Ping...
I'd like to ping for this patch:
https://gcc.gnu.org/ml/gcc-patches/2019-10/msg01459.html
Thanks
Bernd.
On 10/20/19 9:58 PM, Bernd Edlinger wrote:
> Hi,
>
> this fixes an issue with the gdb step-over aka. "n" command.
>
> It can be seen when you debug an optimized stage-3 cc1
> it does not affect -O0 code, though.
>
> This example debug session will explain the effect.
>
> (gdb) b get_alias_set
> Breakpoint 5 at 0xa099f0: file ../../gcc-trunk/gcc/alias.c, line 837.
> (gdb) r
> Breakpoint 5, get_alias_set (t=t@entry=0x7ffff7ff7ab0) at
> ../../gcc-trunk/gcc/alias.c:837
> 837 if (t == error_mark_node
> (gdb) n
> 839 && (TREE_TYPE (t) == 0 || TREE_TYPE (t) == error_mark_node)))
> (gdb) n
> 3382 return __t; <-- now we have a problem: wrong line info here
> (gdb) bt
> #0 get_alias_set (t=t@entry=0x7ffff7ff7ab0) at
> ../../gcc-trunk/gcc/tree.h:3382
> #1 0x0000000000b25dfe in set_mem_attributes_minus_bitpos
> (ref=0x7ffff746f990, t=0x7ffff7ff7ab0, objectp=1, bitpos=...)
> at ../../gcc-trunk/gcc/emit-rtl.c:1957
> #2 0x0000000001137a55 in make_decl_rtl (decl=0x7ffff7ff7ab0) at
> ../../gcc-trunk/gcc/varasm.c:1518
> #3 0x000000000113b6e8 in assemble_variable (decl=0x7ffff7ff7ab0,
> top_level=<optimized out>, at_end=<optimized out>,
> dont_output_data=0) at ../../gcc-trunk/gcc/varasm.c:2246
> #4 0x000000000113f0ea in varpool_node::assemble_decl (this=0x7ffff745b000)
> at ../../gcc-trunk/gcc/varpool.c:584
> #5 0x000000000113fa17 in varpool_node::assemble_decl (this=0x7ffff745b000)
> at ../../gcc-trunk/gcc/varpool.c:750
>
>
> There are at least two problems here:
>
> First you did not want to step into the TREE_TYPE, but it happens all
> the time, even if you use "n" to step over it.
>
> And secondly, from the call stack, you don't know where you are in
> get_alias_set.
> But the code that is executing at this point is actually the x == 0 || x ==
> error_mark_node
> from alias.c, line 839, which contains the inlined body of the TREE_TYPE, but
> the rest of the if. So there is an inconsistency in the
>
> Contents of the .debug_info section:
>
> <2><4f686>: Abbrev Number: 12 (DW_TAG_inlined_subroutine)
> <4f687> DW_AT_abstract_origin: <0x53d4e>
> <4f68b> DW_AT_entry_pc : 0x7280
> <4f693> DW_AT_GNU_entry_view: 1
> <4f695> DW_AT_ranges : 0xb480
> <4f699> DW_AT_call_file : 8 <- alias.c
> <4f69a> DW_AT_call_line : 839
> <4f69c> DW_AT_call_column : 8
> <4f69d> DW_AT_sibling : <0x4f717>
>
> The File Name Table (offset 0x253):
> 8 2 0 0 alias.c
> 10 2 0 0 tree.h
>
> Contents of the .debug_ranges section:
>
> 0000b480 0000000000007280 0000000000007291
> 0000b480 0000000000002764 000000000000277e
> 0000b480 <End of list>
>
> The problem is at pc=0x7291 in the Line Number Section:
>
> Line Number Statements:
>
> [0x00008826] Special opcode 61: advance Address by 4 to 0x7284 and Line by
> 0 to 3380
> [0x00008827] Set is_stmt to 1
> [0x00008828] Special opcode 189: advance Address by 13 to 0x7291 and Line
> by 2 to 3382 (*)
> [0x00008829] Set is_stmt to 0 (**)
> [0x0000882a] Copy (view 1)
> [0x0000882b] Set File Name to entry 8 in the File Name Table <- back to
> alias.c
> [0x0000882d] Set column to 8
> [0x0000882f] Advance Line by -2543 to 839
> [0x00008832] Copy (view 2)
> [0x00008833] Set column to 27
> [0x00008835] Special opcode 61: advance Address by 4 to 0x7295 and Line by
> 0 to 839
> [0x00008836] Set column to 3
> [0x00008838] Set is_stmt to 1 <-- next line info counts: alias.c:847
> [0x00008839] Special opcode 153: advance Address by 10 to 0x729f and Line
> by 8 to 847
> [0x0000883a] Set column to 7
>
> (*) this line is tree.h:3382, but the program counter is *not* within the
> subroutine,
> but exactly at the first instruction *after* the subroutine according to the
> debug_ranges.
>
> What makes it worse, is that (**) makes gdb ignore the new location info
> alias.c:839,
> which means, normally the n command would have continued to pc=0x729f, which
> is at alias.c:847.
>
>
> The problem happens due to a block with only var
> This patch fixes this problem by moving (**) to the first statement with a
> different line number.
>
> In alias.c.316r.final this looks like that:
>
> (note 2884 2883 1995 31 0x7f903a931ba0 NOTE_INSN_BLOCK_BEG)
> (note 1995 2884 2885 31 ../../gcc-trunk/gcc/tree.h:3377
> NOTE_INSN_INLINE_ENTRY)
> (note 2885 1995 1996 31 0x7f903a931c00 NOTE_INSN_BLOCK_BEG)
> [...]
> (note 50 39 59 32 [bb 32] NOTE_INSN_BASIC_BLOCK)
> (note 59 50 60 32 NOTE_INSN_DELETED)
> (note 60 59 1997 32 NOTE_INSN_DELETED)
> (note 1997 60 2239 32 ../../gcc-trunk/gcc/tree.h:3382 NOTE_INSN_BEGIN_STMT)
> (note 2239 1997 2240 32 (var_location __tD.143911 (nil))
> NOTE_INSN_VAR_LOCATION)
> (note 2240 2239 2241 32 (var_location __sD.143912 (nil))
> NOTE_INSN_VAR_LOCATION)
> (note 2241 2240 2242 32 (var_location __fD.143913 (nil))
> NOTE_INSN_VAR_LOCATION)
> (note 2242 2241 2243 32 (var_location __lD.143914 (nil))
> NOTE_INSN_VAR_LOCATION)
> (note 2243 2242 2886 32 (var_location __gD.143915 (nil))
> NOTE_INSN_VAR_LOCATION)
> (note 2886 2243 2887 32 0x7f903a931c00 NOTE_INSN_BLOCK_END)
> (note 2887 2886 57 32 0x7f903a931ba0 NOTE_INSN_BLOCK_END)
> (insn:TI 57 2887 61 32 (set (reg/f:DI 0 ax [orig:87 _7 ] [87])
> (mem/f/j:DI (plus:DI (reg/f:DI 5 di [orig:83 t.85_2 ] [83])
> (const_int 8 [0x8])) [0 t.85_2->typedD.91322.typeD.90828+0 S8
> A64])) "../../gcc-trunk/gcc/alias.c":839:108 66 {*movdi_internal}
> (nil))
>
> So this patch detects the NOTE_INSN_VAR_LOCATION and makes the next location
> with a different file&line info a statement location, which is hopefully
> a real instruction, thus either part of the subroutine, or the first
> instruction after the subroutine, which should have the correct location.
> Once the same address has a second statement-type .loc info, gdb will ignore
> the first one, and the stepping works as expected.
>
> So this is a bit of a heuristic, but it appears to work quite well.
>
> The test case g++.dg/guality/pr55541.C is the only test where this
> change had an effect. But it is not a regression, since previously
> the test case was "unsupported" on any optimization mode, since there
> was no breakpoint at line 11, now the breakpoint works, but the variable
> value is wrong, but basically this was not working before.
>
> I don't know how to make this test xfail when compiled with optimization,
> but the do-skip-if is probably good enough for this kind of test case.
>
>
> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
> Is it OK for trunk?
>
>
> Thanks
> Bernd.
>
