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On 10/27/19 9:14 AM, Bernd Edlinger wrote: > 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. >>
