x86: Use operand size 4 when it would be 2 for cmpxchg8b.

This means the instruction is treated as cmpxchg8b when the effective
operand size is 16 bits.

Change-Id: I4d9bb295f96097e1746a9bbccb2c579d14738fab
Reviewed-on: https://gem5-review.googlesource.com/6603
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

src/arch/x86/isa/insts/general_purpose/semaphores.py

@@ -132,6 +132,7 @@ def macroop XADD_R_R {
 # of dataSize.
 cmpxchg8bCode = '''
 def macroop CMPXCHG8B_%(suffix)s {
+    .adjust_env clampOsz
     %(rdip)s
     lea t1, seg, %(sib)s, disp, dataSize=asz
     ldsplit%(l)s (t2, t3), seg, [1, t0, t1], disp=0

src/arch/x86/isa/microasm.isa

@@ -180,6 +180,11 @@ let {{
         env.dataSize = 4;
     '''
 
+    assembler.symbols["clampOsz"] = '''
+    if (env.dataSize == 2)
+        env.dataSize = 4;
+    '''
+
     def trimImm(width):
         return "adjustedImm = adjustedImm & mask(%s);" % width