[compiler] Always use full version of RecordWrite builtin

When we know that the value in a write barrier is a map, we know that
we are not going to have an old-to-new reference (maps are always in
old generation). Therefore we also don't really need the generational
barrier in RecordWrite. While this is technically correct, we don't
gain much from this optimization. The inline and out-of-line generated
code for the barrier is still the same as in all other cases. Which
means that outside marking we don't even reach the RecordWrite builtin.
Most write barrier executions happen outside incremental marking, hence
performance of the incremental marking barrier isn't critical. This CL
always uses the full RecordWrite builtin using a flag in order to
allow for an easy revert.

This CL is motivated by the shared heap work, which needs an additional
always-on barrier in the future (similar to OLD_TO_NEW) to keep a
OLD_TO_SHARED remembered set up-to-date. While maps are always in the
old generation, they maybe by located in the shared heap.

Bug: v8:11708
Change-Id: I71a6ded2547a0b2bbb9bbbd796dbcae0987b2232
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3471854Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79160}

src/compiler/backend/arm/code-generator-arm.cc

@@ -195,8 +195,10 @@ class OutOfLineRecordWrite final : public OutOfLineCode {
     __ CheckPageFlag(value_, MemoryChunk::kPointersToHereAreInterestingMask, eq,
                      exit());
     RememberedSetAction const remembered_set_action =
-        mode_ > RecordWriteMode::kValueIsMap ? RememberedSetAction::kEmit
-                                             : RememberedSetAction::kOmit;
+        mode_ > RecordWriteMode::kValueIsMap ||
+                FLAG_use_full_record_write_builtin
+            ? RememberedSetAction::kEmit
+            : RememberedSetAction::kOmit;
     SaveFPRegsMode const save_fp_mode = frame()->DidAllocateDoubleRegisters()
                                             ? SaveFPRegsMode::kSave
                                             : SaveFPRegsMode::kIgnore;

src/compiler/backend/arm64/code-generator-arm64.cc

@@ -287,8 +287,10 @@ class OutOfLineRecordWrite final : public OutOfLineCode {
     __ CheckPageFlag(value_, MemoryChunk::kPointersToHereAreInterestingMask, ne,
                      exit());
     RememberedSetAction const remembered_set_action =
-        mode_ > RecordWriteMode::kValueIsMap ? RememberedSetAction::kEmit
-                                             : RememberedSetAction::kOmit;
+        mode_ > RecordWriteMode::kValueIsMap ||
+                FLAG_use_full_record_write_builtin
+            ? RememberedSetAction::kEmit
+            : RememberedSetAction::kOmit;
     SaveFPRegsMode const save_fp_mode = frame()->DidAllocateDoubleRegisters()
                                             ? SaveFPRegsMode::kSave
                                             : SaveFPRegsMode::kIgnore;

src/compiler/backend/ia32/code-generator-ia32.cc

@@ -328,8 +328,10 @@ class OutOfLineRecordWrite final : public OutOfLineCode {
                      exit());
     __ lea(scratch1_, operand_);
     RememberedSetAction const remembered_set_action =
-        mode_ > RecordWriteMode::kValueIsMap ? RememberedSetAction::kEmit
-                                             : RememberedSetAction::kOmit;
+        mode_ > RecordWriteMode::kValueIsMap ||
+                FLAG_use_full_record_write_builtin
+            ? RememberedSetAction::kEmit
+            : RememberedSetAction::kOmit;
     SaveFPRegsMode const save_fp_mode = frame()->DidAllocateDoubleRegisters()
                                             ? SaveFPRegsMode::kSave
                                             : SaveFPRegsMode::kIgnore;

src/compiler/backend/x64/code-generator-x64.cc

@@ -288,8 +288,10 @@ class OutOfLineRecordWrite final : public OutOfLineCode {
     __ leaq(scratch1_, operand_);
 
     RememberedSetAction const remembered_set_action =
-        mode_ > RecordWriteMode::kValueIsMap ? RememberedSetAction::kEmit
-                                             : RememberedSetAction::kOmit;
+        mode_ > RecordWriteMode::kValueIsMap ||
+                FLAG_use_full_record_write_builtin
+            ? RememberedSetAction::kEmit
+            : RememberedSetAction::kOmit;
     SaveFPRegsMode const save_fp_mode = frame()->DidAllocateDoubleRegisters()
                                             ? SaveFPRegsMode::kSave
                                             : SaveFPRegsMode::kIgnore;

src/flags/flag-definitions.h

@@ -482,6 +482,9 @@ DEFINE_BOOL_READONLY(enable_unconditional_write_barriers,
                      V8_ENABLE_UNCONDITIONAL_WRITE_BARRIERS_BOOL,
                      "always use full write barriers")
 
+DEFINE_BOOL(use_full_record_write_builtin, true,
+            "Force use of full version of RecordWrite builtin.")
+
 #ifdef V8_ENABLE_SINGLE_GENERATION
 #define V8_SINGLE_GENERATION_BOOL true
 #else