X64: Remove restriction on using r12 as index register.

Review URL: http://codereview.chromium.org/126045


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2155 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/assembler-x64-inl.h

@@ -260,7 +260,8 @@ void Operand::set_modrm(int mod, Register rm) {
 void Operand::set_sib(ScaleFactor scale, Register index, Register base) {
   ASSERT(len_ == 1);
   ASSERT(is_uint2(scale));
-  // Use SIB with no index register only for base rsp or r12.
+  // Use SIB with no index register only for base rsp or r12. Otherwise we
+  // would skip the SIB byte entirely.
   ASSERT(!index.is(rsp) || base.is(rsp) || base.is(r12));
   buf_[1] = scale << 6 | (index.code() & 0x7) << 3 | (base.code() & 0x7);
   rex_ |= (index.code() >> 3) << 1 | base.code() >> 3;

src/x64/assembler-x64.cc

@@ -96,12 +96,12 @@ Operand::Operand(Register base,
                  Register index,
                  ScaleFactor scale,
                  int32_t disp): rex_(0) {
-  ASSERT(!index.is(rsp) && !index.is(r12));
+  ASSERT(!index.is(rsp));
   len_ = 1;
   set_sib(scale, index, base);
   if (disp == 0 && !base.is(rbp) && !base.is(r13)) {
-    // The call to set_modrm doesn't overwrite the REX.B bit possibly set
-    // by set_sib.
+    // This call to set_modrm doesn't overwrite the REX.B (or REX.X) bits
+    // possibly set by set_sib.
     set_modrm(0, rsp);
   } else if (is_int8(disp)) {
     set_modrm(1, rsp);