Re: CFI for saved argument registers

[Andreas Krebbel <krebbel@linux.ibm.com>]

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.  */