[turbofan] Make {CallDescriptor} fully immutable.

This changes the way {SaveFPRegsMode} is stored in a {CallDescriptor}.
The value is now encoded in the flags field, similar to all other
boolean properties. It removes the last mutable field from the call
descriptor, making the struct fully immutable.

R=neis@chromium.org

Change-Id: I6bca118c7ce0c5bc77e7aa2fcc6541439bd27b3b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1868616
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64512}

src/compiler/backend/instruction-selector.cc

@@ -2780,8 +2780,10 @@ void InstructionSelector::VisitCall(Node* node, BasicBlock* handler) {
   auto call_descriptor = CallDescriptorOf(node->op());
 
   if (call_descriptor->NeedsCallerSavedRegisters()) {
-    Emit(kArchSaveCallerRegisters | MiscField::encode(static_cast<int>(
-                                        call_descriptor->get_save_fp_mode())),
+    SaveFPRegsMode mode = call_descriptor->NeedsCallerSavedFPRegisters()
+                              ? kSaveFPRegs
+                              : kDontSaveFPRegs;
+    Emit(kArchSaveCallerRegisters | MiscField::encode(static_cast<int>(mode)),
          g.NoOutput());
   }
 
@@ -2862,10 +2864,12 @@ void InstructionSelector::VisitCall(Node* node, BasicBlock* handler) {
   EmitPrepareResults(&(buffer.output_nodes), call_descriptor, node);
 
   if (call_descriptor->NeedsCallerSavedRegisters()) {
-    Emit(kArchRestoreCallerRegisters |
-             MiscField::encode(
-                 static_cast<int>(call_descriptor->get_save_fp_mode())),
-         g.NoOutput());
+    SaveFPRegsMode mode = call_descriptor->NeedsCallerSavedFPRegisters()
+                              ? kSaveFPRegs
+                              : kDontSaveFPRegs;
+    Emit(
+        kArchRestoreCallerRegisters | MiscField::encode(static_cast<int>(mode)),
+        g.NoOutput());
   }
 }
 

src/compiler/linkage.h

@@ -212,9 +212,12 @@ class V8_EXPORT_PRIVATE CallDescriptor final
     kFixedTargetRegister = 1u << 7,
     kAllowCallThroughSlot = 1u << 8,
     kCallerSavedRegisters = 1u << 9,
+    // The kCallerSavedFPRegisters only matters (and set) when the more general
+    // flag for kCallerSavedRegisters above is also set.
+    kCallerSavedFPRegisters = 1u << 10,
     // AIX has a function descriptor which we will set to true by default
     // for all CFunction calls (only used for Kind::kCallAddress).
-    kHasFunctionDescriptor = 1u << 10,
+    kHasFunctionDescriptor = 1u << 11,
   };
   using Flags = base::Flags<Flag>;
 
@@ -296,6 +299,9 @@ class V8_EXPORT_PRIVATE CallDescriptor final
   bool NeedsCallerSavedRegisters() const {
     return flags() & kCallerSavedRegisters;
   }
+  bool NeedsCallerSavedFPRegisters() const {
+    return flags() & kCallerSavedFPRegisters;
+  }
   bool HasFunctionDescriptor() const {
     return flags() & kHasFunctionDescriptor;
   }
@@ -354,13 +360,8 @@ class V8_EXPORT_PRIVATE CallDescriptor final
     return allocatable_registers_ != 0;
   }
 
-  void set_save_fp_mode(SaveFPRegsMode mode) { save_fp_mode_ = mode; }
-
-  SaveFPRegsMode get_save_fp_mode() const { return save_fp_mode_; }
-
  private:
   friend class Linkage;
-  SaveFPRegsMode save_fp_mode_ = kSaveFPRegs;
 
   const Kind kind_;
   const MachineType target_type_;

src/compiler/raw-machine-assembler.cc

@@ -715,14 +715,14 @@ Node* CallCFunctionImpl(
   builder.AddReturn(return_type);
   for (const auto& arg : args) builder.AddParam(arg.first);
 
+  bool caller_saved_fp_regs = caller_saved_regs && (mode == kSaveFPRegs);
   CallDescriptor::Flags flags = CallDescriptor::kNoFlags;
   if (caller_saved_regs) flags |= CallDescriptor::kCallerSavedRegisters;
+  if (caller_saved_fp_regs) flags |= CallDescriptor::kCallerSavedFPRegisters;
   if (has_function_descriptor) flags |= CallDescriptor::kHasFunctionDescriptor;
   auto call_descriptor =
       Linkage::GetSimplifiedCDescriptor(rasm->zone(), builder.Build(), flags);
 
-  if (caller_saved_regs) call_descriptor->set_save_fp_mode(mode);
-
   base::SmallVector<Node*, kNumCArgs> nodes(args.size() + 1);
   nodes[0] = function;
   std::transform(