[lldb-dev] lldb subprogram ranges support
David Blaikie via lldb-dev
lldb-dev at lists.llvm.org
Tue Dec 29 14:06:03 PST 2020
On Wed, Dec 23, 2020 at 7:02 PM Sriraman Tallam <tmsriram at google.com> wrote:
>
>
> 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.
>
I'd looked at the DWARF being well-formed & for the most part efficient as
it can be, given the nature of Basic Block Sections - but I haven't done
any debugger testing with it.
You mentioned gdb might already be pretty well setup for functions that are
split into multiple chunks because GCC does this under some circumstances?
But it looks like lldb might not be so well situated.
> How do I test if this breaks with bbsections?
>
Test printing out the value of a variable in a function with more than one
section, eg:
$ ~/dev/llvm/build/default/bin/lldb ./b
(lldb) target create "./b"
Current executable set to '/usr/local/google/home/blaikie/dev/scratch/b'
(x86_64).
(lldb) b main
Breakpoint 1: where = b`main + 15, address = 0x000000000040112f
(lldb) start
*error: *'start' is not a valid command.
(lldb) r
Process 1827628 launched: '/usr/local/google/home/blaikie/dev/scratch/b'
(x86_64)
Process 1827628 stopped
* thread #1, name = 'b', stop reason = breakpoint 1.1
frame #0: 0x000000000040112f b`main at test.cpp:5:7
2 int j = 12;
3 }
4 int main() {
-> 5 int i = 7;
6 if (i)
7 f1();
8 }
(lldb) p i
error: <user expression 0>:1:1: use of undeclared identifier 'i'
i
^
(lldb) ^D
$ clang++-tot test.cpp -g -o b
$ ~/dev/llvm/build/default/bin/lldb ./b
(lldb) target create "./b"
Current executable set to '/usr/local/google/home/blaikie/dev/scratch/b'
(x86_64).
(lldb) b main
Breakpoint 1: where = b`main + 15 at test.cpp:5:7, address =
0x000000000040112f
(lldb) r
Process 1828108 launched: '/usr/local/google/home/blaikie/dev/scratch/b'
(x86_64)
p i
Process 1828108 stopped
* thread #1, name = 'b', stop reason = breakpoint 1.1
frame #0: 0x000000000040112f b`main at test.cpp:5:7
2 int j = 12;
3 }
4 int main() {
-> 5 int i = 7;
6 if (i)
7 f1();
8 }
(lldb) p i
(int) $0 = 0
(lldb) ^D
$ cat test.cpp
void f1() {
int j = 12;
}
int main() {
int i = 7;
if (i)
f1();
}
So, yeah, seems like DW_AT_ranges on a DW_TAG_subprogram is a bit buggy
with lldb & that'll need to be fixed for Propeller to be usable with lldb.
For my "ranges everywhere" feature - nice to fix, but given we/Google/my
use case uses -ffunction-sections, subprogram ranges don't actually ever
get used in that situation (since every function starts at a new relocated
address - subprogram address ranges can't share address pool entries anyway
- so they never get DW_AT_ranges in this case), so I could tweak
ranges-everywhere to not apply to subprogram ranges for now to keep it more
usable/unsurprising.
> 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.
>
This applies to DWARFv4 as well, as shown above - sorry for the confusion
there. I happened to be experimenting with DWARFv5 range features - but it
shows lldb has some problems with ranges on subprograms in general (& even
if the ranges only contains a single range (expressed with a range list,
rather than with low/high pc) it still breaks)
>
>
>>
>> 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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