backend+mu: Add helper to return immediate values

Signed-off-by: Ian Moffett <ian@mirocom.org>

backend/x86_64.c

@@ -1,6 +1,19 @@
 #include <stdio.h>
 #include "rifle/mu.h"
 
+#define retreg(msize)       \
+    (msize >= MSIZE_MAX)    \
+        ? "bad"             \
+        : rettab[(msize)]
+
+/* Size to return register table */
+static const char *rettab[] = {
+    [MSIZE_BYTE]  = "al",
+    [MSIZE_WORD]  = "ax",
+    [MSIZE_DWORD] = "eax",
+    [MSIZE_QWORD] = "rax"
+};
+
 int
 mu_gen_label(struct rifle_state *state, const char *name, bool global)
 {
@@ -25,6 +38,27 @@ mu_gen_label(struct rifle_state *state, const char *name, bool global)
     return 0;
 }
 
+int
+mu_gen_retimm(struct rifle_state *state, ssize_t v, msize_t size)
+{
+    if (state == NULL) {
+        return -1;
+    }
+
+    if (size >= MSIZE_MAX) {
+        return -1;
+    }
+
+    fprintf(
+        state->out_fp,
+        INST("mov %s, %zd"),
+        retreg(size),
+        v
+    );
+
+    return 0;
+}
+
 int
 mu_gen_ret(struct rifle_state *state)
 {