[Bug rtl-optimization/51271] [4.7 Regression] ICE in in maybe_record_trace_start, at dwarf2cfi.c:2244

[vries at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51271

--- Comment #15 from vries at gcc dot gnu.org 2011-12-19 13:17:23 UTC ---
> given this definition, maybe insn 141 should be marked as frame-related, since
> it restores a reg in the epilogue.

It seems to be the other way round: insn 141 is ignored by scan_insn_after,
because:
- it is not frame-related, and 
- doesn't contain a REG_ARGS_SIZE note.
The problematic insn is insn 143. That one has the REG_CFA notes attached, and
is causes changes in cfi state.

In scan_trace, we're handling insn 143 as element in the delay slot of an
annulling branch, executed only on taken branch:
...
(gdb) call debug_rtx (elt)
(insn/s/f 143 79 162 (set (reg/f:SI 29 $sp)
        (plus:SI (reg/f:SI 29 $sp)
            (const_int 16 [0x10]))) 10 {*addsi3}
     (expr_list:REG_CFA_DEF_CFA (reg/f:SI 29 $sp)
        (expr_list:REG_CFA_RESTORE (reg:DI 28 $28)
            (nil))))
...

The row state at that point is:
...
(gdb) call debug_cfi_row (cur_row)
    .cfi_def_cfa 29, 16
    .cfi_offset 28, -8
...

we then execute scan_insn_after to setup the trace at the target of the jump:
...
(gdb) 
2460              scan_insn_after (elt);
....

at which point the REG_CFAs have been taken into account:
...
(gdb) call debug_cfi_row (cur_row)
    .cfi_def_cfa 29, 0
...

we then try to restore for the fallthru path:
...
2467              cur_trace->end_true_args_size = restore_args_size;
(gdb) n
2468              cur_row->cfa = this_cfa;
(gdb) 
...

but the state is not sufficiently restored:
...
(gdb) call debug_cfi_row (cur_row)
    .cfi_def_cfa 29, 16
...
The '.cfi_offset 28, -8' is missing.

This tentative patch fixes the problem for the testcase:
...
Index: src/gcc-mainline/gcc/dwarf2cfi.c
===================================================================
--- src/gcc-mainline/gcc/dwarf2cfi.c (revision 182341)
+++ src/gcc-mainline/gcc/dwarf2cfi.c (working copy)
@@ -2452,10 +2452,12 @@ scan_trace (dw_trace_info *trace)
           if (INSN_FROM_TARGET_P (elt))
         {
           HOST_WIDE_INT restore_args_size;
+          cfi_vec save_row_reg_save;

           add_cfi_insn = NULL;
           restore_args_size = cur_trace->end_true_args_size;
           cur_cfa = &cur_row->cfa;
+          save_row_reg_save = VEC_copy (dw_cfi_ref, gc, cur_row->reg_save);

           scan_insn_after (elt);

@@ -2466,6 +2468,7 @@ scan_trace (dw_trace_info *trace)

           cur_trace->end_true_args_size = restore_args_size;
           cur_row->cfa = this_cfa;
+          cur_row->reg_save = save_row_reg_save;
           cur_cfa = &this_cfa;
           continue;
         }
...