target-arm: A64: Avoid left shifting negative integers in disas_pc_rel_addr

Shifting a negative integer left is undefined behaviour in C.
Avoid it by assembling and shifting the offset fields as
unsigned values and then sign extending as the final action.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 1423233250-15853-4-git-send-email-peter.maydell@linaro.org

target-arm/translate-a64.c

@@ -2662,11 +2662,12 @@ static void disas_pc_rel_adr(DisasContext *s, uint32_t insn)
 {
     unsigned int page, rd;
     uint64_t base;
-    int64_t offset;
+    uint64_t offset;
 
     page = extract32(insn, 31, 1);
     /* SignExtend(immhi:immlo) -> offset */
-    offset = ((int64_t)sextract32(insn, 5, 19) << 2) | extract32(insn, 29, 2);
+    offset = sextract64(insn, 5, 19);
+    offset = offset << 2 | extract32(insn, 29, 2);
     rd = extract32(insn, 0, 5);
     base = s->pc - 4;