[ic] Fix megamorphic stub cache probing on some platforms.

src/ic/arm64/stub-cache-arm64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if V8_TARGET_ARCH_ARM64
 
 #include "src/codegen.h"
 #include "src/ic/ic.h"
 #include "src/ic/stub-cache.h"
 #include "src/interface-descriptors.h"
@@ skipping 29 matching lines @@
   uintptr_t map_off_addr = reinterpret_cast<uintptr_t>(map_offset.address());
 
   Label miss;
 
   DCHECK(!AreAliased(name, offset, scratch, scratch2, scratch3));
 
   // Multiply by 3 because there are 3 fields per entry.
   __ Add(scratch3, offset, Operand(offset, LSL, 1));
 
   // Calculate the base address of the entry.
   __ Mov(scratch, key_offset);

[Jakob Kummerow, 2016/07/20 12:09:45]

Why is this instruction even needed? Wouldn't it suffice to do "Add(scratch,
key_offset, ...)" below?

[Igor Sheludko, 2016/07/20 13:40:26]

Unfortunately we don't have such an instruction on arm64. 

[Jakob Kummerow, 2016/07/20 13:50:01]

Argh, my mistake. Somehow I thought |key_offset| was a register. Never mind.

-  __ Add(scratch, scratch, Operand(scratch3, LSL, kPointerSizeLog2));
+  __ Add(
+      scratch, scratch,
+      Operand(scratch3, LSL, kPointerSizeLog2 - StubCache::kCacheIndexShift));
 
   // Check that the key in the entry matches the name.
   __ Ldr(scratch2, MemOperand(scratch));
   __ Cmp(name, scratch2);
   __ B(ne, &miss);
 
   // Check the map matches.
   __ Ldr(scratch2, MemOperand(scratch, map_off_addr - key_off_addr));
   __ Ldr(scratch3, FieldMemOperand(receiver, HeapObject::kMapOffset));
   __ Cmp(scratch2, scratch3);
@@ skipping 59 matching lines @@
 #endif
 
   Counters* counters = masm->isolate()->counters();
   __ IncrementCounter(counters->megamorphic_stub_cache_probes(), 1, extra2,
                       extra3);
 
   // Check that the receiver isn't a smi.
   __ JumpIfSmi(receiver, &miss);
 
   // Compute the hash for primary table.
-  __ Ldr(scratch, FieldMemOperand(name, Name::kHashFieldOffset));
+  __ Ldr(scratch.W(), FieldMemOperand(name, Name::kHashFieldOffset));
   __ Ldr(extra, FieldMemOperand(receiver, HeapObject::kMapOffset));
   __ Add(scratch, scratch, extra);
   __ Eor(scratch, scratch, flags);
-  // We shift out the last two bits because they are not part of the hash.
-  __ Ubfx(scratch, scratch, kCacheIndexShift,
-          CountTrailingZeros(kPrimaryTableSize, 64));
+  __ Mov(extra, Operand(kPrimaryTableSize - 1));
+  __ And(scratch, scratch, Operand(extra, LSL, kCacheIndexShift));

[Jakob Kummerow, 2016/07/20 12:09:45]

IIUC, "And(scratch, scratch, Operand((kPrimaryTableSize - 1) <<
kCacheIndexShift))" should do the right thing under the hood. Again below.

[Igor Sheludko, 2016/07/20 13:40:26]

Indeed. Done here and everywhere.

 
   // Probe the primary table.
   ProbeTable(this, masm, flags, kPrimary, receiver, name, scratch, extra,
              extra2, extra3);
 
   // Primary miss: Compute hash for secondary table.
-  __ Sub(scratch, scratch, Operand(name, LSR, kCacheIndexShift));
-  __ Add(scratch, scratch, flags >> kCacheIndexShift);
-  __ And(scratch, scratch, kSecondaryTableSize - 1);
+  __ Sub(scratch, scratch, Operand(name));
+  __ Add(scratch, scratch, Operand(flags));
+  __ Mov(extra, Operand(kSecondaryTableSize - 1));
+  __ And(scratch, scratch, Operand(extra, LSL, kCacheIndexShift));
 
   // Probe the secondary table.
   ProbeTable(this, masm, flags, kSecondary, receiver, name, scratch, extra,
              extra2, extra3);
 
   // Cache miss: Fall-through and let caller handle the miss by
   // entering the runtime system.
   __ Bind(&miss);
   __ IncrementCounter(counters->megamorphic_stub_cache_misses(), 1, extra2,
                       extra3);
 }
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_ARM64