Re: [PATCH] Use GCC5/DWARF5 DW_AT_noreturn to mark functions that don't return normally.

[Mark Wielaard <mjw at redhat dot com>]

On Thu, 2014-12-04 at 14:02 +0000, Pedro Alves wrote:
> On 11/27/2014 02:53 PM, Mark Wielaard wrote:
> > Add a flag field is_noreturn to struct func_type. Make calling_convention
> > a small bit field to not increase the size of the struct. Set is_noreturn
> > if the new GCC5/DWARF5 DW_AT_noreturn is set on a DW_TAG_subprogram.
> > Use this information to warn the user before doing a finish or return from
> > a function that does not return normally to its caller.
> > 
> > (gdb) finish
> > Warning. Function endless does not return normally.
> > Try to finish anyway? (y or n)
> > 
> > (gdb) return
> > warning: Function does not return normally to caller!
> > Make endless return now? (y or n)
> 
> I'd suggest making the warnings a bit more consistent.
> 
> - "Warning." vs "warning: "
> 
>   Prefer the latter, as that's what the "warning" function uses.
> 
> - "." vs "!"
> 
>   I'd keep it calm and get rid of the "!".  :-)

Fixed both.

> > 
> > gdb/ChangeLog
> > 
> > 	* dwarf2read.c (read_subroutine_type): Set TYPE_NO_RETURN from
> > 	DW_AT_noreturn.
> > 	* gdbtypes.h (struct func_type): Add is_noreturn field flag. Make
> > 	calling_convention an 8 bit bit field.
> > 	(TYPE_NO_RETURN): New macro.
> > 	* infcmd.c (finish_command): Query if function does not return
> > 	normally.
> > 	* stack.c (return_command): Likewise.
> > 
> > include/ChangeLog
> > 
> > 	* dwarf2.def (DW_AT_noreturn): New DWARF5 attribute.
> 
> I wonder if we could have a test?  Could e.g., make sure we don't
> crash when the user confirms a return in a noreturn function.

I am not sure how to write such a test. This is mainly interactive code,
which will only trigger from_tty. I also am not sure such a test really
tests this new feature. Trying to return from a noreturn function
triggers undefined behavior. GDB probably won't crash, but the inferior
might since the result is unpredictable (that is precisely why I added
this, you forcibly return from a function and end up somewhere
unexpected). Which makes testing the expected output of the user
ignoring the warning somewhat hard.

> > --- a/gdb/infcmd.c
> > +++ b/gdb/infcmd.c
> > @@ -1869,7 +1869,14 @@ finish_command (char *arg, int from_tty)
> >        if (execution_direction == EXEC_REVERSE)
> >  	printf_filtered (_("Run back to call of "));
> >        else
> > -	printf_filtered (_("Run till exit from "));
> > +	{
> > +	  if (function != NULL && TYPE_NO_RETURN (function->type)
> > +	      && ! query (_("Warning. Function %s does not return normally.\n"
> > +			    "Try to finish anyway? "),
> > +			  SYMBOL_PRINT_NAME (function)))
> > +	    error (_("Not confirmed."));
> > +	  printf_filtered (_("Run till exit from "));
> > +	}
> 
> No space between "!" and query:
> 
> 	      && !query ...

Fixed.

> > --- a/gdb/stack.c
> > +++ b/gdb/stack.c
> > @@ -2462,8 +2462,12 @@ return_command (char *retval_exp, int from_tty)
> >  	confirmed = query (_("%sMake selected stack frame return now? "),
> >  			   query_prefix);
> >        else
> > -	confirmed = query (_("%sMake %s return now? "), query_prefix,
> > -			   SYMBOL_PRINT_NAME (thisfun));
> > +	{
> > +	  if (TYPE_NO_RETURN (thisfun->type))
> > +	    warning ("Function does not return normally to caller!");
> 
> i18n / _() .

Added.

Rebased patch attached.

Thanks,

Mark

From 6b454703a5a53bee1355b5a871a10f6bbfcde64e Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mjw@redhat.com>
Date: Tue, 9 Dec 2014 11:45:41 +0100
Subject: [PATCH] Use GCC5/DWARF5 DW_AT_noreturn to mark functions that don't
 return normally.

Add a flag field is_noreturn to struct func_type. Make calling_convention
a small bit field to not increase the size of the struct. Set is_noreturn
if the new GCC5/DWARF5 DW_AT_noreturn is set on a DW_TAG_subprogram.
Use this information to warn the user before doing a finish or return from
a function that does not return normally to its caller.

(gdb) finish
warning: Function endless does not return normally.
Try to finish anyway? (y or n)

(gdb) return
warning: Function does not return normally to caller.
Make endless return now? (y or n)

gdb/ChangeLog

	* dwarf2read.c (read_subroutine_type): Set TYPE_NO_RETURN from
	DW_AT_noreturn.
	* gdbtypes.h (struct func_type): Add is_noreturn field flag. Make
	calling_convention an 8 bit bit field.
	(TYPE_NO_RETURN): New macro.
	* infcmd.c (finish_command): Query if function does not return
	normally.
	* stack.c (return_command): Likewise.

include/ChangeLog

	* dwarf2.def (DW_AT_noreturn): New DWARF5 attribute.

The dwarf2.h addition and the code to emit the new attribute is already in
the gcc tree.
---
 gdb/ChangeLog      | 11 +++++++++++
 gdb/dwarf2read.c   |  6 ++++++
 gdb/gdbtypes.h     | 12 ++++++++++--
 gdb/infcmd.c       |  9 ++++++++-
 gdb/stack.c        |  8 ++++++--
 include/ChangeLog  |  4 ++++
 include/dwarf2.def |  2 ++
 7 files changed, 47 insertions(+), 5 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index ad845a7..43a62af 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,14 @@
+2014-12-09  Mark Wielaard  <mjw@redhat.com>
+
+	* dwarf2read.c (read_subroutine_type): Set TYPE_NO_RETURN from
+	DW_AT_noreturn.
+	* gdbtypes.h (struct func_type): Add is_noreturn field flag. Make
+	calling_convention an 8 bit bit field.
+	(TYPE_NO_RETURN): New macro.
+	* infcmd.c (finish_command): Query if function does not return
+	normally.
+	* stack.c (return_command): Likewise.
+
 2014-12-08  Doug Evans  <dje@google.com>
 
 	* python/py-objfile.c (objfpy_get_owner): Increment refcount of result.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 829611d..90e7893 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -14308,6 +14308,12 @@ read_subroutine_type (struct die_info *die, struct dwarf2_cu *cu)
   else
     TYPE_CALLING_CONVENTION (ftype) = DW_CC_normal;
 
+  /* Record whether the function returns normally to its caller or not
+     if the DWARF producer set that information.  */
+  attr = dwarf2_attr (die, DW_AT_noreturn, cu);
+  if (attr && (DW_UNSND (attr) != 0))
+    TYPE_NO_RETURN (ftype) = 1;
+
   /* We need to add the subroutine type to the die immediately so
      we don't infinitely recurse when dealing with parameters
      declared as the same subroutine type.  */
diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index a56f543..2806c4d 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -1019,9 +1019,16 @@ struct func_type
   {
     /* * The calling convention for targets supporting multiple ABIs.
        Right now this is only fetched from the Dwarf-2
-       DW_AT_calling_convention attribute.  */
+       DW_AT_calling_convention attribute.  The value is one of the
+       DW_CC enum dwarf_calling_convention constants.  */
 
-    unsigned calling_convention;
+    unsigned calling_convention : 8;
+
+    /* * Whether this function normally returns to its caller.  It is
+       set from the DW_AT_noreturn attribute if set on the
+       DW_TAG_subprogram.  */
+
+    unsigned int is_noreturn : 1;
 
     /* * Only those DW_TAG_GNU_call_site's in this function that have
        DW_AT_GNU_tail_call set are linked in this list.  Function
@@ -1245,6 +1252,7 @@ extern void allocate_gnat_aux_type (struct type *);
 #define TYPE_GNAT_SPECIFIC(thistype) TYPE_MAIN_TYPE(thistype)->type_specific.gnat_stuff
 #define TYPE_DESCRIPTIVE_TYPE(thistype) TYPE_GNAT_SPECIFIC(thistype)->descriptive_type
 #define TYPE_CALLING_CONVENTION(thistype) TYPE_MAIN_TYPE(thistype)->type_specific.func_stuff->calling_convention
+#define TYPE_NO_RETURN(thistype) TYPE_MAIN_TYPE(thistype)->type_specific.func_stuff->is_noreturn
 #define TYPE_TAIL_CALL_LIST(thistype) TYPE_MAIN_TYPE(thistype)->type_specific.func_stuff->tail_call_list
 #define TYPE_BASECLASS(thistype,index) TYPE_FIELD_TYPE(thistype, index)
 #define TYPE_N_BASECLASSES(thistype) TYPE_CPLUS_SPECIFIC(thistype)->n_baseclasses
diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index 4415b31..68d9ef9 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -1869,7 +1869,14 @@ finish_command (char *arg, int from_tty)
       if (execution_direction == EXEC_REVERSE)
 	printf_filtered (_("Run back to call of "));
       else
-	printf_filtered (_("Run till exit from "));
+	{
+	  if (function != NULL && TYPE_NO_RETURN (function->type)
+	      && !query (_("warning: Function %s does not return normally.\n"
+			   "Try to finish anyway? "),
+			 SYMBOL_PRINT_NAME (function)))
+	    error (_("Not confirmed."));
+	  printf_filtered (_("Run till exit from "));
+	}
 
       print_stack_frame (get_selected_frame (NULL), 1, LOCATION, 0);
     }
diff --git a/gdb/stack.c b/gdb/stack.c
index 2834801..f7e77e4 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -2462,8 +2462,12 @@ return_command (char *retval_exp, int from_tty)
 	confirmed = query (_("%sMake selected stack frame return now? "),
 			   query_prefix);
       else
-	confirmed = query (_("%sMake %s return now? "), query_prefix,
-			   SYMBOL_PRINT_NAME (thisfun));
+	{
+	  if (TYPE_NO_RETURN (thisfun->type))
+	    warning ("Function does not return normally to caller.");
+	  confirmed = query (_("%sMake %s return now? "), query_prefix,
+			     SYMBOL_PRINT_NAME (thisfun));
+	}
       if (!confirmed)
 	error (_("Not confirmed"));
     }
diff --git a/include/ChangeLog b/include/ChangeLog
index e80d2ec..69ed996 100644
--- a/include/ChangeLog
+++ b/include/ChangeLog
@@ -1,3 +1,7 @@
+2014-12-09  Mark Wielaard  <mjw@redhat.com>
+
+	* dwarf2.def (DW_AT_noreturn): New DWARF5 attribute.
+
 2014-12-06  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* dis-asm.h (print_insn_visium): Declare.
diff --git a/include/dwarf2.def b/include/dwarf2.def
index 8ca143c..8533a3e 100644
--- a/include/dwarf2.def
+++ b/include/dwarf2.def
@@ -308,6 +308,8 @@ DW_AT (DW_AT_data_bit_offset, 0x6b)
 DW_AT (DW_AT_const_expr, 0x6c)
 DW_AT (DW_AT_enum_class, 0x6d)
 DW_AT (DW_AT_linkage_name, 0x6e)
+/* DWARF 5.  */
+DW_AT (DW_AT_noreturn, 0x87)
 
 DW_AT_DUP (DW_AT_lo_user, 0x2000) /* Implementation-defined range start.  */
 DW_AT_DUP (DW_AT_hi_user, 0x3fff) /* Implementation-defined range end.  */
-- 
1.8.3.1