[ptr-compr] Add a switch for branchless/branchful decompression

Bug: v8:7703
Change-Id: Ic6cd8b337813ecff2a0d030aa3a57304e784378a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1511486Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60391}

src/globals.h

@@ -225,6 +225,10 @@ using AtomicTagged_t = base::AtomicWord;
 
 #endif  // V8_COMPRESS_POINTERS
 
+// Defines whether the branchless or branchful implementation of pointer
+// decompression should be used.
+constexpr bool kUseBranchlessPtrDecompression = true;
+
 STATIC_ASSERT(kTaggedSize == (1 << kTaggedSizeLog2));
 
 using AsAtomicTagged = base::AsAtomicPointerImpl<AtomicTagged_t>;

src/ptr-compr-inl.h

@@ -42,10 +42,16 @@ V8_INLINE Address DecompressTaggedAny(Address on_heap_addr,
   // 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) {
     // |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 & GetRootFromOnHeapAddress(on_heap_addr);
     return root_or_zero + static_cast<Address>(value);
+  } else {
+    return HAS_SMI_TAG(value) ? static_cast<Address>(value)
+                              : (GetRootFromOnHeapAddress(on_heap_addr) +
+                                 static_cast<Address>(value));
+  }
 }
 
 #ifdef V8_COMPRESS_POINTERS

src/x64/macro-assembler-x64.cc

@@ -307,6 +307,7 @@ void TurboAssembler::DecompressAnyTagged(Register destination,
   DCHECK(!AreAliased(destination, scratch));
   RecordComment("[ DecompressAnyTagged");
   movsxlq(destination, field_operand);
+  if (kUseBranchlessPtrDecompression) {
     // Branchlessly compute |masked_root|:
     // masked_root = HAS_SMI_TAG(destination) ? 0 : kRootRegister;
     STATIC_ASSERT((kSmiTagSize == 1) && (kSmiTag < 32));
@@ -315,9 +316,15 @@ void TurboAssembler::DecompressAnyTagged(Register destination,
     andl(masked_root, Immediate(kSmiTagMask));
     negq(masked_root);
     andq(masked_root, kRootRegister);
-  // Now this add operation will either leave the value unchanged if it is a smi
-  // or add the isolate root if it is a heap object.
+    // Now this add operation will either leave the value unchanged if it is
+    // a smi or add the isolate root if it is a heap object.
     addq(destination, masked_root);
+  } else {
+    Label done;
+    JumpIfSmi(destination, &done);
+    addq(destination, kRootRegister);
+    bind(&done);
+  }
   RecordComment("]");
 }