[ptr-compr][x64] Tweak isolate root bias value and decompression code

... in order to improve quality of C++ assembly.

This CL also switches C++ code to use branchful decompression.

Bug: v8:9353
Change-Id: Id6a5cc5db2ad729b4394cd541a7ec8035c0d4571
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1677204
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62372}

src/codegen/arm64/macro-assembler-arm64.cc

@@ -2725,7 +2725,7 @@ void TurboAssembler::DecompressAnyTagged(const Register& destination,
                                          const MemOperand& field_operand) {
   RecordComment("[ DecompressAnyTagged");
   Ldrsw(destination, field_operand);
-  if (kUseBranchlessPtrDecompression) {
+  if (kUseBranchlessPtrDecompressionInGeneratedCode) {
     UseScratchRegisterScope temps(this);
     // Branchlessly compute |masked_root|:
     // masked_root = HAS_SMI_TAG(destination) ? 0 : kRootRegister;
@@ -2749,7 +2749,7 @@ void TurboAssembler::DecompressAnyTagged(const Register& destination,
 void TurboAssembler::DecompressAnyTagged(const Register& destination,
                                          const Register& source) {
   RecordComment("[ DecompressAnyTagged");
-  if (kUseBranchlessPtrDecompression) {
+  if (kUseBranchlessPtrDecompressionInGeneratedCode) {
     UseScratchRegisterScope temps(this);
     // Branchlessly compute |masked_root|:
     // masked_root = HAS_SMI_TAG(destination) ? 0 : kRootRegister;

src/codegen/x64/constants-x64.h

@@ -12,7 +12,8 @@ namespace internal {
 // Actual value of root register is offset from the root array's start
 // to take advantage of negative displacement values.
 // TODO(sigurds): Choose best value.
-constexpr int kRootRegisterBias = 128;
+// TODO(ishell): Choose best value for ptr-compr.
+constexpr int kRootRegisterBias = kSystemPointerSize == kTaggedSize ? 128 : 0;
 
 constexpr size_t kMaxPCRelativeCodeRangeInMB = 2048;
 }  // namespace internal

src/codegen/x64/macro-assembler-x64.cc

@@ -317,7 +317,7 @@ void TurboAssembler::DecompressTaggedPointer(Register destination,
 
 void TurboAssembler::DecompressRegisterAnyTagged(Register destination,
                                                  Register scratch) {
-  if (kUseBranchlessPtrDecompression) {
+  if (kUseBranchlessPtrDecompressionInGeneratedCode) {
     // Branchlessly compute |masked_root|:
     // masked_root = HAS_SMI_TAG(destination) ? 0 : kRootRegister;
     STATIC_ASSERT((kSmiTagSize == 1) && (kSmiTag < 32));

src/common/globals.h

@@ -239,7 +239,8 @@ using AtomicTagged_t = base::AtomicWord;
 
 // Defines whether the branchless or branchful implementation of pointer
 // decompression should be used.
-constexpr bool kUseBranchlessPtrDecompression = true;
+constexpr bool kUseBranchlessPtrDecompressionInRuntime = false;
+constexpr bool kUseBranchlessPtrDecompressionInGeneratedCode = true;
 
 STATIC_ASSERT(kTaggedSize == (1 << kTaggedSizeLog2));
 STATIC_ASSERT((kTaggedSize == 8) == TAGGED_SIZE_8_BYTES);

src/common/ptr-compr-inl.h

@@ -40,10 +40,8 @@ V8_INLINE Address GetIsolateRoot<Isolate*>(Isolate* isolate) {
 
 // Decompresses smi value.
 V8_INLINE Address DecompressTaggedSigned(Tagged_t raw_value) {
-  // Current compression scheme requires |raw_value| to be sign-extended
-  // from int32_t to intptr_t.
-  intptr_t value = static_cast<intptr_t>(static_cast<int32_t>(raw_value));
-  return static_cast<Address>(value);
+  // For runtime code the upper 32-bits of the Smi value do not matter.
+  return static_cast<Address>(raw_value);
 }
 
 // Decompresses weak or strong heap object pointer or forwarding pointer,
@@ -62,18 +60,18 @@ V8_INLINE Address DecompressTaggedPointer(TOnHeapAddress on_heap_addr,
 template <typename TOnHeapAddress>
 V8_INLINE Address DecompressTaggedAny(TOnHeapAddress on_heap_addr,
                                       Tagged_t raw_value) {
-  // Current compression scheme requires |raw_value| to be sign-extended
-  // from int32_t to intptr_t.
-  intptr_t value = static_cast<intptr_t>(static_cast<int32_t>(raw_value));
-  if (kUseBranchlessPtrDecompression) {
+  if (kUseBranchlessPtrDecompressionInRuntime) {
+    // Current compression scheme requires |raw_value| to be sign-extended
+    // from int32_t to intptr_t.
+    intptr_t value = static_cast<intptr_t>(static_cast<int32_t>(raw_value));
     // |root_mask| is 0 if the |value| was a smi or -1 otherwise.
     Address root_mask = static_cast<Address>(-(value & kSmiTagMask));
     Address root_or_zero = root_mask & GetIsolateRoot(on_heap_addr);
     return root_or_zero + static_cast<Address>(value);
   } else {
-    return HAS_SMI_TAG(value)
-               ? static_cast<Address>(value)
-               : (GetIsolateRoot(on_heap_addr) + static_cast<Address>(value));
+    return HAS_SMI_TAG(raw_value)
+               ? DecompressTaggedSigned(raw_value)
+               : DecompressTaggedPointer(on_heap_addr, raw_value);
   }
 }