Re: Unwinding CFI gcc practice of assumed `same value' regs

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Dec 11, 2006 at 02:40:22PM -0800, Roland McGrath wrote:
> My reading is that the "ABI authoring body" for GNU systems or the
> "compilation system authoring body" for GNU compilers already specifies
> that the default rule is same_value for callee-saves registers (as chosen
> by each particular ABI), even if this has not been formally documented
> anywhere heretofore.  (This is how I've written ABI support in another
> unwinder implementation I've worked on.)  As you've said, this is the only
> reading by which current CFI is correct and complete for getting the values
> of callee-saves registers.  I presume that GCC's omission of rules for
> those registers is in fact simply because EH unwinding doesn't care and
> people on the generation side just didn't think about it beyond that.
> Regardless of the true reasons for the history, the description above
> applies to the manifest practice that constitutes what we want the formal
> specification to mean.

Well, for satisfying the requirement that undefined retaddr_column
identifies outermost frame it matters whether retaddr_column's default rule
is same_value or undefined.  If it is by default same_value, then
unwind-dw2.c should just handle explicit DW_CFA_undefined retaddr_column
as outermost frame mark, otherwise it would need to handle any unspecified
or explicit DW_CFA_undefined retaddr_column (but not DW_CFA_same_value).
Here is something that would handle by default same_value retaddr_column:

--- gcc/unwind-dw2.h	2006-10-29 21:49:23.000000000 +0100
+++ gcc/unwind-dw2.h	2006-12-12 16:30:29.000000000 +0100
@@ -1,5 +1,5 @@
 /* DWARF2 frame unwind data structure.
-   Copyright (C) 1997, 1998, 1999, 2000, 2001, 2002, 2003
+   Copyright (C) 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2006
    Free Software Foundation, Inc.
 
    This file is part of GCC.
@@ -55,7 +55,8 @@ typedef struct
 	REG_SAVED_REG,
 	REG_SAVED_EXP,
 	REG_SAVED_VAL_OFFSET,
-	REG_SAVED_VAL_EXP
+	REG_SAVED_VAL_EXP,
+	REG_UNDEFINED
       } how;
     } reg[DWARF_FRAME_REGISTERS+1];
 
--- gcc/unwind-dw2.c	2006-12-08 15:57:44.000000000 +0100
+++ gcc/unwind-dw2.c	2006-12-12 16:38:26.000000000 +0100
@@ -887,12 +887,16 @@ execute_cfa_program (const unsigned char
 	  fs->regs.reg[DWARF_REG_TO_UNWIND_COLUMN(reg)].how = REG_UNSAVED;
 	  break;
 
-	case DW_CFA_undefined:
 	case DW_CFA_same_value:
 	  insn_ptr = read_uleb128 (insn_ptr, &reg);
 	  fs->regs.reg[DWARF_REG_TO_UNWIND_COLUMN(reg)].how = REG_UNSAVED;
 	  break;
 
+	case DW_CFA_undefined:
+	  insn_ptr = read_uleb128 (insn_ptr, &reg);
+	  fs->regs.reg[DWARF_REG_TO_UNWIND_COLUMN(reg)].how = REG_UNDEFINED;
+	  break;
+
 	case DW_CFA_nop:
 	  break;
 
@@ -1255,6 +1259,7 @@ uw_update_context_1 (struct _Unwind_Cont
     switch (fs->regs.reg[i].how)
       {
       case REG_UNSAVED:
+      case REG_UNDEFINED:
 	break;
 
       case REG_SAVED_OFFSET:
@@ -1323,10 +1328,21 @@ uw_update_context (struct _Unwind_Contex
 {
   uw_update_context_1 (context, fs);
 
-  /* Compute the return address now, since the return address column
-     can change from frame to frame.  */
-  context->ra = __builtin_extract_return_addr
-    (_Unwind_GetPtr (context, fs->retaddr_column));
+  /* In general this unwinder doesn't make any distinction between
+     undefined and same_value rule.  Call-saved registers are assumed
+     to have same_value rule by default and explicit undefined
+     rule is handled like same_value.  The only exception is
+     DW_CFA_undefined on retaddr_column which is supposed to
+     mark outermost frame in DWARF 3.  */
+  if (fs->regs[fs->retaddr_column].how == REG_UNDEFINED)
+    /* uw_frame_state_for uses context->ra == 0 check to find outermost
+       stack frame.  */
+    context->ra = 0;
+  else
+    /* Compute the return address now, since the return address column
+       can change from frame to frame.  */
+    context->ra = __builtin_extract_return_addr
+      (_Unwind_GetPtr (context, fs->retaddr_column));
 }
 
 static void


	Jakub