[optimizing] Improve x86 parallel moves/swaps

Add a new constructor to ScratchRegisterScope that will supply a
register if there is a free one, but not spill to force one.  Use this
to generated alternate code that doesn't use a temporary, as the
spill/restore of a register generates extra instructions that aren't
necessary on x86.

Here is the benefit for a 32 bit memory-to-memory exchange with no
free registers:
<        50    	       push eax
<        53    	       push ebx
<  8B44244C    	       mov eax, [esp + 76]
<  8B5C246C    	       mov ebx, [esp + 108]
<  8944246C    	       mov [esp + 108], eax
<  895C244C    	       mov [esp + 76], ebx
<        5B    	       pop ebx
<        58    	       pop eax
---
>  FF742444    	       push [esp + 68]
>  FF742468    	       push [esp + 104]
>  8F44244C    	       pop [esp + 72]
>  8F442468    	       pop [esp + 100]

Avoid using xchg instruction, as it is slow on smaller processors.

Change-Id: Id29ee3abd998577baaee552d55d23e60ae0c7871
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/compiler/optimizing/parallel_move_resolver.h b/compiler/optimizing/parallel_move_resolver.h
index 3fa1b37..173cffc 100644
--- a/compiler/optimizing/parallel_move_resolver.h
+++ b/compiler/optimizing/parallel_move_resolver.h
@@ -42,10 +42,15 @@ class ParallelMoveResolver : public ValueObject {
  protected:
   class ScratchRegisterScope : public ValueObject {
    public:
+    // Spill a scratch register if no regs are free.
     ScratchRegisterScope(ParallelMoveResolver* resolver,
                          int blocked,
                          int if_scratch,
                          int number_of_registers);
+    // Grab a scratch register only if available.
+    ScratchRegisterScope(ParallelMoveResolver* resolver,
+                         int blocked,
+                         int number_of_registers);
     ~ScratchRegisterScope();
 
     int GetRegister() const { return reg_; }
@@ -62,6 +67,8 @@ class ParallelMoveResolver : public ValueObject {
   // Allocate a scratch register for performing a move. The method will try to use
   // a register that is the destination of a move, but that move has not been emitted yet.
   int AllocateScratchRegister(int blocked, int if_scratch, int register_count, bool* spilled);
+  // As above, but return -1 if no free register.
+  int AllocateScratchRegister(int blocked, int register_count);
 
   // Emit a move.
   virtual void EmitMove(size_t index) = 0;