[gdb/testsuite] Fix spurious FAILs with examine-backward.exp, again

Commit 59a561480d ("Fix spurious FAILs with examine-backward.exp") describes
the problem that:
...
The test case examine-backward.exp issues the command "x/-s" after the end
of the first string in TestStrings, but without making sure that this
string is preceded by a string terminator.  Thus GDB may spuriously print
some random characters from before that string, and then the test fails.
...

The commit fixes the problem by adding a Barrier variable before the TestStrings
variable:
...
+const char Barrier[] = { 0x0 };
 const char TestStrings[] = {
...

There is however no guarantee that Barrier is placed immediately before
TestStrings.

Before recent commit 169fe7ab54 ("Change gdb.base/examine-backwards.exp for
AIX.") on x86_64-linux, I see:
...
0000000000400660 R Barrier
0000000000400680 R TestStrings
...

So while the Barrier variable is the first before the TestStrings variable,
it's not immediately preceding TestStrings.

After commit 169fe7ab54:
...
0000000000402259 B Barrier
0000000000402020 D TestStrings
...
they're not even in the same section anymore.

Fix this reliably by adding the zero in the array itself:
...
char TestStringsBase[] = {
  0x0,
  ...
};
char *TestStrings = &TestStringsBase[1];
...
and do likewise for TestStringsH and TestStringsW.

Tested on x86_64-linux.

PR testsuite/31064
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31064

gdb/testsuite/gdb.base/examine-backward.c

@@ -32,15 +32,12 @@ literals.  The content of each array is the same as followings:
   };
 */
 
-/* This is here just to ensure we have a null character before
-   TestStrings, to avoid showing garbage when we look for strings
-   backwards from TestStrings.  */
+unsigned char TestStringsBase[] = {
+  /* This is here just to ensure we have a null character before
+     TestStrings, to avoid showing garbage when we look for strings
+     backwards from TestStrings.  */
+  0x0,
 
-unsigned char Barrier[] = {
-  0x00,
-};
-
-unsigned char TestStrings[] = {
   0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
   0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50,
   0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
@@ -54,7 +51,14 @@ unsigned char TestStrings[] = {
   0x00
 };
 
-short TestStringsH[] = {
+unsigned char *TestStrings = &TestStringsBase[1];
+
+short TestStringsHBase[] = {
+  /* This is here just to ensure we have a null character before
+     TestStringsH, to avoid showing garbage when we look for strings
+     backwards from TestStringsH.  */
+  0x0,
+
   0x0041, 0x0042, 0x0043, 0x0044, 0x0045, 0x0046, 0x0047, 0x0048,
   0x0049, 0x004a, 0x004b, 0x004c, 0x004d, 0x004e, 0x004f, 0x0050,
   0x0051, 0x0052, 0x0053, 0x0054, 0x0055, 0x0056, 0x0057, 0x0058,
@@ -67,7 +71,14 @@ short TestStringsH[] = {
   0x0000
 };
 
-int TestStringsW[] = {
+short *TestStringsH = &TestStringsHBase[1];
+
+int TestStringsWBase[] = {
+  /* This is here just to ensure we have a null character before
+     TestStringsW, to avoid showing garbage when we look for strings
+     backwards from TestStringsW.  */
+  0x0,
+
   0x00000041, 0x00000042, 0x00000043, 0x00000044,
   0x00000045, 0x00000046, 0x00000047, 0x00000048,
   0x00000049, 0x0000004a, 0x0000004b, 0x0000004c,
@@ -89,11 +100,13 @@ int TestStringsW[] = {
   0x00000000
 };
 
+int *TestStringsW = &TestStringsWBase[1];
+
 int
 main (void)
 {
   /* Clang++ eliminates the variables if nothing references them.  */
-  int dummy = Barrier[0] + TestStrings[0] + TestStringsH[0] + TestStringsW[0];
+  int dummy = TestStrings[0] + TestStringsH[0] + TestStringsW[0];
 
   /* Backward disassemble test requires at least 20 instructions in
      this function.  Adding a simple bubble sort.  */

gdb/testsuite/gdb.base/examine-backward.exp

@@ -145,7 +145,7 @@ gdb_test_no_output "set charset ASCII"
 
 with_test_prefix "char-width=1, print-max=20" {
     gdb_test_no_output "set print elements 20"
-    gdb_test_sequence "x/6s &TestStrings" "take 6 strings forward" {
+    gdb_test_sequence "x/6s TestStrings" "take 6 strings forward" {
         "\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
         "\"UVWXYZ\""
         "\"\""
@@ -162,7 +162,7 @@ with_test_prefix "char-width=1, print-max=20" {
         "\"[^\"]+\""
         "\"01234567890123456789\"\.\.\."
     }
-    gdb_test_sequence "x/6s &TestStrings" "take 6 strings forward again" {
+    gdb_test_sequence "x/6s TestStrings" "take 6 strings forward again" {
         "\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
         "\"UVWXYZ\""
         "\"\""
@@ -187,7 +187,7 @@ with_test_prefix "char-width=1, print-max=20" {
 
 with_test_prefix "char-width=2, print-max=20" {
     gdb_test_no_output "set print elements 20"
-    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward" {
+    gdb_test_sequence "x/6sh TestStringsH" "take 6 strings forward" {
         "u\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
         "u\"UVWXYZ\""
         "u\"\""
@@ -204,7 +204,7 @@ with_test_prefix "char-width=2, print-max=20" {
         "u\"[^\"]+\""
         "u\"01234567890123456789\"\.\.\."
     }
-    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward again" {
+    gdb_test_sequence "x/6sh TestStringsH" "take 6 strings forward again" {
         "u\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
         "u\"UVWXYZ\""
         "u\"\""
@@ -229,7 +229,7 @@ with_test_prefix "char-width=2, print-max=20" {
 
 with_test_prefix "char-width=4, print-max=20" {
     gdb_test_no_output "set print elements 20"
-    gdb_test_sequence "x/6sw &TestStringsW" "take 6 strings forward" {
+    gdb_test_sequence "x/6sw TestStringsW" "take 6 strings forward" {
         "U\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
         "U\"UVWXYZ\""
         "U\"\""
@@ -246,7 +246,7 @@ with_test_prefix "char-width=4, print-max=20" {
         "U\"[^\"]+\""
         "U\"01234567890123456789\"\.\.\."
     }
-    gdb_test_sequence "x/6sw &TestStringsW" "take 6 strings forward again" {
+    gdb_test_sequence "x/6sw TestStringsW" "take 6 strings forward again" {
         "U\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
         "U\"UVWXYZ\""
         "U\"\""
@@ -271,7 +271,7 @@ with_test_prefix "char-width=4, print-max=20" {
 
 with_test_prefix "char-width=2, print-max=0" {
     gdb_test_no_output "set print elements 0"
-    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward" {
+    gdb_test_sequence "x/6sh TestStringsH" "take 6 strings forward" {
         "u\"ABCDEFGHIJKLMNOPQRSTUVWXYZ\""
         "u\"\""
         "u\"\""
@@ -289,7 +289,7 @@ with_test_prefix "char-width=2, print-max=0" {
         "u\"012345678901234567890123456789\""
         "u\"!!!!!!\""
     }
-    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward again" {
+    gdb_test_sequence "x/6sh TestStringsH" "take 6 strings forward again" {
         "u\"ABCDEFGHIJKLMNOPQRSTUVWXYZ\""
         "u\"\""
         "u\"\""
@@ -314,7 +314,7 @@ with_test_prefix "char-width=2, print-max=0" {
 
 with_test_prefix "char-width=1, print-max=4" {
     gdb_test_no_output "set print elements 4"
-    gdb_test_sequence "x/9s &TestStrings" "take 9 strings forward" {
+    gdb_test_sequence "x/9s TestStrings" "take 9 strings forward" {
         "\"ABCD\"\.\.\."
         "\"EFGH\"\.\.\."
         "\"IJKL\"\.\.\."