[gdb/testsuite] Fix gdb.ada/convvar_comp.exp on s390x-linux

When running test-case gdb.ada/convvar_comp.exp on s390x-linux, I get:
...
(gdb) run ^M
Starting program: pb16_063 ^M
^M
Breakpoint 1, pck.break_me (item=...) at pck.adb:17^M
17        function Break_Me (Item : T) return Boolean is^M
(gdb) print item.started^M
Cannot access memory at address 0x0^M
(gdb) FAIL: gdb.ada/convvar_comp.exp: print item.started
...

This happens as follows.

The parameter item is available in (DW_OP_fbreg: -168):
...
 <2><912>: Abbrev Number: 18 (DW_TAG_formal_parameter)
    <913>   DW_AT_name        : (indirect string, offset: 0x14ca): item
    <919>   DW_AT_type        : <0x929>
    <91d>   DW_AT_location    : 3 byte block: 91 d8 7e  (DW_OP_fbreg: -168)
...
and according to the rules of -O0, it's considered to be available after the
prologue, which looks like this:
...
0000000001002998 <pck__break_me>:
 1002998:       b3 c1 00 2b             ldgr    %f2,%r11
 100299c:       b3 c1 00 0f             ldgr    %f0,%r15
 10029a0:       e3 f0 ff 58 ff 71       lay     %r15,-168(%r15)
 10029a6:       b9 04 00 bf             lgr     %r11,%r15
 10029aa:       e3 20 b0 a0 00 24       stg     %r2,160(%r11)
...

To detect the prologue, gdb checks the line info, which looks like this:
...
pck.adb:
File name                Line number    Starting address    View    Stmt
pck.adb                           17           0x1002998               x
pck.adb                           17           0x1002998       1       x
pck.adb                           19           0x10029b0               x
pck.adb                           20           0x10029b8               x
pck.adb                            -           0x10029c6
...
and gdb concludes that it's an empty prologue, so we stop at 0x1002998 and
try to print parameter item, which is not available yet.

For more details, see this comment in skip_prologue_using_sal:
...
  /* For languages other than assembly, treat two consecutive line
     entries at the same address as a zero-instruction prologue.
...

The same thing happens on x86_64-linux, but it causes no problem there,
because amd64_skip_prologue decides not to trust the result:
...
      struct compunit_symtab *cust = find_pc_compunit_symtab (func_addr);

      /* LLVM backend (Clang/Flang) always emits a line note before the
	 prologue and another one after.  We trust clang and newer Intel
	 compilers to emit usable line notes.  */
      if (post_prologue_pc
	  && (cust != NULL
	      && cust->producer () != nullptr
	      && (producer_is_llvm (cust->producer ())
	      || producer_is_icc_ge_19 (cust->producer ()))))
	return std::max (start_pc, post_prologue_pc);
...
because the producer is GCC.

Work around this by setting a breakpoint on the first statement of
pck.break_me instead.

Tested on s390x-linux.

2 files changed, 4 insertions, 2 deletions

diff --git a/gdb/testsuite/gdb.ada/convvar_comp.exp b/gdb/testsuite/gdb.ada/convvar_comp.exp
index d59a19a..e7ff3ba 100644
--- a/gdb/testsuite/gdb.ada/convvar_comp.exp
+++ b/gdb/testsuite/gdb.ada/convvar_comp.exp
@@ -25,7 +25,9 @@ if {[gdb_compile_ada "${srcfile}" "${binfile}" executable [list debug ]] != "" }
 
 clean_restart ${testfile}
 
-if {![runto "break_me"]} {
+set bp_location [gdb_get_line_number "BREAK" "$testdir/pck.adb"]
+
+if {![runto pck.adb:$bp_location]} {
   return
 }
 
diff --git a/gdb/testsuite/gdb.ada/convvar_comp/pck.adb b/gdb/testsuite/gdb.ada/convvar_comp/pck.adb
index be5d8d7..b7bf3ef 100644
--- a/gdb/testsuite/gdb.ada/convvar_comp/pck.adb
+++ b/gdb/testsuite/gdb.ada/convvar_comp/pck.adb
@@ -16,6 +16,6 @@
 package body Pck is
   function Break_Me (Item : T) return Boolean is
   begin
-     return False;
+     return False; -- BREAK
    end Break_Me;
 end Pck;