On 5/16/22 08:29, Andreas Krebbel via Gcc wrote:
> Hi,
>
> I'm trying to provide a simple dwarf unwinder with access to the
> argument register content. The goal is to make this information
> available for optimized code without having to access debug
> information for things like call site args. The extra overhead
> of saving the values to the stack is acceptable in that case.
>
> For that purpose I save the argument registers to the stack as we
> would do for a variable argument lists. But this time I also provide
> the CFI to allow the unwinder to locate the save slots. Since I never
> actually intend to restore the content there is no matching
> cfi_restore for the cfi_offset and dwarf2cfi complains about the
> traces being inconsistent because of that. I couldn't find a way to
> prevent this.
>
> The only way I see right now is adding a new reg note to invalidate
> the save information in the reg_save array in dwarf2cfi.
>
> Would this be acceptable? Is there perhaps an easier way to achieve that?
Attached is a small patch adding a new reg note REG_CFA_NORESTORE.
I would attach that new note to the register save insn to indicate that this
register will not be
restored. I'll have to emit the REG_CFA_OFFSET note as well then.
An insn saving r2-r6 without restoring them would look like this then:
(insn/f 31 21 32 2 (parallel [
(set/f (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 16 [0x10])) [4 S8 A8])
(reg:DI 2 %r2))
(set/f (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 24 [0x18])) [4 S8 A8])
(reg:DI 3 %r3))
(set/f (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 32 [0x20])) [4 S8 A8])
(reg:DI 4 %r4))
(set/f (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 40 [0x28])) [4 S8 A8])
(reg:DI 5 %r5))
(set/f (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 48 [0x30])) [4 S8 A8])
(reg:DI 6 %r6))
]) "/home/andreas/preserveargs/reduce2/t.c":2:51 -1
(expr_list:REG_CFA_OFFSET (set (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 48 [0x30])) [4 S8 A8])
(reg:DI 6 %r6))
(expr_list:REG_CFA_NO_RESTORE (reg:DI 6 %r6)
(expr_list:REG_CFA_OFFSET (set (mem/c:DI (plus:DI (reg/f:DI 15 %r15)
(const_int 40 [0x28])) [4 S8 A8])
(reg:DI 5 %r5))
(expr_list:REG_CFA_NO_RESTORE (reg:DI 5 %r5)
(expr_list:REG_CFA_OFFSET (set (mem/c:DI (plus:DI (reg/f:DI
15 %r15)
(const_int 32 [0x20])) [4 S8 A8])
(reg:DI 4 %r4))
(expr_list:REG_CFA_NO_RESTORE (reg:DI 4 %r4)
(expr_list:REG_CFA_OFFSET (set (mem/c:DI (plus:DI
(reg/f:DI 15 %r15)
(const_int 24 [0x18])) [4 S8 A8])
(reg:DI 3 %r3))
(expr_list:REG_CFA_NO_RESTORE (reg:DI 3 %r3)
(expr_list:REG_CFA_OFFSET (set (mem/c:DI
(plus:DI (reg/f:DI 15 %r15)
(const_int 16 [0x10])) [4
S8 A8])
(reg:DI 2 %r2))
(expr_list:REG_CFA_NO_RESTORE (reg:DI 2
%r2)
(nil))))))))))))
diff --git a/gcc/dwarf2cfi.c b/gcc/dwarf2cfi.c
index 362ff3fdac27..2cbc2465c3a7 100644
--- a/gcc/dwarf2cfi.c
+++ b/gcc/dwarf2cfi.c
@@ -1346,7 +1346,7 @@ dwarf2out_frame_debug_cfa_val_expression (rtx set)
/* A subroutine of dwarf2out_frame_debug, process a REG_CFA_RESTORE note. */
static void
-dwarf2out_frame_debug_cfa_restore (rtx reg)
+dwarf2out_frame_debug_cfa_restore (rtx reg, bool emit_cfi)
{
gcc_assert (REG_P (reg));
@@ -1354,7 +1354,8 @@ dwarf2out_frame_debug_cfa_restore (rtx reg)
if (!span)
{
unsigned int regno = dwf_regno (reg);
- add_cfi_restore (regno);
+ if (emit_cfi)
+ add_cfi_restore (regno);
update_row_reg_save (cur_row, regno, NULL);
}
else
@@ -1369,7 +1370,8 @@ dwarf2out_frame_debug_cfa_restore (rtx reg)
reg = XVECEXP (span, 0, par_index);
gcc_assert (REG_P (reg));
unsigned int regno = dwf_regno (reg);
- add_cfi_restore (regno);
+ if (emit_cfi)
+ add_cfi_restore (regno);
update_row_reg_save (cur_row, regno, NULL);
}
}
@@ -2155,6 +2157,7 @@ dwarf2out_frame_debug (rtx_insn *insn)
break;
case REG_CFA_RESTORE:
+ case REG_CFA_NO_RESTORE:
n = XEXP (note, 0);
if (n == NULL)
{
@@ -2163,7 +2166,7 @@ dwarf2out_frame_debug (rtx_insn *insn)
n = XVECEXP (n, 0, 0);
n = XEXP (n, 0);
}
- dwarf2out_frame_debug_cfa_restore (n);
+ dwarf2out_frame_debug_cfa_restore (n, REG_NOTE_KIND (note) ==
REG_CFA_RESTORE);
handled_one = true;
break;
diff --git a/gcc/reg-notes.def b/gcc/reg-notes.def
index 995052ebc28e..3e8cc189301c 100644
--- a/gcc/reg-notes.def
+++ b/gcc/reg-notes.def
@@ -167,6 +167,8 @@ REG_CFA_NOTE (CFA_VAL_EXPRESSION)
first pattern is the register to be restored. */
REG_CFA_NOTE (CFA_RESTORE)
+REG_CFA_NOTE (CFA_NO_RESTORE)
+
/* Attached to insns that are RTX_FRAME_RELATED_P, marks insn that sets
vDRAP from DRAP. If vDRAP is a register, vdrap_reg is initalized
to the argument, if it is a MEM, it is ignored. */
