tcg/aarch64: Fix addsub2 for 0+C

When al == xzr, we cannot use addi/subi because that encodes xsp.
Force a zero into the temp register for that (rare) case.
Signed-off-by: NRichard Henderson <rth@twiddle.net>
Message-Id: <20161207180727.6286-2-rth@twiddle.net>

tcg/aarch64/tcg-target.inc.c

@@ -964,6 +964,15 @@ static inline void tcg_out_addsub2(TCGContext *s, int ext, TCGReg rl,
             insn = I3401_SUBSI;
             bl = -bl;
         }
+        if (unlikely(al == TCG_REG_XZR)) {
+            /* ??? We want to allow al to be zero for the benefit of
+               negation via subtraction.  However, that leaves open the
+               possibility of adding 0+const in the low part, and the
+               immediate add instructions encode XSP not XZR.  Don't try
+               anything more elaborate here than loading another zero.  */
+            al = TCG_REG_TMP;
+            tcg_out_movi(s, ext, al, 0);
+        }
         tcg_out_insn_3401(s, insn, ext, rl, al, bl);
     } else {
         tcg_out_insn_3502(s, sub ? I3502_SUBS : I3502_ADDS, ext, rl, al, bl);