ARM: More micro-optimization of megamorphic lookup.

src/arm/stub-cache-arm.cc

Index: src/arm/stub-cache-arm.cc
===================================================================
--- src/arm/stub-cache-arm.cc	(revision 10591)
+++ src/arm/stub-cache-arm.cc	(working copy)
@@ -205,8 +205,9 @@
   // Mask down the eor argument to the minimum to keep the immediate
   // ARM-encodable.
   __ eor(scratch, scratch, Operand(flags & mask));
-  // Prefer ubfx to and_ here because the mask is not ARM-encodable.
-  __ Ubfx(scratch, scratch, kHeapObjectTagSize, kPrimaryTableBits);
+  // Prefer and_ to ubfx here because ubfx takes 2 cycles.
+  __ and_(scratch, scratch, Operand(mask));
+  __ mov(scratch, Operand(scratch, LSR, 1));
 
   // Probe the primary table.
   ProbeTable(isolate,
@@ -215,14 +216,17 @@
              kPrimary,
              name,
              scratch,
-             kHeapObjectTagSize,
+             1,
              extra,
              extra2);
 
   // Primary miss: Compute hash for secondary probe.

[Michael Starzinger, 2012/02/02 15:36:08]

Can the following sequence be rewritten like follows? It is your call whether to
do that.

__ sub(scratch, scratch, Operand(name, LSR, 1));
__ add(scratch, scratch, Operand(flags >> 1));
__ and_(scratch,
        scratch,
        Operand((kSecondaryTableSize - 1) << kHeapObjectTagSize - 1));

-  __ rsb(scratch, name, Operand(scratch, LSL, kHeapObjectTagSize));
+  __ rsb(scratch, name, Operand(scratch, LSL, 1));
   __ add(scratch, scratch, Operand(flags));
-  __ Ubfx(scratch, scratch, kHeapObjectTagSize, kSecondaryTableBits);
+  __ and_(scratch,
+          scratch,
+          Operand((kSecondaryTableSize - 1) << kHeapObjectTagSize));
+  __ mov(scratch, Operand(scratch, LSR, 1));
 
   // Probe the secondary table.
   ProbeTable(isolate,
@@ -231,7 +235,7 @@
              kSecondary,
              name,
              scratch,
-             kHeapObjectTagSize,
+             1,
              extra,
              extra2);