Debugging ARM noreturn functions

Debugging ARM noreturn functions

[Paul Brook]

The patch below addresses a long standing issue when debugging code that 
includes noreturn functions on ARM targets.

For somewhat nebulous performance reasons, the ARM backend decides not to save 
any registers in noreturn functions. While this is technically valid, it makes 
it impossible for the debugger to unwind past this point. A common example of 
this kind of function is abort(), so this happens more than you may first 
think.

This has been discussed a previously [*], and the compromise reached was to 
save just enough information to allow a backtrace to be obtained. In practice 
this means the link register (r14), and common frame pointer registers
(r7, r11). Conveniently the result is a more localized hack than the previous 
code.

Tested on arm-none-eabi.
Applied to SVN head.

Paul

[*] Unfortunately I've lost the link to these discussions.

2009-11-23  Paul Brook  <paul@codesourcery.com>

	gcc/
	* config/arm/arm.c (arm_compute_save_reg0_reg12_mask): Add special
	case for noreturn functions.
	(arm_compute_save_reg_mask): Remove special noreturn handling.

Index: gcc/config/arm/arm.c
===================================================================
--- gcc/config/arm/arm.c	(revision 154433)
+++ gcc/config/arm/arm.c	(working copy)
@@ -12831,6 +12831,20 @@ arm_compute_save_reg0_reg12_mask (void)
 	  && crtl->uses_pic_offset_table)
 	save_reg_mask |= 1 << PIC_OFFSET_TABLE_REGNUM;
     }
+  else if (IS_VOLATILE(func_type))
+    {
+      /* For noreturn functions we historically omitted register saves
+	 altogether.  However this really messes up debugging.  As a
+	 compromise save just the fame pointers.  Combined with the link
+	 register saved elsewhere this should be sufficient to get
+	 a backtrace.  */
+      if (frame_pointer_needed)
+	save_reg_mask |= 1 << HARD_FRAME_POINTER_REGNUM;
+      if (df_regs_ever_live_p (ARM_HARD_FRAME_POINTER_REGNUM))
+	save_reg_mask |= 1 << ARM_HARD_FRAME_POINTER_REGNUM;
+      if (df_regs_ever_live_p (THUMB_HARD_FRAME_POINTER_REGNUM))
+	save_reg_mask |= 1 << THUMB_HARD_FRAME_POINTER_REGNUM;
+    }
   else
     {
       /* In the normal case we only need to save those registers
@@ -12917,11 +12931,6 @@ arm_compute_save_reg_mask (void)
       | (1 << LR_REGNUM)
       | (1 << PC_REGNUM);
 
-  /* Volatile functions do not return, so there
-     is no need to save any other registers.  */
-  if (IS_VOLATILE (func_type))
-    return save_reg_mask;
-
   save_reg_mask |= arm_compute_save_reg0_reg12_mask ();
 
   /* Decide if we need to save the link register.

Re: Debugging ARM noreturn functions

[Richard Earnshaw]

On Mon, 2009-11-23 at 13:03 +0000, Paul Brook wrote:
> +        compromise save just the fame pointers.  Combined with the
> link

Frame pointers

R.

Re: Debugging ARM noreturn functions

[Paul Brook]

On Tuesday 24 November 2009, Richard Earnshaw wrote:
> On Mon, 2009-11-23 at 13:03 +0000, Paul Brook wrote:
> > +        compromise save just the fame pointers.  Combined with the
> 
> Frame pointers

Fixed, thanks.

Paul

2009-11-24  Paul Brook  <paul@codesourcery.com>

	gcc/
	* gcc/config/arm/arm.c (arm_compute_save_reg0_reg12_mask): Fix comment
	typo.

Index: gcc/config/arm/arm.c
===================================================================
--- gcc/config/arm/arm.c	(revision 154446)
+++ gcc/config/arm/arm.c	(working copy)
@@ -12835,7 +12835,7 @@ arm_compute_save_reg0_reg12_mask (void)
     {
       /* For noreturn functions we historically omitted register saves
 	 altogether.  However this really messes up debugging.  As a
-	 compromise save just the fame pointers.  Combined with the link
+	 compromise save just the frame pointers.  Combined with the link
 	 register saved elsewhere this should be sufficient to get
 	 a backtrace.  */
       if (frame_pointer_needed)