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The test gdb.cp/step-and-next-inline.exp creates a test binary called
step-and-next-inline-no-header. This test includes a function
`tree_check` which is inlined 3 times.
When testing with some older versions of gcc (I've tried 8.4.0, 9.3.1)
we see the following DWARF representing one of the inline instances of
tree_check:
<2><8d9>: Abbrev Number: 38 (DW_TAG_inlined_subroutine)
<8da> DW_AT_abstract_origin: <0x9ee>
<8de> DW_AT_entry_pc : 0x401165
<8e6> DW_AT_GNU_entry_view: 0
<8e7> DW_AT_ranges : 0x30
<8eb> DW_AT_call_file : 1
<8ec> DW_AT_call_line : 52
<8ed> DW_AT_call_column : 10
<8ee> DW_AT_sibling : <0x92d>
...
<1><9ee>: Abbrev Number: 46 (DW_TAG_subprogram)
<9ef> DW_AT_external : 1
<9ef> DW_AT_name : (indirect string, offset: 0xe8): tree_check
<9f3> DW_AT_decl_file : 1
<9f4> DW_AT_decl_line : 38
<9f5> DW_AT_decl_column : 1
<9f6> DW_AT_linkage_name: (indirect string, offset: 0x2f2): _Z10tree_checkP4treei
<9fa> DW_AT_type : <0x9e8>
<9fe> DW_AT_inline : 3 (declared as inline and inlined)
<9ff> DW_AT_sibling : <0xa22>
...
Contents of the .debug_ranges section:
Offset Begin End
...
00000030 0000000000401165 0000000000401165 (start == end)
00000030 0000000000401169 0000000000401173
00000030 0000000000401040 0000000000401045
00000030 <End of list>
...
Notice that one of the sub-ranges of tree-check is empty, this is the
line marked 'start == end'. As the end address is the first address
after the range, this range cover absolutely no code.
But notice too that the DW_AT_entry_pc for the inline instance points
at this empty range.
Further, notice that despite the ordering of the sub-ranges, the empty
range is actually in the middle of the region defined by the lowest
address to the highest address. The ordering is not a problem, the
DWARF spec doesn't require that ranges be in any particular order.
However, this empty range is causing issues with GDB newly acquire
DW_AT_entry_pc support.
GDB already rejects, and has done for a long time, empty sub-ranges,
after all, the DWARF spec is clear that such a range covers no code.
The recent DW_AT_entry_pc patch also had GDB reject an entry-pc which
was outside of the low/high bounds of a block.
But in this case, the entry-pc value is within the bounds of a block,
it's just not within any useful sub-range. As a consequence, GDB is
storing the entry-pc value, and making use of it, but when GDB stops,
and tries to work out which block the inferior is in, it fails to spot
that the inferior is within tree_check, and instead reports the
function into which tree_check was inlined.
I've tested with newer versions of gcc (12.2.0 and 14.2.0) and with
these versions gcc is still generating the empty sub-range, but now
this empty sub-range is no longer the entry point. Here's the
corresponding ranges table from gcc 14.2.0:
Contents of the .debug_rnglists section:
Table at Offset: 0:
Length: 0x56
DWARF version: 5
Address size: 8
Segment size: 0
Offset entries: 0
Offset Begin End
...
00000021 0000000000401165 000000000040116f
0000002b 0000000000401040 (base address)
00000034 0000000000401040 0000000000401040 (start == end)
00000037 0000000000401041 0000000000401046
0000003a <End of list>
...
The DW_AT_entry_pc is 0x401165, but this is not the empty sub-range,
as a result, when GDB stops at the entry-pc, GDB will correctly spot
that the inferior is in the tree_check function.
The fix I propose here is, instead of rejecting entry-pc values that
are outside the block's low/high range, instead reject entry-pc values
that are not inside any of the block's sub-ranges.
Now, GDB will ignore the prescribed entry-pc, and will instead select
a suitable default entry-pc based on either the block's low-pc value,
or the first address of the first range.
I have extended the gdb.cp/step-and-next-inline.exp test to check this
case, but this does depend on the compiler version being used (newer
compilers will always pass, even without the fix).
So I have also added a DWARF assembler test to cover this case.
Reviewed-By: Kevin Buettner <kevinb@redhat.com>