target/s390x: fix PACK reading 1 byte less and writing 1 byte more

PACK fails on the test from the Principles of Operation: F1F2F3F4
becomes 0000234C instead of 0001234C due to an off-by-one error.
Furthermore, it overwrites one extra byte to the left of F1.

If len_dest is 0, then we only want to flip the 1st byte and never loop
over the rest. Therefore, the loop condition should be > and not >=.

If len_src is 1, then we should flip the 1st byte and pack the 2nd.
Since len_src is already decremented before the loop, the first
condition should be >=, and not >.

Likewise for len_src == 2 and the second condition.
Signed-off-by: NPavel Zbitskiy <pavel.zbitskiy@gmail.com>
Message-Id: <20180821025104.19604-7-pavel.zbitskiy@gmail.com>
Reviewed-by: NDavid Hildenbrand <david@redhat.com>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

target/s390x/mem_helper.c

@@ -1019,15 +1019,15 @@ void HELPER(pack)(CPUS390XState *env, uint32_t len, uint64_t dest, uint64_t src)
     len_src--;
 
     /* now pack every value */
-    while (len_dest >= 0) {
+    while (len_dest > 0) {
         b = 0;
 
-        if (len_src > 0) {
+        if (len_src >= 0) {
             b = cpu_ldub_data_ra(env, src, ra) & 0x0f;
             src--;
             len_src--;
         }
-        if (len_src > 0) {
+        if (len_src >= 0) {
             b |= cpu_ldub_data_ra(env, src, ra) << 4;
             src--;
             len_src--;

tests/tcg/s390x/Makefile.target

@@ -5,3 +5,4 @@ TESTS+=csst
 TESTS+=ipm
 TESTS+=exrl-trt
 TESTS+=exrl-trtr
+TESTS+=pack

tests/tcg/s390x/pack.c

+#include <unistd.h>
+
+int main(void)
+{
+    char data[] = {0xaa, 0xaa, 0xf1, 0xf2, 0xf3, 0xc4, 0xaa, 0xaa};
+    char exp[] = {0xaa, 0xaa, 0x00, 0x01, 0x23, 0x4c, 0xaa, 0xaa};
+    int i;
+
+    asm volatile(
+        "    pack 2(4,%[data]),2(4,%[data])\n"
+        :
+        : [data] "r" (&data[0])
+        : "memory");
+    for (i = 0; i < 8; i++) {
+        if (data[i] != exp[i]) {
+            write(1, "bad data\n", 9);
+            return 1;
+        }
+    }
+    return 0;
+}