Use a fixed input register where we require a byte register.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
===================================================================
--- src/x64/lithium-x64.cc	(revision 7402)
+++ src/x64/lithium-x64.cc	(working copy)
@@ -1873,11 +1873,22 @@
   ASSERT(instr->key()->representation().IsInteger32());
 
   LOperand* external_pointer = UseRegister(instr->external_pointer());
-  bool val_is_temp_register = array_type == kExternalPixelArray ||
-      array_type == kExternalFloatArray;
-  LOperand* val = val_is_temp_register
-      ? UseTempRegister(instr->value())
-      : UseRegister(instr->key());
+  LOperand* val = NULL;
+  switch (array_type) {
+    case kExternalPixelArray:

[danno, 2011/03/29 14:25:34]

Doesn't pixel array still need to be a temp, since the clamping code overwrites
rax and you don't want to accidentally use it later expecting the contents to
still be val. 

[fschneider, 2011/03/29 14:43:58]

That would be ok, since UseFixed is always a fixed writable input register. The
naming of UseFixed is however not ideal.

+    case kExternalByteArray:
+    case kExternalUnsignedByteArray:
+      // We need a byte register in these cases.
+      val = UseFixed(instr->value(), rax);

[danno, 2011/03/29 14:25:34]

Do you really need to use rax? On 64 bit the byte store should work for any
register with the REX prefix that's handled by the assembler?

[fschneider, 2011/03/29 14:43:58]

You're right. I didn't realize x64 allows that. I don't need to change anything
on x64 then.

+      break;
+    case kExternalFloatArray:
+      // We need a writable register here.
+      val = UseTempRegister(instr->value());
+      break;
+    default:
+      val = UseRegister(instr->value());
+      break;
+  }
   LOperand* key = UseRegister(instr->key());
 
   return new LStoreKeyedSpecializedArrayElement(external_pointer,