[PATCH][gdb/breakpoint] Fix stepping past non-stmt line-table entries

[Tom de Vries <tdevries@suse.de>]

Hi,

Consider the test-case small.c:
...
$ cat -n small.c
     1  __attribute__ ((noinline, noclone))
     2  int foo (char *c)
     3  {
     4    asm volatile ("" : : "r" (c) : "memory");
     5    return 1;
     6  }
     7
     8  int main ()
     9  {
    10    char tpl1[20] = "/tmp/test.XXX";
    11    char tpl2[20] = "/tmp/test.XXX";
    12    int fd1 = foo (tpl1);
    13    int fd2 = foo (tpl2);
    14    if (fd1 == -1) {
    15      return 1;
    16    }
    17
    18    return 0;
    19  }
...

Compiled with gcc-8 and optimization:
...
$ gcc-8 -O2 -g small.c
...

We step through the calls to foo, but fail to visit line 13:
...
12	  int fd1 = foo (tpl1);
(gdb) step
foo (c=c@entry=0x7fffffffdea0 "/tmp/test.XXX") at small.c:5
5	  return 1;
(gdb) step
foo (c=c@entry=0x7fffffffdec0 "/tmp/test.XXX") at small.c:5
5	  return 1;
(gdb) step
main () at small.c:14
14	  if (fd1 == -1) {
(gdb)
...

This is caused by the following.  The calls to foo are implemented by these
insns:
....
  4003df:       0f 29 04 24             movaps %xmm0,(%rsp)
  4003e3:       0f 29 44 24 20          movaps %xmm0,0x20(%rsp)
  4003e8:       e8 03 01 00 00          callq  4004f0 <foo>
  4003ed:       48 8d 7c 24 20          lea    0x20(%rsp),%rdi
  4003f2:       89 c2                   mov    %eax,%edx
  4003f4:       e8 f7 00 00 00          callq  4004f0 <foo>
  4003f9:       31 c0                   xor    %eax,%eax
...
with corresponding line table entries:
...
INDEX  LINE   ADDRESS            IS-STMT
8      12     0x00000000004003df Y
9      10     0x00000000004003df
10     11     0x00000000004003e3
11     12     0x00000000004003e8
12     13     0x00000000004003ed
13     12     0x00000000004003f2
14     13     0x00000000004003f4 Y
15     13     0x00000000004003f4
16     14     0x00000000004003f9 Y
17     14     0x00000000004003f9
...

Once we step out of the call to foo at 4003e8, we land at 4003ed, and gdb
enters process_event_stop_test to figure out what to do.

That entry has is-stmt=n, so it's not the start of a line, so we don't stop
there.  However, we do update ecs->event_thread->current_line to line 13,
because the frame has changed (because we stepped out of the function).

Next we land at 4003f2.  Again the entry has is-stmt=n, so it's not the start
of a line, so we don't stop there.  However, because the frame hasn't changed,
we don't update update ecs->event_thread->current_line, so it stays 13.

Next we land at 4003f4.  Now is-stmt=y, so it's the start of a line, and we'd
like to stop here.

But we don't stop because this test fails:
...
  if ((ecs->event_thread->suspend.stop_pc == stop_pc_sal.pc)
      && (ecs->event_thread->current_line != stop_pc_sal.line
          || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
    {
...
because ecs->event_thread->current_line == 13 and stop_pc_sal.line == 13.

Fix this by resetting ecs->event_thread->current_line to 0 if is-stmt=n and
the frame has changed, such that we have:
...
12        int fd1 = foo (tpl1);
(gdb) step
foo (c=c@entry=0x7fffffffdbc0 "/tmp/test.XXX") at small.c:5
5         return 1;
(gdb) step
main () at small.c:13
13        int fd2 = foo (tpl2);
(gdb)
...

Tested on x86_64-linux, with gcc-7 and gcc-8.

Any comments?

Thanks,
- Tom

[gdb/breakpoint] Fix stepping past non-stmt line-table entries

gdb/ChangeLog:

2021-01-14  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/26063
	* infrun.c (process_event_stop_test): Reset
	ecs->event_thread->current_line to 0 if is-stmt=n and frame has
	changed.

gdb/testsuite/ChangeLog:

2021-01-14  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/26063
	* gdb.dwarf2/dw2-step-out-of-function-no-stmt.c: New test.
	* gdb.dwarf2/dw2-step-out-of-function-no-stmt.exp: New file.

---
 gdb/infrun.c                                       |  29 +++--
 .../gdb.dwarf2/dw2-step-out-of-function-no-stmt.c  |  44 +++++++
 .../dw2-step-out-of-function-no-stmt.exp           | 126 +++++++++++++++++++++
 3 files changed, 192 insertions(+), 7 deletions(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 940f575e22a..57a70b5aa96 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -7000,12 +7000,15 @@ process_event_stop_test (struct execution_control_state *ecs)
       && (ecs->event_thread->current_line != stop_pc_sal.line
  	  || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
     {
+      /* We are at a different line.  */
+
       if (stop_pc_sal.is_stmt)
 	{
-	  /* We are at the start of a different line.  So stop.  Note that
-	     we don't stop if we step into the middle of a different line.
-	     That is said to make things like for (;;) statements work
-	     better.  */
+	  /* We are at the start of a statement.
+
+	     So stop.  Note that we don't stop if we step into the middle of a
+	     statement.  That is said to make things like for (;;) statements
+	     work better.  */
 	  infrun_debug_printf ("stepped to a different line");
 	  end_stepping_range (ecs);
 	  return;
@@ -7013,14 +7016,26 @@ process_event_stop_test (struct execution_control_state *ecs)
       else if (frame_id_eq (get_frame_id (get_current_frame ()),
 			    ecs->event_thread->control.step_frame_id))
 	{
-	  /* We are at the start of a different line, however, this line is
-	     not marked as a statement, and we have not changed frame.  We
-	     ignore this line table entry, and continue stepping forward,
+	  /* We are not at the start of a statement, and we have not changed
+	     frame.
+
+	     We ignore this line table entry, and continue stepping forward,
 	     looking for a better place to stop.  */
 	  refresh_step_info = false;
 	  infrun_debug_printf ("stepped to a different line, but "
 			       "it's not the start of a statement");
 	}
+      else
+	{
+	  /* We are not the start of a statement, and we have changed frame.
+
+	     We ignore this line table entry, and continue stepping forward,
+	     looking for a better place to stop.  Keep refresh_step_info at
+	     true to note that the frame has changed, but ignore the line
+	     number to make sure we don't ignore a subsequent entry with the
+	     same line number.  */
+	  stop_pc_sal.line = 0;
+	}
     }
 
   /* We aren't done stepping.
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-step-out-of-function-no-stmt.c b/gdb/testsuite/gdb.dwarf2/dw2-step-out-of-function-no-stmt.c
new file mode 100644
index 00000000000..a8184f0c1bb
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-step-out-of-function-no-stmt.c
@@ -0,0 +1,44 @@
+/*
+   Copyright 2021 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+void
+foo (int x)
+{
+
+}
+
+void
+bar (void)
+{
+  asm ("bar_label: .globl bar_label");
+}
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+
+  bar ();
+
+  asm ("main_label_2: .globl main_label_2");
+
+  foo (10);
+
+  asm ("main_label_3: .globl main_label_3");
+
+  return 0;
+}
+
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-step-out-of-function-no-stmt.exp b/gdb/testsuite/gdb.dwarf2/dw2-step-out-of-function-no-stmt.exp
new file mode 100644
index 00000000000..9249eb590f7
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-step-out-of-function-no-stmt.exp
@@ -0,0 +1,126 @@
+# Copyright 2021 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Check whether stepping out of a function works, in case:
+# - the first insn after the call has an is-stmt=no entry
+# - the next insn has an is-stmt=yes entry, for the same line number
+#
+# This sort of thing can occur in optimized code, f.i. here a slightly more
+# elaborate case with another is-stmt=no entry (the one with line number 12)
+# inbetween:
+# INDEX  LINE   ADDRESS            IS-STMT
+# 12     13     0x00000000004003ed
+# 13     12     0x00000000004003f2
+# 14     13     0x00000000004003f4 Y
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    verbose "Skipping $gdb_test_file_name."
+    return 0
+}
+
+# The .c files use __attribute__.
+if [get_compiler_info] {
+    return -1
+}
+if !$gcc_compiled {
+    verbose "Skipping $gdb_test_file_name."
+    return 0
+}
+
+standard_testfile .c -dw.S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    declare_labels Llines
+    global srcdir subdir srcfile
+
+    lassign [function_range main [list ${srcdir}/${subdir}/$srcfile]] \
+	main_start main_len
+    set main_end "$main_start + $main_len"
+
+    lassign [function_range main [list ${srcdir}/${subdir}/$srcfile]] \
+	bar_start bar_len
+    set bar_end "$bar_start + $bar_len"
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name $srcfile}
+	    {stmt_list $Llines DW_FORM_sec_offset}
+	} {
+	    subprogram {
+		{external 1 flag}
+		{MACRO_AT_func {main}}
+	    }
+	    subprogram {
+		{external 1 flag}
+		{MACRO_AT_func {bar}}
+	    }
+	}
+    }
+
+    lines {version 2} Llines {
+	include_dir "${srcdir}/${subdir}"
+	file_name "$srcfile" 1
+
+	program {
+	    {DW_LNE_set_address bar_label}
+	    {line 26}
+	    {DW_LNS_copy}
+
+	    {DW_LNE_set_address $bar_end}
+	    {DW_LNE_end_sequence}
+
+	    {DW_LNE_set_address main_label}
+	    {line 32}
+	    {DW_LNS_copy}
+
+	    {DW_LNE_set_address main_label_2}
+	    {line 36}
+	    {DW_LNS_negate_stmt}
+	    {DW_LNS_copy}
+	    {DW_LNS_negate_stmt}
+
+	    {DW_LNE_set_address main_label_3}
+	    {line 36}
+	    {DW_LNS_copy}
+
+	    {DW_LNE_set_address $main_end}
+	    {DW_LNE_end_sequence}
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+# Step into bar.
+gdb_breakpoint "bar"
+gdb_continue_to_breakpoint "bar"
+
+# Step out of bar.
+gdb_test "step" [multi_line \
+		     "main \\(\\) at \[^\r\n\]*$srcfile:36" \
+		     "36\t\[^\r\n\]*"]
+