Alexey Lapshin via llvm-dev
2019-Sep-11 22:31 UTC
[llvm-dev] Remove obsolete debug info while garbage collecting
Debuginfo and linker folks, we (AccessSoftek) would like to suggest a proposal for removing obsolete debug info. If you find it useful we will be happy to work on improving it. Thank you for any opinions and suggestions. Alexey. Currently when the linker does garbage collection a lot of abandoned debug info is left behind (see Appendix A for documentation). Besides inflated debug info size, we ended up with overlapping address ranges and no way to say valid vs garbage ranges. We propose removing debug info along with removing code. This would reduce debug info size and make sure debug info accuracy. There are several approaches which could be used to solve that problem: 1. Require dwarf producers to generate fragmented debug data according to DWARF5 specification: "E.3.3 Single-function-per-DWARF-compilation-unit" page 388. That approach assumes fragmenting the whole debug info per function basis and glue fragmented sections at the link time using section groups. 2. Use an additional tool, which would optimize out unnecessary debug data, something similar to dwz (dwarf compressor tool), dsymutil (links the DWARF debug information). This approach assumes additional post-link binaries processing. 3. Teach the linker to parse debug data and let it remove unused debug data. In this proposal, we focus on approach #3. We show that this approach is viable and discuss some preliminary results, leaving particular implementation out of the scope. We attach the Proof of Concept (PoC) implementation(https://reviews.llvm.org/D67469) for illustrative purposes. Please keep in mind that it is not final, and there is room for improvements (see Appendix B). However, the achieved results look quite promising and demonstrate up to 2 times size reduction and performance overhead is 30% of linking time (which is in the same ballpark as the already done section compressing (see table 2 point F)). A straightforward implementation would fully parse DWARF, create an in-memory hierarchy of DWARF objects, optimize them, and generate new sections content. Thus, it would require too much memory and would take noticeable time to process. Instead, the proposed solution is a combination of "fragmented DWARF" (#1 above) and "Optimise parsed DWARF at link stage" (#3 above). However, there is no preliminary DWARF data fragmentation step. Instead, the data is parsed at the link time, and then pieces, that correspond to live debug data, are copied into resulting sections. Essentially, the patch skips the debug info (subprograms, address ranges, line sentences) that corresponds to the dead sections. Two command-line options are added to lld: 1. --gc-debuginfo removes pieces of debug information related to the discarded sections. 2. --gc-debuginfo-types does alternative type deduplication while doing --gc-debuginfo. For the purpose of simplicity, some shortcuts were used in this PoC implementation: 1. Same types always use the same abbreviations. Full implementation should take different abbreviations into account. 2. Split DWARF is not supported. 3. Only .debug_abbrev, .debug_info, .debug_ranges, .debug_rnglists, .debug_lines tables are processed. 4. DWARF64 is not supported. We also note that the proposed approach is quite universal and could be used for other debug info optimization tasks. F.e. there exists an alternative solution for data types deduplication (other than using COMDAT sections to keep types (-fdebug-types-section)): parse DWARF, cut out duplicated types, patch type references to point to the single type definition. I.e., it uses the same approach as used for deleting unused debug info - cut out unneeded debug section content. This alternative implementation should not necessarily replace -fdebug-types-section, but it shows that this approach could be used for the type deduplication as well. This solution (combined with the global type table, which is not implemented by this patch) has some advantages though. It could reduce the number of references inside .debug_info section. It could reduce the size of the type information by deduplicating base and DW_FORM_ref_sig8 types. There are several things which would have been approved by the DWARF standard will help this implementation to work better: 1. Minimize or entirely avoid references from subprograms into other parts of .debug_info section. That would simplify splitting and removing subprograms out in that sense that it would minimize the number of references that should be parsed and followed. (DW_FORM_ref_subroutine instead of DW_FORM_ref_*, ?) 2. Create additional section - global types table (.debug_types_table). That would significantly reduce the number of references inside .debug_info section. It also makes it possible to have a 4-byte reference in this section instead of 8-bytes reference into type unit (DW_FORM_ref_types instead of DW_FORM_ref_sig8). It also makes it possible to place base types into this section and avoid per-compile unit duplication of them. Additionally, there could be achieved size reduction by not generating type unit header. Note, that new section - .debug_types_table - differs from DWARF4 section .debug_types in that sense that: it contains unique type descriptors referenced by offsets instead of list of type units referenced by DW_FORM_ref_sig8; all table entries share the same abbreviations and do not have type unit headers. 3. Define the limited scope for line programs which could be removed independently. I.e. currently .debug_line section contains a program in byte-coded language for a state machine. That program actually represents a matrix [instruction][line information]. In general, it is hard to cut out part of that program and to keep the whole program correct. Thus it would be good to specify separate scopes (related to address ranges) which could be easily removed from the program body. We evaluated the approach on LLVM and Clang codebases. The results obtained are summarized in the tables below: Abbreviations: LLVM bin size - size of llvm build/bin directory. LLVM build time - compilation time for building llvm. Clang size - size of clang binary. link time - time for linking clang binary. Errors - number of errors reported by llvm-dwarfdump --verify for clang binary. gc-dbginfo - linker option added by this patch. Spelled as "-gc-debuginfo". gc-dbgtypes - linker option added by this patch. Spelled as "-gc-debuginfo-types" Table 1. LLVM codebase. ------------------------------------------------------------------- | | CL options | LLVM bin size | LLVM build time | ------------------------------------------------------------------- | A | (default set of options*) | 100.0%(17.0GB)| 100.0%(47m26s) | | | | | | | B | A +gc-dbginfo | 82.4%(14.0GB)| 108.3%(51m24s) | | | | | | | C | A +gc-dbginfo +gc-dbgtypes| 56.5%( 9.6GB)| 117.2%(55m36s) | | | | | | | D | A +fdebug-types-section | 64.7%(11.0GB)| 98.4%(46m41s) | | | | | | | E | D +gc-dbginfo | 45.9%( 7.8GB)| 98.5%(46m43s) | | | | | | | F | D +gc-dbginfo +gc-dbgtypes| 45.9%( 7.8GB)| 99.9%(47m25s) | -------------------------------------------------------------------- Even larger size reduction could be further achieved via -ccompress-debug-sections=zlib : -------------------------------------------------------------------- | | CL options | LLVM bin size | LLVM build time | -------------------------------------------------------------------- | G | A +gc-dbginfo +gc-dbgtypes| 30.6%( 5.2GB)| 118.5%(56m11s) | | | | | | | H | D + gc-dbginfo+gc-dbgtypes| 24.7%( 4,2GB)| 102.2%(48m28s) | -------------------------------------------------------------------- Table 2. Clang binary. --------------------------------------------------------------------- | | CL options | Clang size | link time | Errors | --------------------------------------------------------------------- | A | (default set of options*)|100.0%(1,46GB)| 100%(23s)|2.5mln(**)| | | | | | | | B | A +gc-dbginfo | 87.0%(1.27GB)| 417%( 96s)| 0.12mln | | | | | | | | C | A +gc-dbginfo+gc-dbgtypes| 56.2%(0.82GB)| 530%(122s)| 0.12mln | | | | | | | | D | A +fdebug-types-section | 54.8%(0.80GB)| 74%( 17s)| 3.60mln | | | | | | | | E | D +gc-dbginfo | 43.2%(0.63GB)| 117%( 27s)| 1.30mln | | | | | | | | F | D +gc-dbginfo+gc-dbgtypes| 42.5%(0.62GB)| 121%( 28s)| 0.50mln | -------------------------------------------------------------------- Even larger size reduction could be further achieved via -ccompress-debug-sections=zlib : --------------------------------------------------------------------- | | CL options | Clang size | link time | Errors | --------------------------------------------------------------------- | G | A +gc-dbginfo+gc-dbgtypes| 31.5%(0.46GB)| 613%(141s)| 0.12mln | | | | | | | | H | D +gc-dbginfo+gc-dbgtypes| 24.7%(0.36GB)| 173%( 40s)| 0.50mln | -------------------------------------------------------------------- (*) LLVM_TARGETS_TO_BUILD X86;AArch64 LLVM_TOOL_CLANG_BUILD=ON LLVM_TOOL_LLD_BUILD=ON LLVM_USE_LINKER=lld CMAKE_CXX_FLAGS=--ffunction-sections CMAKE_C_FLAGS=--ffunction-sections CMAKE_EXE_LINKER_FLAGS=--Wl,--gc-sections CMAKE_MODULE_LINKER_FLAGS=-Wl,--gc-sections CMAKE_SHARED_LINKER_FLAGS=-Wl,--gc-sections (**) Significantly large number of errors for non-patched clang is due to error “overlapping ranges”. (***) HW configuration: OS Ubuntu 18.04 CPU Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz RAM 32018 MiB Storage SSD ====================================================================Appendix A. Documentation: previous topics on llvm-dev, links to Dwarf wiki, related reviews. These links do not relate only to -function-sections case but for other DWARF reducing questions also. 1. [llvm-dev] [lldb-dev] [LLD] How to get rid of debug info of sections deleted by garbage collector http://lists.llvm.org/pipermail/llvm-dev/2018-September/126282.html 2. [lldb-dev] LLDB behaviour for GCed sections http://lists.llvm.org/pipermail/lldb-dev/2017-March/012081.html 3. [llvm-dev] [RFC] - Deduplication of debug information in linkers (LLD). http://lists.llvm.org/pipermail/llvm-dev/2017-December/119470.html 4. [llvm-dev] [DWARF] De-segregating type units and compile units http://lists.llvm.org/pipermail/llvm-dev/2018-July/124819.html 5. [LLD][ELF][DebugInfo] llvm-symbolizer shows incorrect source line info if --gc-sections used https://reviews.llvm.org/D59553 6. Discard debuginfo for object files empty after GC https://reviews.llvm.org/D54747 7. [ELF] Add --strip-debug-non-line option https://reviews.llvm.org/D46628 8. Monolithic input section handling https://groups.google.com/forum/#!msg/generic-abi/A-1rbP8hFCA/EDA7Sf3KBwAJ 9. Using COMDAT Sections to Reduce the Size of DWARF Debug Information http://wiki.dwarfstd.org/index.php?title=COMDAT_Type_Sections 10. DWARF Extensions for Unwinding Across Merged Functions http://wiki.dwarfstd.org/index.php?title=ICF 11. Type Unit Merge http://dwarfstd.org/ShowIssue.php?issue=130526.1 12. DWARF Extensions for Separate Debug Information Files https://gcc.gnu.org/wiki/DebugFission 13. dwz dwarf compressor http://sourceware.org/git/dwz.git 14. DWARF5 standard http://dwarfstd.org/Dwarf5Std.php ====================================================================Appendix B. List of improvements which are not done by this patch. 1. Type hash calculation should not be done at the linkage stage. DWARF producer should do it. As it is already done in -fdebug-types-section implementation and defined in DWARF standard: "A type signature is computed only by a DWARF producer; a consumer need only compare two type signatures to check for equality." 2. Alternative types deduplication implementation should use global type table to store types. 3. DWARF parsing classes could be improved to parse DIEs faster. 4. Processing could probably be done per compilation unit(i.e., not loading all units in memory). 5. Better impact on the resulting size of binary could be achieved by optimizing more debug info sections. 6. This implementation is not optimized for speed. Run-time performance could be improved if optimization would be done. -------------- next part -------------- An HTML attachment was scrubbed... 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David Blaikie via llvm-dev
2019-Sep-17 00:12 UTC
[llvm-dev] Remove obsolete debug info while garbage collecting
On Wed, Sep 11, 2019 at 3:32 PM Alexey Lapshin via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Debuginfo and linker folks, we (AccessSoftek) would like to suggest a > proposal for removing obsolete debug info. If you find it useful we will be > happy to work on improving it. Thank you for any opinions and suggestions. > > Alexey. > > Currently when the linker does garbage collection a lot of abandoned > debug info is left behind (see Appendix A for documentation). Besides > inflated debug info size, we ended up with overlapping address ranges and > no way to say valid vs garbage ranges. We propose removing debug info along > with removing code. This would reduce debug info size and make sure debug > info accuracy. > > There are several approaches which could be used to solve that problem: > > 1. Require dwarf producers to generate fragmented debug data according to > DWARF5 specification: "E.3.3 Single-function-per-DWARF-compilation-unit" > page 388. That approach assumes fragmenting the whole debug info per > function basis and glue fragmented sections at the link time using section > groups. > > 2. Use an additional tool, which would optimize out unnecessary debug > data, something similar to dwz (dwarf compressor tool), dsymutil (links the > DWARF debug information). This approach assumes additional post-link > binaries processing. > > 3. Teach the linker to parse debug data and let it remove unused debug > data. > > In this proposal, we focus on approach #3. We show that this approach is > viable and discuss some preliminary results, leaving particular > implementation out of the scope. We attach the Proof of Concept (PoC) > implementation(https://reviews.llvm.org/D67469) for illustrative > purposes. Please keep in mind that it is not final, and there is room for > improvements (see Appendix B). However, the achieved results look quite > promising and demonstrate up to 2 times size reduction and performance > overhead is 30% of linking time (which is in the same ballpark as the > already done section compressing (see table 2 point F)). >Have you considered/tried reusing the DWARF minimization/deduplication/linking logic that's already in llvm's dsymutil implementation? If we're going to do that having a singular implementation would be desirable. (bonus points if we could do something like the dsymutil approach when using Split DWARF and building a DWP - taking some address table output from the linker, and using that to help trim things (or, even when having no input from the linker - at least doing more aggressive deduplication during DWP construction than can be currently done with only type units (& potentially removing/avoiding type unit overhead too))> > A straightforward implementation would fully parse DWARF, create an > in-memory hierarchy of DWARF objects, optimize them, and generate new > sections content. Thus, it would require too much memory and would take > noticeable time to process. Instead, the proposed solution is a combination > of "fragmented DWARF" (#1 above) and "Optimise parsed DWARF at link stage" > (#3 above). However, there is no preliminary DWARF data fragmentation step. > Instead, the data is parsed at the link time, and then pieces, that > correspond to live debug data, are copied into resulting sections. > Essentially, the patch skips the debug info (subprograms, address ranges, > line sentences) that corresponds to the dead sections. > > Two command-line options are added to lld: > > 1. --gc-debuginfo removes pieces of debug information related to the > discarded sections. > > 2. --gc-debuginfo-types does alternative type deduplication while doing > --gc-debuginfo. > > For the purpose of simplicity, some shortcuts were used in this PoC > implementation: > > 1. Same types always use the same abbreviations. Full implementation > should take different abbreviations into account. > 2. Split DWARF is not supported. > 3. Only .debug_abbrev, .debug_info, .debug_ranges, .debug_rnglists, > .debug_lines tables are processed. > 4. DWARF64 is not supported. > > We also note that the proposed approach is quite universal and could be > used for other debug info optimization tasks. F.e. there exists an > alternative solution for data types deduplication (other than using COMDAT > sections to keep types (-fdebug-types-section)): parse DWARF, cut out > duplicated types, patch type references to point to the single type > definition. I.e., it uses the same approach as used for deleting unused > debug info - cut out unneeded debug section content. This alternative > implementation should not necessarily replace -fdebug-types-section, but it > shows that this approach could be used for the type deduplication as well. > This solution (combined with the global type table, which is not > implemented by this patch) has some advantages though. It could reduce the > number of references inside .debug_info section. It could reduce the size > of the type information by deduplicating base and DW_FORM_ref_sig8 types. > > There are several things which would have been approved by the DWARF > standard will help this implementation to work better: > > 1. Minimize or entirely avoid references from subprograms into other parts > of .debug_info section. That would simplify splitting and removing > subprograms out in that sense that it would minimize the number of > references that should be parsed and followed. (DW_FORM_ref_subroutine > instead of DW_FORM_ref_*, ?) >Not sure I follow - by "other parts of the .debug_info section" do you mean in the same CU, or cross CU references? Any particular references you have in mind? Or encountered in practice?> 2. Create additional section - global types table (.debug_types_table). > That would significantly reduce the number of references inside .debug_info > section. It also makes it possible to have a 4-byte reference in this > section instead of 8-bytes reference into type unit (DW_FORM_ref_types > instead of DW_FORM_ref_sig8). It also makes it possible to place base types > into this section and avoid per-compile unit duplication of them. > Additionally, there could be achieved size reduction by not generating type > unit header. Note, that new section - .debug_types_table - differs from > DWARF4 section .debug_types in that sense that: it contains unique type > descriptors referenced by offsets instead of list of type units referenced > by DW_FORM_ref_sig8; all table entries share the same abbreviations and do > not have type unit headers. >What do you mean when you say "global types table" the phrasing in the above paragraph is present-tense, as though this thing exists but doesn't seem to describe what it actually is and how it achieves the things the text says it achieves. Perhaps I've missed some context here.> 3. Define the limited scope for line programs which could be removed > independently. I.e. currently .debug_line section contains a program in > byte-coded language for a state machine. That program actually represents a > matrix [instruction][line information]. In general, it is hard to cut out > part of that program and to keep the whole program correct. Thus it would > be good to specify separate scopes (related to address ranges) which could > be easily removed from the program body. >In my experience line tables are /tiny/ - have you prototyped any change in this space to have a sense of whether it would have significant savings? (it'd potentially help address the address ambiguity issues when the linker discards code, though - so might be a correctness issue rather than a size performance issue)> > We evaluated the approach on LLVM and Clang codebases. The results > obtained are summarized in the tables below: >Memory usage statistics (& confidence intervals for the build time) would probably be especially useful for comparing these tradeoffs. Doubly so when using compression (since the decompression would need to use more memory, as would the recompression - so, two different tradeoffs (compressed input, compressed output, and then both at the same time))> > Abbreviations: > > LLVM bin size - size of llvm build/bin directory. > LLVM build time - compilation time for building llvm. > Clang size - size of clang binary. > link time - time for linking clang binary. > Errors - number of errors reported by llvm-dwarfdump --verify for clang > binary. > gc-dbginfo - linker option added by this patch. Spelled as "-gc-debuginfo". > gc-dbgtypes - linker option added by this patch. Spelled as > "-gc-debuginfo-types" > > Table 1. LLVM codebase. > > ------------------------------------------------------------------- > | | CL options | LLVM bin size | LLVM build time | > ------------------------------------------------------------------- > | A | (default set of options*) | 100.0%(17.0GB)| 100.0%(47m26s) | > | | | | | > | B | A +gc-dbginfo | 82.4%(14.0GB)| 108.3%(51m24s) | > | | | | | > | C | A +gc-dbginfo +gc-dbgtypes| 56.5%( 9.6GB)| 117.2%(55m36s) | > | | | | | > | D | A +fdebug-types-section | 64.7%(11.0GB)| 98.4%(46m41s) | > | | | | | > | E | D +gc-dbginfo | 45.9%( 7.8GB)| 98.5%(46m43s) | > | | | | | > | F | D +gc-dbginfo +gc-dbgtypes| 45.9%( 7.8GB)| 99.9%(47m25s) | > -------------------------------------------------------------------- > > Even larger size reduction could be further achieved via > -ccompress-debug-sections=zlib : > > -------------------------------------------------------------------- > | | CL options | LLVM bin size | LLVM build time | > -------------------------------------------------------------------- > | G | A +gc-dbginfo +gc-dbgtypes| 30.6%( 5.2GB)| 118.5%(56m11s) | > | | | | | > | H | D + gc-dbginfo+gc-dbgtypes| 24.7%( 4,2GB)| 102.2%(48m28s) | > -------------------------------------------------------------------- > > Table 2. Clang binary. > > --------------------------------------------------------------------- > | | CL options | Clang size | link time | Errors | > --------------------------------------------------------------------- > | A | (default set of options*)|100.0%(1,46GB)| 100%(23s)|2.5mln(**)| > | | | | | | > | B | A +gc-dbginfo | 87.0%(1.27GB)| 417%( 96s)| 0.12mln | > | | | | | | > | C | A +gc-dbginfo+gc-dbgtypes| 56.2%(0.82GB)| 530%(122s)| 0.12mln | > | | | | | | > | D | A +fdebug-types-section | 54.8%(0.80GB)| 74%( 17s)| 3.60mln | > | | | | | | > | E | D +gc-dbginfo | 43.2%(0.63GB)| 117%( 27s)| 1.30mln | > | | | | | | > | F | D +gc-dbginfo+gc-dbgtypes| 42.5%(0.62GB)| 121%( 28s)| 0.50mln | > -------------------------------------------------------------------- > > Even larger size reduction could be further achieved via > -ccompress-debug-sections=zlib : > > --------------------------------------------------------------------- > | | CL options | Clang size | link time | Errors | > --------------------------------------------------------------------- > | G | A +gc-dbginfo+gc-dbgtypes| 31.5%(0.46GB)| 613%(141s)| 0.12mln | > | | | | | | > | H | D +gc-dbginfo+gc-dbgtypes| 24.7%(0.36GB)| 173%( 40s)| 0.50mln | > -------------------------------------------------------------------- > > (*) > LLVM_TARGETS_TO_BUILD X86;AArch64 > LLVM_TOOL_CLANG_BUILD=ON > LLVM_TOOL_LLD_BUILD=ON > LLVM_USE_LINKER=lld > CMAKE_CXX_FLAGS=--ffunction-sections > CMAKE_C_FLAGS=--ffunction-sections > CMAKE_EXE_LINKER_FLAGS=--Wl,--gc-sections > CMAKE_MODULE_LINKER_FLAGS=-Wl,--gc-sections > CMAKE_SHARED_LINKER_FLAGS=-Wl,--gc-sections > > (**) Significantly large number of errors for non-patched clang is due to > error “overlapping ranges”. > > (***) HW configuration: > > OS Ubuntu 18.04 > CPU Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz > RAM 32018 MiB > Storage SSD > > ====================================================================> Appendix A. Documentation: previous topics on llvm-dev, links to Dwarf > wiki, related reviews. These links do not relate only to -function-sections > case but for other DWARF reducing questions also. > > 1. [llvm-dev] [lldb-dev] [LLD] How to get rid of debug info of sections > deleted > by garbage collector > http://lists.llvm.org/pipermail/llvm-dev/2018-September/126282.html > > 2. [lldb-dev] LLDB behaviour for GCed sections > http://lists.llvm.org/pipermail/lldb-dev/2017-March/012081.html > > 3. [llvm-dev] [RFC] - Deduplication of debug information in linkers (LLD). > http://lists.llvm.org/pipermail/llvm-dev/2017-December/119470.html > > 4. [llvm-dev] [DWARF] De-segregating type units and compile units > http://lists.llvm.org/pipermail/llvm-dev/2018-July/124819.html > > 5. [LLD][ELF][DebugInfo] llvm-symbolizer shows incorrect source line info > if > --gc-sections used > https://reviews.llvm.org/D59553 > > 6. Discard debuginfo for object files empty after GC > https://reviews.llvm.org/D54747 > > 7. [ELF] Add --strip-debug-non-line option > https://reviews.llvm.org/D46628 > > 8. Monolithic input section handling > > https://groups.google.com/forum/#!msg/generic-abi/A-1rbP8hFCA/EDA7Sf3KBwAJ > > 9. Using COMDAT Sections to Reduce the Size of DWARF Debug Information > http://wiki.dwarfstd.org/index.php?title=COMDAT_Type_Sections > > 10. DWARF Extensions for Unwinding Across Merged Functions > http://wiki.dwarfstd.org/index.php?title=ICF > > 11. Type Unit Merge > http://dwarfstd.org/ShowIssue.php?issue=130526.1 > > 12. DWARF Extensions for Separate Debug Information Files > https://gcc.gnu.org/wiki/DebugFission > > 13. dwz dwarf compressor > http://sourceware.org/git/dwz.git > > 14. DWARF5 standard > http://dwarfstd.org/Dwarf5Std.php > > ====================================================================> Appendix B. List of improvements which are not done by this patch. > > 1. Type hash calculation should not be done at the linkage stage. DWARF > producer should do it. As it is already done in -fdebug-types-section > implementation and defined in DWARF standard: "A type signature is computed > only by a DWARF producer; a consumer need only compare two type signatures > to check for equality." > > 2. Alternative types deduplication implementation should use global type > table to store types. > > 3. DWARF parsing classes could be improved to parse DIEs faster. > > 4. Processing could probably be done per compilation unit(i.e., not > loading all units in memory). > > 5. Better impact on the resulting size of binary could be achieved by > optimizing more debug info sections. > > 6. This implementation is not optimized for speed. Run-time performance > could be improved if optimization would be done. > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190916/0a28c4cc/attachment.html>
Alexey Lapshin via llvm-dev
2019-Sep-18 14:25 UTC
[llvm-dev] Remove obsolete debug info while garbage collecting
17.09.2019 3:12, David Blaikie пишет:> > > On Wed, Sep 11, 2019 at 3:32 PM Alexey Lapshin via llvm-dev > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote: > > Debuginfo and linker folks, we (AccessSoftek) would like to > suggest a proposal for removing obsolete debug info. If you find > it useful we will be happy to work on improving it. Thank you for > any opinions and suggestions. > > Alexey. > > Currently when the linker does garbage collection a lot of > abandoned debug info is left behind (see Appendix A for > documentation). Besides inflated debug info size, we ended up with > overlapping address ranges and no way to say valid vs garbage > ranges. We propose removing debug info along with removing code. > This would reduce debug info size and make sure debug info accuracy. > > There are several approaches which could be used to solve that > problem: > > 1. Require dwarf producers to generate fragmented debug data > according to DWARF5 specification: "E.3.3 > Single-function-per-DWARF-compilation-unit" page 388. That > approach assumes fragmenting the whole debug info per function > basis and glue fragmented sections at the link time using section > groups. > > 2. Use an additional tool, which would optimize out unnecessary > debug data, something similar to dwz (dwarf compressor tool), > dsymutil (links the DWARF debug information). This approach > assumes additional post-link binaries processing. > > 3. Teach the linker to parse debug data and let it remove unused > debug data. > > In this proposal, we focus on approach #3. We show that this > approach is viable and discuss some preliminary results, leaving > particular implementation out of the scope. We attach the Proof of > Concept (PoC) implementation(https://reviews.llvm.org/D67469) for > illustrative purposes. Please keep in mind that it is not final, > and there is room for improvements (see Appendix B). However, the > achieved results look quite promising and demonstrate up to 2 > times size reduction and performance overhead is 30% of linking > time (which is in the same ballpark as the already done section > compressing (see table 2 point F)). > > > Have you considered/tried reusing the DWARF > minimization/deduplication/linking logic that's already in llvm's > dsymutil implementation? If we're going to do that having a singular > implementation would be desirable. > > (bonus points if we could do something like the dsymutil approach when > using Split DWARF and building a DWP - taking some address table > output from the linker, and using that to help trim things (or, even > when having no input from the linker - at least doing more aggressive > deduplication during DWP construction than can be currently done with > only type units (& potentially removing/avoiding type unit overhead too))Generally speaking, dsymutil does a very similar thing. It parses DWARF DIEs, analyzes relocations, scans through references and throws out unused DIEs. But it`s current interface does not allow to use it at link stage. I think it would be perfect to have a singular implementation. Though I did not analyze how easy or is it possible to reuse its code at the link stage, it looked like it needs a significant rework. Implementation from this proposal does removing of obsolete debug info at link stage. And so has benefits of already loaded object files, already created liveness information, generating an optimized binary from scratch. If dsymutil could be refactored in such manner that could be used at the link stage, then it`s implementation could be reused. I would research the possibility of such a refactoring.> 1. Minimize or entirely avoid references from subprograms into > other parts of .debug_info section. That would simplify splitting > and removing subprograms out in that sense that it would minimize > the number of references that should be parsed and followed. > (DW_FORM_ref_subroutine instead of DW_FORM_ref_*, ?) > > > Not sure I follow - by "other parts of the .debug_info section" do you > mean in the same CU, or cross CU references? Any particular references > you have in mind? Or encountered in practice?I mean here all kinds of references into .debug_info section. Going through references is the time-consuming task. Thus the fewer references there should be followed then the faster it works. For the cross CU references - It requires to load referenced CU. I do not know use cases where cross CU references are used. If that is the specific case and is not used inside subprograms usually, then probably it is possible to avoid it. For the same CU - there could probably be cases when references could be ignored: https://reviews.llvm.org/P8165> > 2. Create additional section - global types table > (.debug_types_table). That would significantly reduce the number > of references inside .debug_info section. It also makes it > possible to have a 4-byte reference in this section instead of > 8-bytes reference into type unit (DW_FORM_ref_types instead of > DW_FORM_ref_sig8). It also makes it possible to place base types > into this section and avoid per-compile unit duplication of them. > Additionally, there could be achieved size reduction by not > generating type unit header. Note, that new section - > .debug_types_table - differs from DWARF4 section .debug_types in > that sense that: it contains unique type descriptors referenced by > offsets instead of list of type units referenced by > DW_FORM_ref_sig8; all table entries share the same abbreviations > and do not have type unit headers. > > > What do you mean when you say "global types table" the phrasing in the > above paragraph is present-tense, as though this thing exists but > doesn't seem to describe what it actually is and how it achieves the > things the text says it achieves. Perhaps I've missed some context here.The "global types table" does not exist yet. It could be created if the discussed approach would be considered useful. Please check the comparison of possible "global types table" and currently existed type units: https://reviews.llvm.org/P8164 The benefit of using "global types table" is that it saves the space required to keep types comparing with type units solution.> 3. Define the limited scope for line programs which could be > removed independently. I.e. currently .debug_line section contains > a program in byte-coded language for a state machine. That program > actually represents a matrix [instruction][line information]. In > general, it is hard to cut out part of that program and to keep > the whole program correct. Thus it would be good to specify > separate scopes (related to address ranges) which could be easily > removed from the program body. > > > In my experience line tables are /tiny/ - have you prototyped any > change in this space to have a sense of whether it would have > significant savings? (it'd potentially help address the address > ambiguity issues when the linker discards code, though - so might be a > correctness issue rather than a size performance issue)I did not measure the value of size reduction for line table, though I think that it would be a small value. The more important thing is a correctness issue. Line table could contain information for overlapping address ranges. There is another attempt to fix that issue - https://reviews.llvm.org/D59553.> > We evaluated the approach on LLVM and Clang codebases. The results > obtained are summarized in the tables below: > > > Memory usage statistics (& confidence intervals for the build time) > would probably be especially useful for comparing these tradeoffs. > Doubly so when using compression (since the decompression would need > to use more memory, as would the recompression - so, two different > tradeoffs (compressed input, compressed output, and then both at the > same time))I would measure memory impact for that PoC implementation, but I expect it would be significant. Memory usage was not optimized yet. There are several things which might be done to reduce memory footprint: do not load all compile units into memory, avoid adding Parent field to all DIEs. Alexey.> > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190918/c0d0d2ed/attachment.html>
Rui Ueyama via llvm-dev
2019-Sep-24 05:26 UTC
[llvm-dev] Remove obsolete debug info while garbage collecting
Hi Alexey, Thank you for sharing this proposal. Reducing the size of debug info is generally a good thing, and I believe you'd see more debug info size reduction in Rust programs than in C++ programs, because I heard that the Rust compiler driver passes a lot of object files to the linker, expecting that the linker would remove most of them, which leaves dead debug info. On Thu, Sep 12, 2019 at 7:32 AM Alexey Lapshin via llvm-dev < llvm-dev at lists.llvm.org> wrote:> Debuginfo and linker folks, we (AccessSoftek) would like to suggest a > proposal for removing obsolete debug info. If you find it useful we will be > happy to work on improving it. Thank you for any opinions and suggestions. > > Alexey. > > Currently when the linker does garbage collection a lot of abandoned > debug info is left behind (see Appendix A for documentation). Besides > inflated debug info size, we ended up with overlapping address ranges and > no way to say valid vs garbage ranges. We propose removing debug info along > with removing code. This would reduce debug info size and make sure debug > info accuracy. > > There are several approaches which could be used to solve that problem: > > 1. Require dwarf producers to generate fragmented debug data according to > DWARF5 specification: "E.3.3 Single-function-per-DWARF-compilation-unit" > page 388. That approach assumes fragmenting the whole debug info per > function basis and glue fragmented sections at the link time using section > groups. > > 2. Use an additional tool, which would optimize out unnecessary debug > data, something similar to dwz (dwarf compressor tool), dsymutil (links the > DWARF debug information). This approach assumes additional post-link > binaries processing. > > 3. Teach the linker to parse debug data and let it remove unused debug > data. > > In this proposal, we focus on approach #3. We show that this approach is > viable and discuss some preliminary results, leaving particular > implementation out of the scope. We attach the Proof of Concept (PoC) > implementation(https://reviews.llvm.org/D67469) for illustrative > purposes. Please keep in mind that it is not final, and there is room for > improvements (see Appendix B). However, the achieved results look quite > promising and demonstrate up to 2 times size reduction and performance > overhead is 30% of linking time (which is in the same ballpark as the > already done section compressing (see table 2 point F)). >I believe #1 was added to DWARF5 to make link-time debug info GC possible, so could you tell me a little bit about why you chose to do #3? Is this because you want to do this for DWARF4? A straightforward implementation would fully parse DWARF, create an> in-memory hierarchy of DWARF objects, optimize them, and generate new > sections content. Thus, it would require too much memory and would take > noticeable time to process. Instead, the proposed solution is a combination > of "fragmented DWARF" (#1 above) and "Optimise parsed DWARF at link stage" > (#3 above). However, there is no preliminary DWARF data fragmentation step. > Instead, the data is parsed at the link time, and then pieces, that > correspond to live debug data, are copied into resulting sections. > Essentially, the patch skips the debug info (subprograms, address ranges, > line sentences) that corresponds to the dead sections. > > Two command-line options are added to lld: > > 1. --gc-debuginfo removes pieces of debug information related to the > discarded sections. > > 2. --gc-debuginfo-types does alternative type deduplication while doing > --gc-debuginfo. > > For the purpose of simplicity, some shortcuts were used in this PoC > implementation: > > 1. Same types always use the same abbreviations. Full implementation > should take different abbreviations into account. > 2. Split DWARF is not supported. > 3. Only .debug_abbrev, .debug_info, .debug_ranges, .debug_rnglists, > .debug_lines tables are processed. > 4. DWARF64 is not supported. > > We also note that the proposed approach is quite universal and could be > used for other debug info optimization tasks. F.e. there exists an > alternative solution for data types deduplication (other than using COMDAT > sections to keep types (-fdebug-types-section)): parse DWARF, cut out > duplicated types, patch type references to point to the single type > definition. I.e., it uses the same approach as used for deleting unused > debug info - cut out unneeded debug section content. This alternative > implementation should not necessarily replace -fdebug-types-section, but it > shows that this approach could be used for the type deduplication as well. > This solution (combined with the global type table, which is not > implemented by this patch) has some advantages though. It could reduce the > number of references inside .debug_info section. It could reduce the size > of the type information by deduplicating base and DW_FORM_ref_sig8 types. > > There are several things which would have been approved by the DWARF > standard will help this implementation to work better: > > 1. Minimize or entirely avoid references from subprograms into other parts > of .debug_info section. That would simplify splitting and removing > subprograms out in that sense that it would minimize the number of > references that should be parsed and followed. (DW_FORM_ref_subroutine > instead of DW_FORM_ref_*, ?) > > 2. Create additional section - global types table (.debug_types_table). > That would significantly reduce the number of references inside .debug_info > section. It also makes it possible to have a 4-byte reference in this > section instead of 8-bytes reference into type unit (DW_FORM_ref_types > instead of DW_FORM_ref_sig8). It also makes it possible to place base types > into this section and avoid per-compile unit duplication of them. > Additionally, there could be achieved size reduction by not generating type > unit header. Note, that new section - .debug_types_table - differs from > DWARF4 section .debug_types in that sense that: it contains unique type > descriptors referenced by offsets instead of list of type units referenced > by DW_FORM_ref_sig8; all table entries share the same abbreviations and do > not have type unit headers. > > 3. Define the limited scope for line programs which could be removed > independently. I.e. currently .debug_line section contains a program in > byte-coded language for a state machine. That program actually represents a > matrix [instruction][line information]. In general, it is hard to cut out > part of that program and to keep the whole program correct. Thus it would > be good to specify separate scopes (related to address ranges) which could > be easily removed from the program body. > > We evaluated the approach on LLVM and Clang codebases. The results > obtained are summarized in the tables below: > > Abbreviations: > > LLVM bin size - size of llvm build/bin directory. > LLVM build time - compilation time for building llvm. > Clang size - size of clang binary. > link time - time for linking clang binary. > Errors - number of errors reported by llvm-dwarfdump --verify for clang > binary. > gc-dbginfo - linker option added by this patch. Spelled as "-gc-debuginfo". > gc-dbgtypes - linker option added by this patch. Spelled as > "-gc-debuginfo-types" > > Table 1. LLVM codebase. > > ------------------------------------------------------------------- > | | CL options | LLVM bin size | LLVM build time | > ------------------------------------------------------------------- > | A | (default set of options*) | 100.0%(17.0GB)| 100.0%(47m26s) | > | | | | | > | B | A +gc-dbginfo | 82.4%(14.0GB)| 108.3%(51m24s) | > | | | | | > | C | A +gc-dbginfo +gc-dbgtypes| 56.5%( 9.6GB)| 117.2%(55m36s) | > | | | | | > | D | A +fdebug-types-section | 64.7%(11.0GB)| 98.4%(46m41s) | > | | | | | > | E | D +gc-dbginfo | 45.9%( 7.8GB)| 98.5%(46m43s) | > | | | | | > | F | D +gc-dbginfo +gc-dbgtypes| 45.9%( 7.8GB)| 99.9%(47m25s) | > -------------------------------------------------------------------- > > Even larger size reduction could be further achieved via > -ccompress-debug-sections=zlib : > > -------------------------------------------------------------------- > | | CL options | LLVM bin size | LLVM build time | > -------------------------------------------------------------------- > | G | A +gc-dbginfo +gc-dbgtypes| 30.6%( 5.2GB)| 118.5%(56m11s) | > | | | | | > | H | D + gc-dbginfo+gc-dbgtypes| 24.7%( 4,2GB)| 102.2%(48m28s) | > -------------------------------------------------------------------- > > Table 2. Clang binary. > > --------------------------------------------------------------------- > | | CL options | Clang size | link time | Errors | > --------------------------------------------------------------------- > | A | (default set of options*)|100.0%(1,46GB)| 100%(23s)|2.5mln(**)| > | | | | | | > | B | A +gc-dbginfo | 87.0%(1.27GB)| 417%( 96s)| 0.12mln | > | | | | | | > | C | A +gc-dbginfo+gc-dbgtypes| 56.2%(0.82GB)| 530%(122s)| 0.12mln | > | | | | | | > | D | A +fdebug-types-section | 54.8%(0.80GB)| 74%( 17s)| 3.60mln | > | | | | | | > | E | D +gc-dbginfo | 43.2%(0.63GB)| 117%( 27s)| 1.30mln | > | | | | | | > | F | D +gc-dbginfo+gc-dbgtypes| 42.5%(0.62GB)| 121%( 28s)| 0.50mln | > -------------------------------------------------------------------- > > Even larger size reduction could be further achieved via > -ccompress-debug-sections=zlib : > > --------------------------------------------------------------------- > | | CL options | Clang size | link time | Errors | > --------------------------------------------------------------------- > | G | A +gc-dbginfo+gc-dbgtypes| 31.5%(0.46GB)| 613%(141s)| 0.12mln | > | | | | | | > | H | D +gc-dbginfo+gc-dbgtypes| 24.7%(0.36GB)| 173%( 40s)| 0.50mln | > -------------------------------------------------------------------- > > (*) > LLVM_TARGETS_TO_BUILD X86;AArch64 > LLVM_TOOL_CLANG_BUILD=ON > LLVM_TOOL_LLD_BUILD=ON > LLVM_USE_LINKER=lld > CMAKE_CXX_FLAGS=--ffunction-sections > CMAKE_C_FLAGS=--ffunction-sections > CMAKE_EXE_LINKER_FLAGS=--Wl,--gc-sections > CMAKE_MODULE_LINKER_FLAGS=-Wl,--gc-sections > CMAKE_SHARED_LINKER_FLAGS=-Wl,--gc-sections > > (**) Significantly large number of errors for non-patched clang is due to > error “overlapping ranges”. > > (***) HW configuration: > > OS Ubuntu 18.04 > CPU Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz > RAM 32018 MiB > Storage SSD > > ====================================================================> Appendix A. Documentation: previous topics on llvm-dev, links to Dwarf > wiki, related reviews. These links do not relate only to -function-sections > case but for other DWARF reducing questions also. > > 1. [llvm-dev] [lldb-dev] [LLD] How to get rid of debug info of sections > deleted > by garbage collector > http://lists.llvm.org/pipermail/llvm-dev/2018-September/126282.html > > 2. [lldb-dev] LLDB behaviour for GCed sections > http://lists.llvm.org/pipermail/lldb-dev/2017-March/012081.html > > 3. [llvm-dev] [RFC] - Deduplication of debug information in linkers (LLD). > http://lists.llvm.org/pipermail/llvm-dev/2017-December/119470.html > > 4. [llvm-dev] [DWARF] De-segregating type units and compile units > http://lists.llvm.org/pipermail/llvm-dev/2018-July/124819.html > > 5. [LLD][ELF][DebugInfo] llvm-symbolizer shows incorrect source line info > if > --gc-sections used > https://reviews.llvm.org/D59553 > > 6. Discard debuginfo for object files empty after GC > https://reviews.llvm.org/D54747 > > 7. [ELF] Add --strip-debug-non-line option > https://reviews.llvm.org/D46628 > > 8. Monolithic input section handling > > https://groups.google.com/forum/#!msg/generic-abi/A-1rbP8hFCA/EDA7Sf3KBwAJ > > 9. Using COMDAT Sections to Reduce the Size of DWARF Debug Information > http://wiki.dwarfstd.org/index.php?title=COMDAT_Type_Sections > > 10. DWARF Extensions for Unwinding Across Merged Functions > http://wiki.dwarfstd.org/index.php?title=ICF > > 11. Type Unit Merge > http://dwarfstd.org/ShowIssue.php?issue=130526.1 > > 12. DWARF Extensions for Separate Debug Information Files > https://gcc.gnu.org/wiki/DebugFission > > 13. dwz dwarf compressor > http://sourceware.org/git/dwz.git > > 14. DWARF5 standard > http://dwarfstd.org/Dwarf5Std.php > > ====================================================================> Appendix B. List of improvements which are not done by this patch. > > 1. Type hash calculation should not be done at the linkage stage. DWARF > producer should do it. As it is already done in -fdebug-types-section > implementation and defined in DWARF standard: "A type signature is computed > only by a DWARF producer; a consumer need only compare two type signatures > to check for equality." > > 2. Alternative types deduplication implementation should use global type > table to store types. > > 3. DWARF parsing classes could be improved to parse DIEs faster. > > 4. Processing could probably be done per compilation unit(i.e., not > loading all units in memory). > > 5. Better impact on the resulting size of binary could be achieved by > optimizing more debug info sections. > > 6. This implementation is not optimized for speed. Run-time performance > could be improved if optimization would be done. > > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190924/a3d78224/attachment-0001.html>
Alexey Lapshin via llvm-dev
2019-Sep-24 22:05 UTC
[llvm-dev] Remove obsolete debug info while garbage collecting
24.09.2019 8:26, Rui Ueyama пишет:> Hi Alexey, > > Thank you for sharing this proposal. Reducing the size of debug info > is generally a good thing, and I believe you'd see more debug info > size reduction in Rust programs than in C++ programs, because I heard > that the Rust compiler driver passes a lot of object files to the > linker, expecting that the linker would remove most of them, which > leaves dead debug info. >Hi Rui, Thanks!> On Thu, Sep 12, 2019 at 7:32 AM Alexey Lapshin via llvm-dev > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote: > > Debuginfo and linker folks, we (AccessSoftek) would like to > suggest a proposal for removing obsolete debug info. If you find > it useful we will be happy to work on improving it. Thank you for > any opinions and suggestions. > > Alexey. > > Currently when the linker does garbage collection a lot of > abandoned debug info is left behind (see Appendix A for > documentation). Besides inflated debug info size, we ended up with > overlapping address ranges and no way to say valid vs garbage > ranges. We propose removing debug info along with removing code. > This would reduce debug info size and make sure debug info accuracy. > > There are several approaches which could be used to solve that > problem: > > 1. Require dwarf producers to generate fragmented debug data > according to DWARF5 specification: "E.3.3 > Single-function-per-DWARF-compilation-unit" page 388. That > approach assumes fragmenting the whole debug info per function > basis and glue fragmented sections at the link time using section > groups. > > 2. Use an additional tool, which would optimize out unnecessary > debug data, something similar to dwz (dwarf compressor tool), > dsymutil (links the DWARF debug information). This approach > assumes additional post-link binaries processing. > > 3. Teach the linker to parse debug data and let it remove unused > debug data. > > In this proposal, we focus on approach #3. We show that this > approach is viable and discuss some preliminary results, leaving > particular implementation out of the scope. We attach the Proof of > Concept (PoC) implementation(https://reviews.llvm.org/D67469) for > illustrative purposes. Please keep in mind that it is not final, > and there is room for improvements (see Appendix B). However, the > achieved results look quite promising and demonstrate up to 2 > times size reduction and performance overhead is 30% of linking > time (which is in the same ballpark as the already done section > compressing (see table 2 point F)). > > > I believe #1 was added to DWARF5 to make link-time debug info GC > possible, so could you tell me a little bit about why you chose to do > #3? Is this because you want to do this for DWARF4? > >No, that proposal is not DWARF-4 specific. The proposal is for DWARF-5 also. The solution added to DWARF-5("E.3.3 Single-function-per-DWARF-compilation-unit" page 388.) is not a complete solution. This is a recommendation which needs to have an additional specification. There is -fdebug-types-section implementation which follows that recommendation. Other cases(other than type units) do not easily fit into this recommendation. There are tables which have a common header. F.e. .debug_line, .debug_rnglists, .debug_addr. It is not clear how these tables could be separated between section groups. The more important thing is the fragmentation itself. Dividing debug tables into pieces would increase debug info size. It also would significantly complicate code working with debug info. F.e. include/llvm/DebugInfo/DWARF/DWARFObject.h has interface for class DWARFObject. It currently is not ready for the case when there could be multiple tables. Patch introducing support for multiple tables would be massive change affected many places in llvm codebase. Another thing is that not only the llvm code base but all other DWARF consumers should be changed to support fragmented debug info. Shortly, if all debug tables would be fragmented then working with debug info would be significantly complicated. Thus the reasons to select #3 are : 1. It could be done in a single place, not affecting other parts of the llvm code base. 2. It does not require other DWARF consumers to implement support for it. 3. Avoiding fragmentation would save space. 4. Processing of not fragmented debug info is faster. 5. No need to adapt DWARF tables for fragmentation. They could be handled with their current state. Alexey> > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190925/f032e665/attachment.html>
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