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

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

Index: src/ic/arm64/stub-cache-arm64.cc
diff --git a/src/ic/arm64/stub-cache-arm64.cc b/src/ic/arm64/stub-cache-arm64.cc
index bd219b0e33851d8735e69df1c77256ab99a49f1c..ba9df0453e4132fc8c17102759f07ee9710b6a00 100644
--- a/src/ic/arm64/stub-cache-arm64.cc
+++ b/src/ic/arm64/stub-cache-arm64.cc
@@ -48,7 +48,9 @@ static void ProbeTable(StubCache* stub_cache, MacroAssembler* masm,
 
   // 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));
@@ -128,22 +130,22 @@ void StubCache::GenerateProbe(MacroAssembler* masm, Register receiver,
   __ 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,