general: Avoid shift undefined behavior

Shifts by 64 are not allowed, so add a conditional guard. Likewise,
left-shifting negative numbers is not allowed in C (unlike C++20, where
it is defined), so avoid such cases.

decode.c

@@ -24,7 +24,8 @@ static uint64_t immlogical(unsigned sf, unsigned N, unsigned immr,
   unsigned esize = 1 << len;
   uint64_t welem = ((uint64_t)1 << (s + 1)) - 1;
   // ROR(welem, r) as bits(esize)
-  welem = (welem >> r) | (welem << (esize - r));
+  if (r)
+    welem = (welem >> r) | (welem << (esize - r));
   if (esize < 64)
     welem &= ((uint64_t)1 << esize) - 1;
   // Replicate(ROR(welem, r))

parse.py

@@ -548,10 +548,10 @@ def add_mnem(self, grp: str, name: str, grpdesc: Desc, descmask: MaskedVal, ops:
         # use XOR s.t. immadd can toggle between add and sub.
         buildexprs = ["(" + "^".join([f"{descmask.val:#x}"] + grandExpansions) + ")"]
         for de in grpdesc:
-            bitmask = ((1 << de.size) - 1) << de.idx
+            bitmask = (1 << de.size) - 1
             if de.name in assignments:
-                buildexprs.append(f"(({assignments[de.name]})<<{de.idx}&{bitmask:#x})")
-            elif descmask.mask & bitmask != bitmask:
+                buildexprs.append(f"((({assignments[de.name]})&{bitmask:#x})<<{de.idx})")
+            elif descmask.mask & (bitmask << de.idx) != (bitmask << de.idx):
                 print("Encode desc missing for", grp, name, de.name, assignments)
                 return
         expr = "|".join(buildexprs)