gdb: Add maint set ignore-prologue-end-flag

The previous patch added support for the DWARF prologue-end flag in line
table. This flag can be used by DWARF producers to indicate where to
place breakpoints past a function prologue.  However, this takes
precedence over prologue analyzers. So if we have to debug a program
with erroneous debug information, the overall debugging experience will
be degraded.

This commit proposes to add a maintenance command to instruct GDB to
ignore the prologue_end flag.

Tested on x86_64-gnu-linux.

Change-Id: Idda6d1b96ba887f4af555b43d9923261b9cc6f82

gdb/NEWS

@@ -14,6 +14,15 @@
   emit to indicate where a breakpoint should be placed to break in a function
   past its prologue.
 
+* New commands
+
+maintenance set ignore-prologue-end-flag on|off
+maintenance show ignore-prologue-end-flag
+  This setting, which is off by default, controls whether GDB ignores the
+  PROLOGUE-END flag from the line-table when skipping prologue.  This can be
+  used to force GDB to use prologue analyzers if the line-table is constructed
+  from erroneous debug information.
+
 * Changed commands
 
 maintenance info line-table

gdb/doc/gdb.texinfo

@@ -19857,6 +19857,19 @@ when collecting performance data.  The command @code{maint
 flush-symbol-cache} is deprecated in favor of @code{maint flush
 symbol-cache}..
 
+@kindex maint set ignore-prologue-end-flag
+@cindex prologue-end
+@item maint set ignore-prologue-end-flag [on|off]
+Enable or disable the use of the @samp{PROLOGUE-END} flag from the line-table.
+When @samp{off} (the default), @value{GDBN} uses the @samp{PROLOGUE-END} flag
+to place breakpoints past the end of a function prologue.  When @samp{on},
+@value{GDBN} ignores the flag and relies on prologue analyzers to skip function
+prologues.
+
+@kindex maint show ignore-prologue-end-flag
+@item maint show ignore-prologue-end-flag
+Show whether @value{GDBN} will ignore the @samp{PROLOGUE-END} flag.
+
 @end table
 
 @node Altering

gdb/symtab.c

@@ -288,6 +288,10 @@ static const char *const multiple_symbols_modes[] =
 };
 static const char *multiple_symbols_mode = multiple_symbols_all;
 
+/* When TRUE, ignore the prologue-end flag in linetable_entry when searching
+   for the SAL past a function prologue.  */
+static bool ignore_prologue_end_flag = false;
+
 /* Read-only accessor to AUTO_SELECT_MODE.  */
 
 const char *
@@ -3941,7 +3945,7 @@ skip_prologue_sal (struct symtab_and_line *sal)
 
       /* Check if the compiler explicitly indicated where a breakpoint should
          be placed to skip the prologue.  */
-      if (skip)
+      if (!ignore_prologue_end_flag && skip)
 	{
 	  gdb::optional<CORE_ADDR> linetable_pc
 	    = skip_prologue_using_linetable (pc);
@@ -7113,6 +7117,19 @@ If zero then the symbol cache is disabled."),
 			     &maintenance_set_cmdlist,
 			     &maintenance_show_cmdlist);
 
+  add_setshow_boolean_cmd ("ignore-prologue-end-flag", no_class,
+			   &ignore_prologue_end_flag,
+			   _("Set if the PROLOGUE-END flag is ignored."),
+			   _("Show if the PROLOGUE-END flag is ignored."),
+			   _("\
+The PROLOGUE-END flag from the line-table entries is used to place \
+breakpoints past the prologue of functions.  Disabeling its use use forces \
+the use of prologue scanners."),
+			   nullptr, nullptr,
+			   &maintenance_set_cmdlist,
+			   &maintenance_show_cmdlist);
+
+
   add_cmd ("symbol-cache", class_maintenance, maintenance_print_symbol_cache,
 	   _("Dump the symbol cache for each program space."),
 	   &maintenanceprintlist);

gdb/testsuite/gdb.dwarf2/dw2-prologue-end.exp

@@ -89,3 +89,17 @@ if ![runto_main] {
 
 set prologue_end_line [gdb_get_line_number "main assign o"]
 gdb_test "frame" ".*main \\\(\\\) at \[^\r\n\]*:$prologue_end_line\r\n.*"
+
+with_test_prefix "ignore-prologue-end" {
+    clean_restart $binfile
+    gdb_test_no_output "maintenance set ignore-prologue-end-flag on"
+
+    if ![runto_main] {
+	return -1
+    }
+
+    # If we ignore the prologue-end flag, we should stop at the first statement
+    # of main which assigns m.
+    set prologue_end_line [gdb_get_line_number "main assign m"]
+    gdb_test "frame" ".*main \\\(\\\) at \[^\r\n\]*:$prologue_end_line\r\n.*"
+}