[llvm-dev] lldb subprogram ranges support

[Sriraman Tallam via llvm-dev]

On Wed, Dec 23, 2020 at 4:46 PM David Blaikie <dblaikie at gmail.com> wrote:

> Hey folks,
>
> So I've been doing some more testing/implementation work on various
> address pool reduction strategies previously discussed back in January (
> http://lists.llvm.org/pipermail/llvm-dev/2020-January/thread.html#138029
> ).
>
> I've committed a -mllvm flag to allow experimenting with the first of
> these strategies: Always using ranges in DWARFv5 (the flag has no effect
> pre-v5). Since ranges can use address pool entries, this allows significant
> address reuse (clang opt split-dwarf 13% reduction in object file size,
> specifically a reduction in aggregate .rela.debug_addr size from 78MB to
> 16MB - the lowest this could go is approximately 8MB (this is the size of
> .rela.debug_line)).
>
> It causes one lldb test to
> fail lldb/test/SymbolFile/DWARF/Output/debug-types-expressions.test which
> reveals that lldb has some trouble with ranges on DW_TAG_subprograms.
>
> Anyone happen to have ideas about what the problem might be? Anyone
> interested in fixing this? (Jordan, maybe?)
>
> Sri: Sounded like you folks had done some testing of Propeller with lldb -
> and I'd expect it to trip over this same problem, since it'll cause ranges
> to be used for DW_TAG_subprograms to an even greater degree. Have you come
> across anything like this?
>

Not sure David.  I think you tested basic block sections for v5 a while
back.  How do I test if this breaks with bbsections? I can give you a
simple program with bb sections that would create a lot of ranges. Any
pointers? My understanding of DWARF v5 is near zero so please bear with me.
Thanks.


>
> Here's a small example:
>
> (the test has an inline function to force the output file to have more
> than one section (otherwise it'll all be in the text section, the CU's
> low_pc will be relocatable and all the other addresses will be relative to
> that - so there won't be any benefit to using ranges) and 'main' is the
> second function, so it starts at an offset relative to the address in the
> address pool (which will be f2's starting address) and benefit from using
> ranges to share that address)
>
> $ cat test.cpp
>
> inline __attribute__((noinline)) void f1() { }
>
> void f2() {
>
> }
>
> int main() {
>
>   int i = 7;
>
>   f1();
>
> }
> $ ~/dev/llvm/build/default/bin/lldb ./a
>
> (lldb) target create "./a"
>
> Current executable set to
> '/usr/local/google/home/blaikie/dev/scratch/always_ranges/a' (x86_64).
>
> (lldb) b main
>
> Breakpoint 1: where = a`main + 8 at test.cpp:5:7, address =
> 0x0000000000401128
>
> (lldb) r
>
> Process 2271305 launched:
> '/usr/local/google/home/blaikie/dev/scratch/always_ranges/a' (x86_64)
>
> p iProcess 2271305 stopped
>
> * thread #1, name = 'a', stop reason = breakpoint 1.1
>
>     frame #0: 0x0000000000401128 a`main at test.cpp:5:7
>
>    2    void f2() {
>
>    3    }
>
>    4    int main() {
>
> -> 5      int i = 7;
>
>    6      f1();
>
>    7    }
>
> (lldb) p i
>
> (int) $0 = 0
>
> $ ~/dev/llvm/build/default/bin/lldb ./b
>
> (lldb) target create "./b"
>
> Current executable set to
> '/usr/local/google/home/blaikie/dev/scratch/always_ranges/b' (x86_64).
>
> (lldb) b main
>
> Breakpoint 1: where = b`main + 8, address = 0x0000000000401128
>
> (lldb) r
>
> Process 2271759 launched:
> '/usr/local/google/home/blaikie/dev/scratch/always_ranges/b' (x86_64)
>
> Process 2271759 stopped
>
> * thread #1, name = 'b', stop reason = breakpoint 1.1
>
>     frame #0: 0x0000000000401128 b`main at test.cpp:5:7
>
>    2    void f2() {
>
>    3    }
>
>    4    int main() {
>
> -> 5      int i = 7;
>
>    6      f1();
>
>    7    }
>
> (lldb) p i
>
> error: <user expression 0>:1:1: use of undeclared identifier 'i'
>
> i
>
> ^
>
> $ diff <(llvm-dwarfdump-tot a | sed -e "s/0x[0-9a-f]\{8\}//g")
> <(llvm-dwarfdump-tot b | sed -e "s/0x[0-9a-f]\{8\}//g")
>
> 1c1
>
> < a:    file format elf64-x86-64
>
> ---
>
> > b:    file format elf64-x86-64
>
> 14c14
>
> <               DW_AT_ranges    (indexed (0x0) rangelist =
>
> ---
>
> >               DW_AT_ranges    (indexed (0x1) rangelist =
>
> 31,32c31,32
>
> <                 DW_AT_low_pc  (00401120)
>
> <                 DW_AT_high_pc (0040113c)
>
> ---
>
> >                 DW_AT_ranges  (indexed (0x0) rangelist =
>
> >                    [00401120, 0040113c))
>
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