i#1569 AArch64: Implement move (immediate) to zero register. (#2423)

Moving a value to the zero register is a no-op but we have seen
ADR XZR,... used by Node.js (libv8). Having these API functions,
instrlist_insert_mov_{immed_ptrsz,instr_addr}, create no instructions
potentially adds a case for the caller to handle but most uses of
those functions set the last two arguments to NULL in any case. If it
were a problem we could create a NOP instruction instead. Creating
actual moves to XZR seems less good and would be awkward if in
generating a constant value we ever used instructions that cannot
target XZR.

core/arch/aarchxx/mangle.c

@@ -1225,26 +1225,36 @@ insert_mov_immed_arch(dcontext_t *dcontext, instr_t *src_inst, byte *encode_esti
 {
 #ifdef AARCH64
     instr_t *mov;
-    uint rt;
     int i;
 
     CLIENT_ASSERT(opnd_is_reg(dst),
                   "AArch64 cannot store an immediate direct to memory");
-    rt = opnd_get_reg(dst) - DR_REG_X0;
-    ASSERT(rt < 31);
+
+    if (opnd_get_reg(dst) == DR_REG_XZR) {
+        /* Moving a value to the zero register is a no-op. We insert nothing,
+         * so *first and *last are set to NULL. Caller beware!
+         */
+        if (first != NULL)
+            *first = NULL;
+        if (last != NULL)
+            *last = NULL;
+        return;
+    }
+
+    ASSERT((uint)(opnd_get_reg(dst) - DR_REG_X0) < 31);
     if (src_inst != NULL)
         val = (ptr_int_t)encode_estimate;
 
-    /* movz x(rt), #(val & 0xffff) */
-    mov = INSTR_CREATE_movz(dcontext, opnd_create_reg(DR_REG_X0 + rt),
+    /* movz x(dst), #(val & 0xffff) */
+    mov = INSTR_CREATE_movz(dcontext, dst,
                             OPND_CREATE_INT16(val & 0xffff), OPND_CREATE_INT8(0));
     PRE(ilist, instr, mov);
     if (first != NULL)
         *first = mov;
     for (i = 1; i < 4; i++) {
         if ((val >> (16 * i) & 0xffff) != 0) {
-            /* movk x(rt), #(val >> sh & 0xffff), lsl #(sh) */
-            mov = INSTR_CREATE_movk(dcontext, opnd_create_reg(DR_REG_X0 + rt),
+            /* movk x(dst), #(val >> sh & 0xffff), lsl #(sh) */
+            mov = INSTR_CREATE_movk(dcontext, dst,
                                     OPND_CREATE_INT16((val >> 16 * i) & 0xffff),
                                     OPND_CREATE_INT8(i * 16));
             PRE(ilist, instr, mov);

core/lib/instrument_api.h

@@ -5546,6 +5546,8 @@ DR_API
  * Pointers to the first and last created meta instructions are returned
  * in \p first and \p last, unless only one meta instruction is created,
  * in which case NULL is returned in last.
+ * If the instruction is a no-op (when dst is the zero register on AArch64)
+ * then no instructions are created and NULL is returned in first and last.
  */
 void
 instrlist_insert_mov_immed_ptrsz(void *drcontext, ptr_int_t val, opnd_t dst,
@@ -5578,6 +5580,8 @@ DR_API
  * Pointers to the first and last created meta instructions are returned
  * in \p first and \p last, unless only one meta instruction is created,
  * in which case NULL is returned in last.
+ * If the instruction is a no-op (when dst is the zero register on AArch64)
+ * then no instructions are created and NULL is returned in first and last.
  */
 void
 instrlist_insert_mov_instr_addr(void *drcontext, instr_t *src_inst,