[lldb-dev] lldb subprogram ranges support

[David Blaikie via lldb-dev]

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?

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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