On Thu, Dec 12, 2024 at 9:41 AM David Anderson via Dwarf-discuss <
dwarf-discuss@lists.dwarfstd.org> wrote:
> On 12/11/24 16:59, Alex via Dwarf-discuss wrote:
> > ## Proposed Changes
> >
> > In Section 2.17 (Location Descriptions), replace the text on p. 53, line
> 15:
> >
> > From:
> > ```
> > Bounded range entries in a range list may not overlap. There is no
> > requirement that the entries be ordered in any particular way.
> > ```
>
> Neither the proposal nor the html give a clear self-contained example
> of the overlap case and I think that would be important to add.
>
Aggressive linker identical code folding is the main source of potentially
overlapping CU ranges (within a single CU, or across two CUs) (& then
combined with fragmented functions (what clang calls
"-fbasic-block-sections", for instance) - causing one subprogram's ranges
to overlap with itself or another subprogram's ranges (& then that causes
overlapping at the CU level too))
eg:
void f1() { }
void f2() { }
int main() { f1(); f2(); }
$ clang++-tot test.cpp -g -ffunction-sections -fuse-ld=gold -Wl,--icf=all
&& llvm-dwarfdump-tot a.out | grep DW_AT_ranges -A3
DW_AT_ranges (indexed (0x0) rangelist = 0x00000010
[0x0000000000000670, 0x0000000000000676)
[0x0000000000000670, 0x0000000000000676)
[0x0000000000000680, 0x0000000000000692))
Admittedly, lld's icf doesn't have this result - it resolves one copy to an
address, and the other copy is resolved to 0:
$ clang++-tot test.cpp -g -ffunction-sections -fuse-ld=lld -Wl,--icf=all &&
llvm-dwarfdump-tot a.out | grep DW_AT_ranges -A3
DW_AT_ranges (indexed (0x0) rangelist = 0x00000010
[0x0000000000001700, 0x0000000000001706)
[0x0000000000000000, 0x0000000000000006)
[0x0000000000001710, 0x0000000000001722))
(& I'm not sure if/what support bfd ld has for ICF)
Combine this with basic block sections (nodebug to simplify the DWARF
output, but not necessary to reproduce the issue):
__attribute__((nodebug)) void f1() { }
void f2(bool b) {
if (b)
f1();
}
void f3(bool b) {
if (!b)
f1();
}
__attribute__((nodebug)) int main() {
}
CU ranges:
DW_AT_ranges (indexed (0x2) rangelist = 0x0000002c
[0x0000000000000680, 0x000000000000069f)
[0x000000000000069f, 0x00000000000006a9)
[0x00000000000006a9, 0x00000000000006af)
[0x00000000000006b0, 0x00000000000006cf)
[0x000000000000069f, 0x00000000000006a9)
[0x00000000000006a9, 0x00000000000006af))
Subprogram ranges:
f2:
DW_AT_ranges (indexed (0x0) rangelist = 0x00000018
[0x0000000000000680, 0x000000000000069f)
[0x000000000000069f, 0x00000000000006a9)
[0x00000000000006a9, 0x00000000000006af))
f3:
DW_AT_ranges (indexed (0x1) rangelist = 0x00000022
[0x00000000000006b0, 0x00000000000006cf)
[0x000000000000069f, 0x00000000000006a9)
[0x00000000000006a9, 0x00000000000006af))
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