[llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug info in lld.

Fri May 8 06:18:13 PDT 2020

Folks, we work on optimization of binary size and improvement of debug info quality.
To reduce the size of the binary we use -ffunction-sections so that unused code would be garbage collected.
When the linker does garbage collection, a lot of abandoned debug info is left behind.
Besides inflated debug info size, we ended up with overlapping address ranges and no way to say valid vs garbage ranges(D59553).
To resolve these two problems, we use implementation extracted from dsymutil https://reviews.llvm.org/D74169.
It adds --gc-debuginfo command line option to the linker to remove obsolete debug info.
Currently, it has the following limitations: does not support DWARF5, modules, -fdebug-types-section, type units, .debug_types, multiple .debug_info sections, split DWARF, thin lto.

Following are size/performance results for the D74169:

A: --function-sections --gc-sections
B: --function-sections --gc-sections --gc-debuginfo
C: --function-sections --gc-sections --fdebug-types-section
D: --function-sections --gc-sections --gsplit-dwarf
E: --function-sections --gc-sections --gc-debuginfo --compress-debug-sections=zlib

LLVM code base:
--------------------------------------------------------------
| Options |    build time   |    bin size   |    lib size    |
--------------------------------------------------------------
|    A    |    54min(100%)  |   19.0G(100%) |  15.0G(100.0%) |
--------------------------------------------------------------
|    B    |    65min(120%)  |    9.7G( 51%) |  12.0G( 80.0%) |
--------------------------------------------------------------
|    C    |    53min( 98%)  |   12.0G( 63%) |  15.0G(100.0%) |
--------------------------------------------------------------
|    D    |    52min( 96%)  |   12.0G( 63%) |   8.2G( 55.0%) |
--------------------------------------------------------------
|    E    |    64min(118%)  |    5.3G( 28%) |  12.0G( 80.0%) |
--------------------------------------------------------------

Clang binary:
-------------------------------------------------------------
| Options |      size      |     link time  |  used memory  |
-------------------------------------------------------------
|    A    |    1.50G(100%) |    9sec(100%)  |  9307MB(100%) |
-------------------------------------------------------------
|    B    |    0.76G( 50%) |   68sec(755%)  | 15055MB(161%) |
-------------------------------------------------------------
|    C    |    0.82G( 54%) |    8sec( 89%)  |  8402MB( 90%) |
-------------------------------------------------------------
|    D    |    0.96G( 64%) |    6sec( 67%)  |  4273MB( 46%) |
-------------------------------------------------------------
|    E    |    0.43G( 29%) |   77sec(855%)  | 15000MB(161%) |
-------------------------------------------------------------

lldb loading time:
--------------------------------------------
| Options |      time     |   used memory  |
--------------------------------------------
|    A    |  6.4sec(100%) |  1495MB(100%)  |
--------------------------------------------
|    B    |  4.0sec( 63%) |   826MB( 55%)  |
--------------------------------------------
|    C    |  3.7sec( 58%) |   877MB( 59%)  |
--------------------------------------------
|    D    |  4.3sec( 67%) |  1023MB( 69%)  |
--------------------------------------------
|    E    |  2.1sec( 33%) |   478MB( 32%)  |
--------------------------------------------

I want to discuss the results and to decide whether it is worth to integrate of D74169:

improvements:

1. Reduces the size of debug info(50%).
2. Resolves overlapping of address ranges(D59553).
3. Reduced size of debug info allows tools to work faster and to require less memory.

drawbacks and not implemented features:

1. linking time is increased(755%).

  The --gc-debuginfo option is off by default. So it would affect only those who need it and explicitly specified it.

  I think the current DWARFLinker code could be optimized more to improve performance results.

2. Support of type units.

  That could be implemented further.

3. DWARF5.

   Current DWARFEmitter/DWARFStreamer has an implementation for DWARF generation, which does not support
DWARF5(only debug_names table). At the same time, there already exists code in CodeGen/AsmPrinter/DwarfDebug.h,
which implements most of DWARF5. It seems that DWARFEmitter/DWARFStreamer should be rewritten using
DwarfDebug/DwarfFile. Though I am not sure whether it would be easy to re-use DwarfDebug/DwarfFile.
It would probably be necessary to separate some intermediate level of DwarfDebug/DwarfFile.

4. split DWARF support.

   This solution does not work with split DWARF currently. But it could be useful for the split dwarf in two ways:

   a) The generation of skeleton file could be changed in such a way that address ranges pointing to garbage
collected code would be replaced with lowpc=0, highpc=0. That would solve the problem of overlapping address
ranges(D59553).

   b) The approach similar to dsymutil implementation could be used to generate monolithic debuginfo created
from .dwo files. That suggestion is from - https://reviews.llvm.org/D74169#1888386.
      i.e., DWARFLinker could be taught to generate the same output as D74169 but for split DWARF as the source.

5. -fmodules-debuginfo

   That problem was described in this review - https://reviews.llvm.org/D54747#1505462 . Currently, DWARFLinker/dsymutil has the same problem. It could be solved using the fact that DWARFLinker analyzes debuginfo. It could recognize debug info generated for the module and keep it(compile units containing debug info for modules do not have low_pc, high_pc).

6. -flto=thin

   That problem was described in this review https://reviews.llvm.org/D54747#1503720. It also exists in current DWARFLinker/dsymutil implementation. I think that problem should be discussed more: it could probably be fixed by avoiding generation of such incomplete declaration during thinlto, or, alternatively, DWARFLinker could recognize such situation and copy missed type declaration.

=======================================================================================

Debuginfo, Linker folks, What do you think about current results and future directions?

It introduces quite a significant linking time increase(6x-8x). But it would affect only those who use that feature.

Thus the users will be able to decide whether that linking time increase is acceptable or not.
Resolving all 1-6 points is quite a significant work. But, in the result, debug info is more correct and compact.

Do you think that it would be good to integrate it and to start to work on improving?

Thank you, Alexey.

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