test: Add tests on register management for strjoin()

The current register management in the code generator needs overhaul.
Here are some tests to check operations in strjoin().  Some of these
tests are marked @@xfail until the overhaul has been done.

For example, here is what happens in tst.reg_spilling5.d.

The D script is:

  trace(strjoin("abc",
                strjoin("def",
                        strjoin("ghi",
                                strjoin("jkl",
                                        strjoin("mno",
                                                "pqrstuvwx"
                                               )
                                       )
                               )
                       )
               )
       );

The generated code basically does this:
    r8 = pointer to "abc"
    r7 = pointer to "def"
    r6 = pointer to "ghi"
    r5 = pointer to "jkl"
    r4 = pointer to "mno"
    r3 = pointer to "pqrstuvwx"

Then it does the innermost strjoin:
    r2 = pointer to tstring(0)
    spill %r2
    spill %r3
    spill %r4
    spill %r5
    dt_strjoin(tstring(0), "mno", "pqrstuvwx");
    restore %r2
    restore %r5

Then it starts the next strjoin:
    r4 = pointer to tstring(1)
    spill %r2
    spill %r4
    spill %r5

Next, it must fill the arguments for the dt_strjoin(dst, s1, s2) call.
It uses the following code from dt_cg_subr_strjoin():
    BPF_MOV_REG(BPF_REG_1, dnp->dn_reg);
    BPF_MOV_REG(BPF_REG_2, s1->dn_reg);
    dt_regset_free(s1->dn_reg);
    BPF_MOV_REG(BPF_REG_3, s2->dn_reg);
    dt_regset_free(s2->dn_reg);

At this point, however:
    r4  is  dst
    r5  is  s1
    r2  is  s2
So the code is:
    r1 = r4        // loads dst into r1, okay
    r2 = r5        // loads s1 into r2, but overwrites s2!
    fill r5        // okay, we do not care
    r3 = r2        // uses overwritten value!
    fill r2        // overwrites a function arg!

Careful register management in dt_cg_subr_strjoin() could fix this
problem, but resolution will be left for the larger overhaul.

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

test/unittest/codegen/tst.reg_spilling2.d

@@ -0,0 +1,23 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION: Test that the code generator's spill/fill works with strjoin().
+ */
+
+#pragma D option quiet
+
+BEGIN
+{
+  trace(strjoin("abc",
+                strjoin("def",
+                        "ghijklmnopqrstuvwx"
+                       )
+               )
+       );
+  exit(0);
+}

test/unittest/codegen/tst.reg_spilling2.r

@@ -0,0 +1 @@
+abcdefghijklmnopqrstuvwx

test/unittest/codegen/tst.reg_spilling3.d

@@ -0,0 +1,25 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION: Test that the code generator's spill/fill works with strjoin().
+ */
+
+#pragma D option quiet
+
+BEGIN
+{
+  trace(strjoin("abc",
+                strjoin("def",
+                        strjoin("ghi",
+                                "jklmnopqrstuvwx"
+                               )
+                       )
+               )
+       );
+  exit(0);
+}

test/unittest/codegen/tst.reg_spilling3.r

@@ -0,0 +1 @@
+abcdefghijklmnopqrstuvwx

test/unittest/codegen/tst.reg_spilling4.d

@@ -0,0 +1,27 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION: Test that the code generator's spill/fill works with strjoin().
+ */
+
+#pragma D option quiet
+
+BEGIN
+{
+  trace(strjoin("abc",
+                strjoin("def",
+                        strjoin("ghi",
+                                strjoin("jkl",
+                                        "mnopqrstuvwx"
+                                       )
+                               )
+                       )
+               )
+       );
+  exit(0);
+}

test/unittest/codegen/tst.reg_spilling4.r

@@ -0,0 +1 @@
+abcdefghijklmnopqrstuvwx

test/unittest/codegen/tst.reg_spilling5.d

@@ -0,0 +1,30 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION: Test that the code generator's spill/fill works with strjoin().
+ */
+/* @@xfail: DTv2 register management */
+
+#pragma D option quiet
+
+BEGIN
+{
+  trace(strjoin("abc",
+                strjoin("def",
+                        strjoin("ghi",
+                                strjoin("jkl",
+                                        strjoin("mno",
+                                                "pqrstuvwx"
+                                               )
+                                       )
+                               )
+                       )
+               )
+       );
+  exit(0);
+}

test/unittest/codegen/tst.reg_spilling5.r

@@ -0,0 +1 @@
+abcdefghijklmnopqrstuvwx

test/unittest/codegen/tst.reg_spilling6.d

@@ -0,0 +1,32 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION: Test that the code generator's spill/fill works with strjoin().
+ */
+/* @@xfail: DTv2 register management */
+
+#pragma D option quiet
+
+BEGIN
+{
+  trace(strjoin("abc",
+                strjoin("def",
+                        strjoin("ghi",
+                                strjoin("jkl",
+                                        strjoin("mno",
+                                                strjoin("pqr",
+                                                        "stuvwx"
+                                                       )
+                                               )
+                                       )
+                               )
+                       )
+               )
+       );
+  exit(0);
+}

test/unittest/codegen/tst.reg_spilling6.r

@@ -0,0 +1 @@
+abcdefghijklmnopqrstuvwx

test/unittest/codegen/tst.reg_spilling7.d

@@ -0,0 +1,32 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION: Test that the code generator's spill/fill works with strjoin().
+ */
+/* @@xfail: DTv2 register management */
+
+#pragma D option quiet
+
+BEGIN
+{
+  trace(strjoin("abc",
+                strjoin("def",
+                        strjoin("ghi",
+                                strjoin("jkl",
+                                        strjoin("mno",
+                                                strjoin("pqr",
+                                                        strjoin("stu", "vwx")
+                                                       )
+                                               )
+                                       )
+                               )
+                       )
+               )
+       );
+  exit(0);
+}

test/unittest/codegen/tst.reg_spilling7.r

@@ -0,0 +1 @@
+abcdefghijklmnopqrstuvwx