const int first_unused_stack_slot =

src/compiler/linkage.cc

@@ -77,21 +77,6 @@ MachineSignature* CallDescriptor::GetMachineSignature(Zone* zone) const {
   return zone->New<MachineSignature>(return_count, param_count, types);
 }
 
-int CallDescriptor::GetFirstUnusedStackSlot() const {
-  int slots_above_sp = 0;
-  for (size_t i = 0; i < InputCount(); ++i) {
-    LinkageLocation operand = GetInputLocation(i);
-    if (!operand.IsRegister()) {
-      int new_candidate =
-          -operand.GetLocation() + operand.GetSizeInPointers() - 1;
-      if (new_candidate > slots_above_sp) {
-        slots_above_sp = new_candidate;
-      }
-    }
-  }
-  return slots_above_sp;
-}
-
 int CallDescriptor::GetStackParameterDelta(
     CallDescriptor const* tail_caller) const {
   // In the IsTailCallForTierUp case, the callee has
@@ -100,8 +85,8 @@ int CallDescriptor::GetStackParameterDelta(
   // inputs to the TailCall node, since they already exist on the stack.
   if (IsTailCallForTierUp()) return 0;
 
-  int callee_slots_above_sp = GetFirstUnusedStackSlot();
-  int tail_caller_slots_above_sp = tail_caller->GetFirstUnusedStackSlot();
+  int callee_slots_above_sp = GetOffsetToReturns();
+  int tail_caller_slots_above_sp = tail_caller->GetOffsetToReturns();
   int stack_param_delta = callee_slots_above_sp - tail_caller_slots_above_sp;
   if (ShouldPadArguments(stack_param_delta)) {
     if (callee_slots_above_sp % 2 != 0) {
@@ -120,9 +105,37 @@ int CallDescriptor::GetStackParameterDelta(
 }
 
 int CallDescriptor::GetOffsetToReturns() const {
-  int offset = static_cast<int>(StackParameterCount());
-  if (ShouldPadArguments(offset)) offset++;
-  return offset;
+  // If there are return stack slots, return the first slot of the last one.
+  constexpr int kNoReturnSlot = std::numeric_limits<int>::max();
+  int end_of_returns = kNoReturnSlot;
+  for (size_t i = 0; i < ReturnCount(); ++i) {
+    LinkageLocation operand = GetReturnLocation(i);
+    if (!operand.IsRegister()) {
+      // Reverse, since returns have negative offsets in the frame.
+      int reverse_location = -operand.GetLocation() - 1;
+      DCHECK_GE(reverse_location, 0);
+      end_of_returns = std::min(end_of_returns, reverse_location);
+    }
+  }
+  if (end_of_returns != kNoReturnSlot) return end_of_returns;
+
+  // Otherwise, return the first unused slot before the parameters, with any
+  // additional padding slot if it exists.
+  int start_of_args = 0;
+  for (size_t i = 0; i < InputCount(); ++i) {
+    LinkageLocation operand = GetInputLocation(i);
+    if (!operand.IsRegister()) {
+      // Reverse, since arguments have negative offsets in the frame.
+      int reverse_location =
+          -operand.GetLocation() + operand.GetSizeInPointers() - 1;
+      DCHECK_GE(reverse_location, 0);
+      start_of_args = std::max(start_of_args, reverse_location);
+    }
+  }
+
+  if (ShouldPadArguments(start_of_args)) start_of_args++;
+  DCHECK_EQ(start_of_args == 0, StackParameterCount() == 0);
+  return start_of_args;
 }
 
 int CallDescriptor::GetTaggedParameterSlots() const {
@@ -138,11 +151,12 @@ int CallDescriptor::GetTaggedParameterSlots() const {
 
 bool CallDescriptor::CanTailCall(const CallDescriptor* callee) const {
   if (ReturnCount() != callee->ReturnCount()) return false;
-  const int stack_param_delta = callee->GetStackParameterDelta(this);
+  const int stack_returns_delta =
+      GetOffsetToReturns() - callee->GetOffsetToReturns();
   for (size_t i = 0; i < ReturnCount(); ++i) {
     if (GetReturnLocation(i).IsCallerFrameSlot() &&
         callee->GetReturnLocation(i).IsCallerFrameSlot()) {
-      if (GetReturnLocation(i).AsCallerFrameSlot() - stack_param_delta !=
+      if (GetReturnLocation(i).AsCallerFrameSlot() + stack_returns_delta !=
           callee->GetReturnLocation(i).AsCallerFrameSlot()) {
         return false;
       }

src/compiler/linkage.h

@@ -389,12 +389,11 @@ class V8_EXPORT_PRIVATE CallDescriptor final
 
   bool UsesOnlyRegisters() const;
 
-  // Returns the first stack slot that is not used by the stack parameters.
-  int GetFirstUnusedStackSlot() const;
-
   int GetStackParameterDelta(const CallDescriptor* tail_caller) const;
 
-  // Returns the number of slots to the first return value slot.
+  // If there are return stack slots, returns the first slot of the last one.
+  // Otherwise, return the first unused slot before the parameters. This is the
+  // slot where returns would go if there were any.
   int GetOffsetToReturns() const;
 
   int GetTaggedParameterSlots() const;

test/unittests/compiler/linkage-tail-call-unittest.cc

@@ -29,16 +29,26 @@ class LinkageTailCall : public TestWithZone {
     DCHECK(arraysize(kMachineTypes) >=
            locations->return_count() + locations->parameter_count());
     USE(kMachineTypes);
+    size_t stack_arguments = 0;
+    for (size_t i = 0; i < locations->parameter_count(); ++i) {
+      if (locations->GetParam(i).IsCallerFrameSlot()) stack_arguments++;
+    }
+    size_t stack_returns = 0;
+    for (size_t i = 0; i < locations->return_count(); ++i) {
+      if (locations->GetReturn(i).IsCallerFrameSlot()) stack_returns++;
+    }
     return zone()->New<CallDescriptor>(
         CallDescriptor::kCallCodeObject, MachineType::AnyTagged(),
         LinkageLocation::ForAnyRegister(MachineType::Pointer()),
         locations,  // location_sig
-        0,                         // js_parameter_count
+        stack_arguments,
         Operator::kNoProperties,   // properties
         0,                         // callee-saved
         0,                         // callee-saved fp
         CallDescriptor::kNoFlags,  // flags,
-        "");
+        "", StackArgumentOrder::kDefault,
+        0,  // allocatable_registers
+        stack_returns);
   }
 
   LinkageLocation StackLocation(int loc) {