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2020-03-26[AIX] discard the label in the csect of function description and use ↵diggerlin1-3/+5
qualname for linkage SUMMARY: SUMMARY for a source file "test.c" void foo() {}; llc will generate assembly code as (assembly patch) .globl foo .globl .foo .csect foo[DS] foo: .long .foo .long TOC[TC0] .long 0 and symbol table as (xcoff object file) [4] m 0x00000004 .data 1 unamex foo [5] a4 0x0000000c 0 0 SD DS 0 0 [6] m 0x00000004 .data 1 extern foo [7] a4 0x00000004 0 0 LD DS 0 0 After first patch, the assembly will be as .globl foo[DS] # -- Begin function foo .globl .foo .align 2 .csect foo[DS] .long .foo .long TOC[TC0] .long 0 and symbol table will as [6] m 0x00000004 .data 1 extern foo [7] a4 0x00000004 0 0 DS DS 0 0 Change the code for the assembly path and xcoff objectfile patch for llc. Reviewers: Jason Liu Subscribers: wuzish, nemanjai, hiraditya Differential Revision: https://reviews.llvm.org/D76162
2020-03-20[X86] Reland D71360 Clean up UseInitArray initialization for ↵Fangrui Song1-0/+1
X86ELFTargetObjectFile -fuse-init-array is now the CC1 default but TargetLoweringObjectFileELF::UseInitArray still defaults to false. The following two unknown OS target triples continue using .ctors/.dtors because InitializeELF is not called. clang -target i386 -c a.c clang -target x86_64 -c a.c This cleanup fixes this as a bonus. X86SpeculativeLoadHardeningPass::tracePredStateThroughCall can call MCContext::createTempSymbol before TargetLoweringObjectFileELF::Initialize(). We need to call TargetLoweringObjectFileELF::Initialize() ealier. test/CodeGen/X86/speculative-load-hardening-indirect.ll Differential Revision: https://reviews.llvm.org/D71360
2020-03-20Temporarily Revert "[X86] Reland D71360 Clean up UseInitArray initialization ↵Eric Christopher1-1/+0
for X86ELFTargetObjectFile" as it's causing msan failures. This reverts commit 7899fe9da8d8df6f19ddcbbb877ea124d711c54b.
2020-03-20[X86] Reland D71360 Clean up UseInitArray initialization for ↵Fangrui Song1-0/+1
X86ELFTargetObjectFile UseInitArray is now the CC1 default but TargetLoweringObjectFileELF::UseInitArray still defaults to false. The following two unknown OS target triples continue using .ctors/.dtors because InitializeELF is not called. clang -target i386 -c a.c clang -target x86_64 -c a.c This cleanup fixes this as a bonus. Differential Revision: https://reviews.llvm.org/D71360
2020-03-16Basic Block Sections support in LLVM.Sriraman Tallam1-1/+53
This is the second patch in a series of patches to enable basic block sections support. This patch adds support for: * Creating direct jumps at the end of basic blocks that have fall through instructions. * New pass, bbsections-prepare, that analyzes placement of basic blocks in sections. * Actual placing of a basic block in a unique section with special handling of exception handling blocks. * Supports placing a subset of basic blocks in a unique section. * Support for MIR serialization and deserialization with basic block sections. Parent patch : D68063 Differential Revision: https://reviews.llvm.org/D73674
2020-03-11Fix conflict value for metadata "Objective-C Garbage Collection" in the mix ↵Jin Lin1-0/+9
of swift and Objective-C bitcode Summary: The change is to fix conflict value for metadata "Objective-C Garbage Collection" in the mix of swift and Objective-C bitcode. The purpose is to provide the support of LTO for swift and Objective-C mixed project. Reviewers: rjmccall, ahatanak, steven_wu Reviewed By: rjmccall, steven_wu Subscribers: manmanren, mehdi_amini, hiraditya, dexonsmith, llvm-commits, jinlin Tags: #llvm Differential Revision: https://reviews.llvm.org/D71219
2020-03-06[AIX] Handle LinkOnceODRLinkage and AppendingLinkage for static init gloabl ↵Xiangling Liao1-0/+4
arrays Handle LinkOnceODRLinkage; Handle AppendingLinkage type for llvm.global_ctors/dtors static init global arrays; Differential Revision: https://reviews.llvm.org/D75305
2020-02-29[MC] Add MCStreamer::emitInt{8,16,32,64}Fangrui Song1-8/+8
Similar to AsmPrinter::emitInt{8,16,32,64}.
2020-02-28Reland: [Coverage] Revise format to reduce binary sizeVedant Kumar1-0/+2
Try again with an up-to-date version of D69471 (99317124 was a stale revision). --- Revise the coverage mapping format to reduce binary size by: 1. Naming function records and marking them `linkonce_odr`, and 2. Compressing filenames. This shrinks the size of llc's coverage segment by 82% (334MB -> 62MB) and speeds up end-to-end single-threaded report generation by 10%. For reference the compressed name data in llc is 81MB (__llvm_prf_names). Rationale for changes to the format: - With the current format, most coverage function records are discarded. E.g., more than 97% of the records in llc are *duplicate* placeholders for functions visible-but-not-used in TUs. Placeholders *are* used to show under-covered functions, but duplicate placeholders waste space. - We reached general consensus about giving (1) a try at the 2017 code coverage BoF [1]. The thinking was that using `linkonce_odr` to merge duplicates is simpler than alternatives like teaching build systems about a coverage-aware database/module/etc on the side. - Revising the format is expensive due to the backwards compatibility requirement, so we might as well compress filenames while we're at it. This shrinks the encoded filenames in llc by 86% (12MB -> 1.6MB). See CoverageMappingFormat.rst for the details on what exactly has changed. Fixes PR34533 [2], hopefully. [1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118428.html [2] https://bugs.llvm.org/show_bug.cgi?id=34533 Differential Revision: https://reviews.llvm.org/D69471
2020-02-28Revert "[Coverage] Revise format to reduce binary size"Vedant Kumar1-2/+0
This reverts commit 99317124e1c772e9a9de41a0cd56e1db049b4ea4. This is still busted on Windows: http://lab.llvm.org:8011/builders/lld-x86_64-win7/builds/40873 The llvm-cov tests report 'error: Could not load coverage information'.
2020-02-28[Coverage] Revise format to reduce binary sizeVedant Kumar1-0/+2
Revise the coverage mapping format to reduce binary size by: 1. Naming function records and marking them `linkonce_odr`, and 2. Compressing filenames. This shrinks the size of llc's coverage segment by 82% (334MB -> 62MB) and speeds up end-to-end single-threaded report generation by 10%. For reference the compressed name data in llc is 81MB (__llvm_prf_names). Rationale for changes to the format: - With the current format, most coverage function records are discarded. E.g., more than 97% of the records in llc are *duplicate* placeholders for functions visible-but-not-used in TUs. Placeholders *are* used to show under-covered functions, but duplicate placeholders waste space. - We reached general consensus about giving (1) a try at the 2017 code coverage BoF [1]. The thinking was that using `linkonce_odr` to merge duplicates is simpler than alternatives like teaching build systems about a coverage-aware database/module/etc on the side. - Revising the format is expensive due to the backwards compatibility requirement, so we might as well compress filenames while we're at it. This shrinks the encoded filenames in llc by 86% (12MB -> 1.6MB). See CoverageMappingFormat.rst for the details on what exactly has changed. Fixes PR34533 [2], hopefully. [1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118428.html [2] https://bugs.llvm.org/show_bug.cgi?id=34533 Differential Revision: https://reviews.llvm.org/D69471
2020-02-20[WebAssembly] Remove unneeded getWasmKindForNamedSection functionSam Clegg1-12/+0
I believe this was carried over from getELFKindForNamedSection since the wasm backend originally used ELF object writing as a template. Differential Revision: https://reviews.llvm.org/D74565
2020-02-14[MCStreamer] De-capitalize EmitValue EmitIntValue{,InHex}Fangrui Song1-8/+8
2020-02-14[MC] De-capitalize another set of MCStreamer::Emit* functionsFangrui Song1-6/+6
Emit{ValueTo,Code}Alignment Emit{DTP,TP,GP}* EmitSymbolValue etc
2020-02-14[MC] De-capitalize some MCStreamer::Emit* functionsFangrui Song1-7/+7
2020-02-10[AIX][XCOFF] Support Mergeable2ByteCString and Mergeable4ByteCStringdiggerlin1-3/+0
SUMMARY: The patch is enable to support Mergeable2ByteCString and Mergeable4ByteCString Reviewers: daltenty Subscribers: wuzish, nemanjai, hiraditya Differential Revision: https://reviews.llvm.org/D74164
2020-02-06[MC][ELF] Rename MC related "Associated" to "LinkedToSym"Fangrui Song1-10/+10
"linked-to section" is used by the ELF spec. By analogy, "linked-to symbol" is a good name for the signature symbol. The word "linked-to" implies a directed edge and makes it clear its relation with "sh_link", while one can argue that "associated" means an undirected edge. Also, combine tests and add precise SMLoc to improve diagnostics. Reviewed By: eugenis, grimar, jhenderson Differential Revision: https://reviews.llvm.org/D74082
2020-01-28Make llvm::StringRef to std::string conversions explicit.Benjamin Kramer1-2/+2
This is how it should've been and brings it more in line with std::string_view. There should be no functional change here. This is mostly mechanical from a custom clang-tidy check, with a lot of manual fixups. It uncovers a lot of minor inefficiencies. This doesn't actually modify StringRef yet, I'll do that in a follow-up.
2020-01-22[NFC][XCOFF] Refactor Csect creation into TargetLoweringObjectFileDavid Tenty1-0/+30
Summary: We create a number of standard types of control sections in multiple places for things like the function descriptors, external references and the TOC anchor among others, so it is possible for their properties to be defined inconsistently in different places. This refactor moves their creation and properties into functions in the TargetLoweringObjectFile class hierarchy, where functions for retrieving various special types of sections typically seem to reside. Note: There is one case in PPCISelLowering which is specific to function entry points which we don't address since we don't have access to the TLOF there. Reviewers: DiggerLin, jasonliu, hubert.reinterpretcast Reviewed By: jasonliu, hubert.reinterpretcast Subscribers: wuzish, nemanjai, hiraditya, kbarton, jsji, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D72347
2020-01-14[AIX][XCOFF] Supporting the ReadOnlyWithRel SectionKnddiggerlin1-1/+4
SUMMARY: In this patch we put the global variable in a Csect which's SectionKind is "ReadOnlyWithRel" into Data Section. Reviewers: hubert.reinterpretcast,jasonliu,Xiangling_L Subscribers: wuzish, nemanjai, hiraditya Differential Revision: https://reviews.llvm.org/D72461
2020-01-07[AIX][XCOFF]Implement mergeable constdiggerlin1-1/+1
SUMMARY: In this patch, we map mergeable const objects to the read-only section in the same manner as const objects that are not mergeable. Reviewers: hubert.reinterpretcast,jasonliu Subscribers: wuzish, nemanjai, hiraditya Differential Revision: https://reviews.llvm.org/D71551
2020-01-01[NFC] Fixes -Wrange-loop-analysis warningsMark de Wever1-4/+4
This avoids new warnings due to D68912 adds -Wrange-loop-analysis to -Wall. Differential Revision: https://reviews.llvm.org/D71857
2019-12-17Revert "Honor -fuse-init-array when os is not specified on x86"Mitch Phillips1-1/+0
This reverts commit aa5ee8f244441a8ea103a7e0ed8b6f3e74454516. This change broke the sanitizer buildbots. See comments at the patchset (https://reviews.llvm.org/D71360) for more information.
2019-12-16Honor -fuse-init-array when os is not specified on x86Kamlesh Kumar1-0/+1
Currently -fuse-init-array option is not effective when target triple does not specify os, on x86,x86_64. i.e. // -fuse-init-array is not honored. $ clang -target i386 -fuse-init-array test.c -S // -fuse-init-array is honored. $ clang -target i386-linux -fuse-init-array test.c -S This patch fixes first case. And does cleanup. Reviewers: rnk, craig.topper, fhahn, echristo Reviewed By: rnk Differential Revision: https://reviews.llvm.org/D71360
2019-12-04Revert "[Coverage] Revise format to reduce binary size"Vedant Kumar1-2/+0
This reverts commit e18531595bba495946aa52c0a16b9f9238cff8bc. On Windows, there is an error: http://lab.llvm.org:8011/builders/sanitizer-windows/builds/54963/steps/stage%201%20check/logs/stdio error: C:\b\slave\sanitizer-windows\build\stage1\projects\compiler-rt\test\profile\Profile-x86_64\Output\instrprof-merging.cpp.tmp.v1.o: Failed to load coverage: Malformed coverage data
2019-12-04[Coverage] Revise format to reduce binary sizeVedant Kumar1-0/+2
Revise the coverage mapping format to reduce binary size by: 1. Naming function records and marking them `linkonce_odr`, and 2. Compressing filenames. This shrinks the size of llc's coverage segment by 82% (334MB -> 62MB) and speeds up end-to-end single-threaded report generation by 10%. For reference the compressed name data in llc is 81MB (__llvm_prf_names). Rationale for changes to the format: - With the current format, most coverage function records are discarded. E.g., more than 97% of the records in llc are *duplicate* placeholders for functions visible-but-not-used in TUs. Placeholders *are* used to show under-covered functions, but duplicate placeholders waste space. - We reached general consensus about giving (1) a try at the 2017 code coverage BoF [1]. The thinking was that using `linkonce_odr` to merge duplicates is simpler than alternatives like teaching build systems about a coverage-aware database/module/etc on the side. - Revising the format is expensive due to the backwards compatibility requirement, so we might as well compress filenames while we're at it. This shrinks the encoded filenames in llc by 86% (12MB -> 1.6MB). See CoverageMappingFormat.rst for the details on what exactly has changed. Fixes PR34533 [2], hopefully. [1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118428.html [2] https://bugs.llvm.org/show_bug.cgi?id=34533 Differential Revision: https://reviews.llvm.org/D69471
2019-11-20A fix of the bug introduced by previous lowering in asm patch.Xiangling Liao1-2/+1
Differential Revision: https://reviews.llvm.org/D70243
2019-11-20[AIX][XCOFF] Add support for generating assembly code for one-byte mergable ↵Xing Xue1-2/+20
strings This patch adds support for generating assembly code for one-byte mergeable strings. Generating assembly code for multi-byte mergeable strings and the `XCOFF` object code for mergeable strings will be supported later. Reviewers: hubert.reinterpretcast, jasonliu, daltenty, sfertile, DiggerLin, Xiangling_L Reviewed by: daltenty Subscribers: wuzish, nemanjai, hiraditya, kbarton, jsji, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D70310
2019-11-20[AIX] Lowering jump table, constant pool and block address in asmXiangling Liao1-1/+20
This patch lowering jump table, constant pool and block address in assembly. 1. On AIX, jump table index is always relative; 2. Put CPI and JTI into ReadOnlySection until we support unique data sections; 3. Create the temp symbol for block address symbol; 4. Update MIR testcases and add related assembly part; Differential Revision: https://reviews.llvm.org/D70243
2019-11-15Add read-only data assembly writing for aixdiggerlin1-0/+4
SUMMARY: The patch will emit read-only variable assembly code for aix. Reviewers: daltenty,Xiangling_Liao Subscribers: rupprecht, seiyai,hiraditya Differential Revision: https://reviews.llvm.org/D70182
2019-11-11[PowerPC][XCOFF] Add support for zero initialized global values.Sean Fertile1-0/+6
For XCOFF, globals mapped into the .bss section are linked as COMMON definitions. This behaviour is incorrect for zero initialized data, so emit those to the .data section instead. Differential Revision: https://reviews.llvm.org/D69528
2019-10-15Added support for "#pragma clang section relro=<name>"Dmitry Mikulin1-0/+2
Differential Revision: https://reviews.llvm.org/D68806 llvm-svn: 374934
2019-09-23[Alignment][NFC] DataLayout migration to llvm::AlignGuillaume Chatelet1-1/+1
Summary: This is patch is part of a series to introduce an Alignment type. See this thread for context: http://lists.llvm.org/pipermail/llvm-dev/2019-July/133851.html See this patch for the introduction of the type: https://reviews.llvm.org/D64790 Reviewers: courbet Subscribers: jholewinski, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D67914 llvm-svn: 372596
2019-09-12AArch64: support arm64_32, an ILP32 slice for watchOS.Tim Northover1-0/+1
This is the main CodeGen patch to support the arm64_32 watchOS ABI in LLVM. FastISel is mostly disabled for now since it would generate incorrect code for ILP32. llvm-svn: 371722
2019-08-25[PowerPC][AIX] Adds support for writing the .data section in assembly filesXing Xue1-0/+3
Summary: Adds support for generating the .data section in assembly files for global variables with a non-zero initialization. The support for writing the .data section in XCOFF object files will be added in a follow-on patch. Any relocations are not included in this patch. Reviewers: hubert.reinterpretcast, sfertile, jasonliu, daltenty, Xiangling_L Reviewed by: hubert.reinterpretcast Subscribers: nemanjai, hiraditya, kbarton, MaskRay, jsji, wuzish, shchenz, DiggerLin, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D66154 llvm-svn: 369869
2019-08-22[MachO][TLOF] Use hasLocalLinkage to determine if indirect symbol is localFrancis Visoiu Mistrih1-7/+5
Local symbols in the indirect symbol table contain the value `INDIRECT_SYMBOL_LOCAL` and the corresponding __pointers entry must contain the address of the target. In r349060, I added support for local symbols in the indirect symbol table, which was checking if the symbol `isDefined` && `!isExternal` to determine if the symbol is local or not. It turns out that `isDefined` will return false if the user of the symbol comes before its definition, and we'll again generate .long 0 which will be the symbol at the adress 0x0. Instead of doing that, use GlobalValue::hasLocalLinkage() to check if the symbol is local. Differential Revision: https://reviews.llvm.org/D66563 llvm-svn: 369671
2019-08-22[COFF] Fix section name for constants larger than 64 bits on WindowsFangrui Song1-1/+2
APIntToHexString returns wrong value ("0000000000000000ffffffffffffffff") for integer larger than 64 bits, and thus TargetLoweringObjectFileCOFF::getSectionForConstant returns same section name for all numbers larger than 64 bits. This patch tries to fix it. Differential Revision: https://reviews.llvm.org/D66458 Patch by Senran Zhang llvm-svn: 369610
2019-08-20Adds support for writing the .bss section for XCOFF object files.Sean Fertile1-1/+19
Adds Wrapper classes for MCSymbol and MCSection into the XCOFF target object writer. Also adds a class to represent the top-level sections, which we materialize in the ObjectWriter. executePostLayoutBinding will map all csects into the appropriate container depending on its storage mapping class, and map all symbols into their containing csect. Once all symbols have been processed we - Assign addresses and symbol table indices. - Calaculte section sizes. - Build the section header table. - Assign the sections raw-pointer value for non-virtual sections. Since the .bss section is virtual, writing the header table is enough to add support. Writing of a sections raw data, or of any relocations is not included in this patch. Testing is done by dumping the section header table, but it needs to be extended to include dumping the symbol table once readobj support for dumping auxiallary entries lands. Differential Revision: https://reviews.llvm.org/D65159 llvm-svn: 369454
2019-08-19Fixed placement of llvm.global_dtors on Windows.Vyacheslav Zakharin1-1/+2
Differential revision: https://reviews.llvm.org/D66373 llvm-svn: 369299
2019-08-08Enable assembly output of local commons for AIXDavid Tenty1-3/+4
Summary: This patch enable assembly output of local commons for AIX using .lcomm directives. Adds a EmitXCOFFLocalCommonSymbol to MCStreamer so we can emit the AIX version of .lcomm assembly directives which include a csect name. Handle the case of BSS locals in PPCAIXAsmPrinter by using EmitXCOFFLocalCommonSymbol. Adds a test for generating .lcomm on AIX Targets. Reviewers: cebowleratibm, hubert.reinterpretcast, Xiangling_L, jasonliu, sfertile Reviewed By: sfertile Subscribers: wuzish, nemanjai, hiraditya, kbarton, MaskRay, jsji, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D64825 llvm-svn: 368306
2019-08-07Revert r367501 "Create unique, but identically-named ELF sections..."Bob Haarman1-17/+2
This reverts commit fbc563e2cb6c5f1d0200b390513506b6aca4d2e9 "Create unique, but identically-named ELF sections for explicitly-sectioned functions and globals when using -function-sections and -data-sections." Reason for revert: sections are created with potentially wrong attributes. llvm-svn: 368204
2019-08-01Create unique, but identically-named ELF sections for explicitly-sectioned ↵Peter Collingbourne1-2/+17
functions and globals when using -function-sections and -data-sections. This allows functions and globals to to be reordered later in the linking phase (using the -symbol-ordering-file) even though reordering will be limited to the scope of the explicit section. Patch by Rahman Lavaee! Differential Revision: https://reviews.llvm.org/D65478 llvm-svn: 367501
2019-07-30Address post commit review comments on revision 366727.Sean Fertile1-5/+5
Addresses number of comment made on D64652 after commiting: - Reorders function decls in the TargetLoweringObjectFileXCOFF class. - Fix comment in MCSectionXCOFF to include description of external reference csects. - Convert several llvm_unreachables to report_fatal_error - Convert several dyn_casts to casts as they are expected not to fail. - Avoid copying DataLayout object. llvm-svn: 367324
2019-07-26[PowerPC][AIX]Add lowering of MCSymbol MachineOperand.Sean Fertile1-0/+3
Adds machine operand lowering for MCSymbolSDNodes to the PowerPC backend. This is needed to produce call instructions in assembly for AIX because the callee operand is a MCSymbolSDNode. The test is XFAIL'ed for asserts due to a (valid) assertion in PEI that the AIX ABI isn't supported yet. Differential Revision: https://reviews.llvm.org/D63738 llvm-svn: 367133
2019-07-22Stubs out TLOF for AIX and add support for common vars in assembly output.Sean Fertile1-0/+55
Stubs out a TargetLoweringObjectFileXCOFF class, implementing only SelectSectionForGlobal for common symbols. Also adds an override of EmitGlobalVariable in PPCAIXAsmPrinter which adds a number of defensive errors and adds support for emitting common globals. llvm-svn: 366727
2019-07-18CodeGen: Allow !associated metadata to point to aliases.Peter Collingbourne1-2/+2
This is a small extension of !associated, mostly useful for the implementation convenience of instrumentation passes that RAUW globals with aliases, such as LowerTypeTests. Differential Revision: https://reviews.llvm.org/D64951 llvm-svn: 366502
2019-07-17[AsmPrinter] Make the encoding of call sites in .gcc_except_table ↵Alex Bradbury1-0/+2
configurable and use for RISC-V The original behavior was to always emit the offsets to each call site in the call site table as uleb128 values, however on some architectures (eg RISCV) these uleb128 offsets into the code cannot always be resolved until link time (because relaxation will invalidate any calculated offsets), and there are no appropriate relocations for uleb128 values. As a consequence it needs to be possible to specify an alternative. This also switches RISCV to use DW_EH_PE_udata4 for call side encodings in .gcc_except_table Differential Revision: https://reviews.llvm.org/D63415 Patch by Edward Jones. llvm-svn: 366329
2019-07-17[RISCV] Set correct encodings for DWARF exception handlingAlex Bradbury1-0/+8
This patch sets correct encodings for DWARF exception handling for RISC-V (other than call site encoding, which must be udata4 rather than uleb128 and is handled by D63415). This has the same intend as D63409, except this version matches GCC/binutils behaviour which uses the same encodings regardless of PIC/non-PIC and medlow/medany code model. llvm-svn: 366327
2019-07-08Standardize on MSVC behavior for triples with no environmentReid Kleckner1-5/+3
Summary: This makes it so that IR files using triples without an environment work out of the box, without normalizing them. Typically, the MSVC behavior is more desirable. For example, it tends to enable things like constant merging, use of associative comdats, etc. Addresses PR42491 Reviewers: compnerd Subscribers: hiraditya, dexonsmith, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D64109 llvm-svn: 365387
2019-05-17[ELF] Implement Dependent Libraries FeatureBen Dunbobbin1-0/+13
This patch implements a limited form of autolinking primarily designed to allow either the --dependent-library compiler option, or "comment lib" pragmas ( https://docs.microsoft.com/en-us/cpp/preprocessor/comment-c-cpp?view=vs-2017) in C/C++ e.g. #pragma comment(lib, "foo"), to cause an ELF linker to automatically add the specified library to the link when processing the input file generated by the compiler. Currently this extension is unique to LLVM and LLD. However, care has been taken to design this feature so that it could be supported by other ELF linkers. The design goals were to provide: - A simple linking model for developers to reason about. - The ability to to override autolinking from the linker command line. - Source code compatibility, where possible, with "comment lib" pragmas in other environments (MSVC in particular). Dependent library support is implemented differently for ELF platforms than on the other platforms. Primarily this difference is that on ELF we pass the dependent library specifiers directly to the linker without manipulating them. This is in contrast to other platforms where they are mapped to a specific linker option by the compiler. This difference is a result of the greater variety of ELF linkers and the fact that ELF linkers tend to handle libraries in a more complicated fashion than on other platforms. This forces us to defer handling the specifiers to the linker. In order to achieve a level of source code compatibility with other platforms we have restricted this feature to work with libraries that meet the following "reasonable" requirements: 1. There are no competing defined symbols in a given set of libraries, or if they exist, the program owner doesn't care which is linked to their program. 2. There may be circular dependencies between libraries. The binary representation is a mergeable string section (SHF_MERGE, SHF_STRINGS), called .deplibs, with custom type SHT_LLVM_DEPENDENT_LIBRARIES (0x6fff4c04). The compiler forms this section by concatenating the arguments of the "comment lib" pragmas and --dependent-library options in the order they are encountered. Partial (-r, -Ur) links are handled by concatenating .deplibs sections with the normal mergeable string section rules. As an example, #pragma comment(lib, "foo") would result in: .section ".deplibs","MS",@llvm_dependent_libraries,1 .asciz "foo" For LTO, equivalent information to the contents of a the .deplibs section can be retrieved by the LLD for bitcode input files. LLD processes the dependent library specifiers in the following way: 1. Dependent libraries which are found from the specifiers in .deplibs sections of relocatable object files are added when the linker decides to include that file (which could itself be in a library) in the link. Dependent libraries behave as if they were appended to the command line after all other options. As a consequence the set of dependent libraries are searched last to resolve symbols. 2. It is an error if a file cannot be found for a given specifier. 3. Any command line options in effect at the end of the command line parsing apply to the dependent libraries, e.g. --whole-archive. 4. The linker tries to add a library or relocatable object file from each of the strings in a .deplibs section by; first, handling the string as if it was specified on the command line; second, by looking for the string in each of the library search paths in turn; third, by looking for a lib<string>.a or lib<string>.so (depending on the current mode of the linker) in each of the library search paths. 5. A new command line option --no-dependent-libraries tells LLD to ignore the dependent libraries. Rationale for the above points: 1. Adding the dependent libraries last makes the process simple to understand from a developers perspective. All linkers are able to implement this scheme. 2. Error-ing for libraries that are not found seems like better behavior than failing the link during symbol resolution. 3. It seems useful for the user to be able to apply command line options which will affect all of the dependent libraries. There is a potential problem of surprise for developers, who might not realize that these options would apply to these "invisible" input files; however, despite the potential for surprise, this is easy for developers to reason about and gives developers the control that they may require. 4. This algorithm takes into account all of the different ways that ELF linkers find input files. The different search methods are tried by the linker in most obvious to least obvious order. 5. I considered adding finer grained control over which dependent libraries were ignored (e.g. MSVC has /nodefaultlib:<library>); however, I concluded that this is not necessary: if finer control is required developers can fall back to using the command line directly. RFC thread: http://lists.llvm.org/pipermail/llvm-dev/2019-March/131004.html. Differential Revision: https://reviews.llvm.org/D60274 llvm-svn: 360984