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, ®);
fs->regs.reg[DWARF_REG_TO_UNWIND_COLUMN(reg)].how = REG_UNSAVED;
break;
+ case DW_CFA_undefined:
+ insn_ptr = read_uleb128 (insn_ptr, ®);
+ 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
