mirror of
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6cf6be73f3
Summary:
An upgrade of LLVM for CrOS [0] containing [1] triggered a bunch of
errors related to writing to reserved registers for a Linux kernel's
arm64 compat vdso (which is a aarch32 image).
After a discussion on LKML [2], it was determined that
-f{no-}omit-frame-pointer was not being specified. Comparing GCC and
Clang [3], it becomes apparent that GCC defaults to omitting the frame
pointer implicitly when optimizations are enabled, and Clang does not.
ie. setting -O1 (or above) implies -fomit-frame-pointer. Clang was
defaulting to -fno-omit-frame-pointer implicitly unless -fomit-frame-pointer
was set explicitly.
Why this becomes a problem is that the Linux kernel's arm64 compat vdso
contains code that uses r7. r7 is used sometimes for the frame pointer
(for example, when targeting thumb (-mthumb)). See useR7AsFramePointer()
in llvm/llvm-project/llvm/lib/Target/ARM/ARMSubtarget.h. This is mostly
for legacy/compatibility reasons, and the 2019 Q4 revision of the ARM
AAPCS looks to standardize r11 as the frame pointer for aarch32, though
this is not yet implemented in LLVM.
Users that are reliant on the implicit value if unspecified when
optimizations are enabled should explicitly choose -fomit-frame-pointer
(new behavior) or -fno-omit-frame-pointer (old behavior).
[0] https://bugs.chromium.org/p/chromium/issues/detail?id=1084372
[1] https://reviews.llvm.org/D76848
[2] https://lore.kernel.org/lkml/20200526173117.155339-1-ndesaulniers@google.com/
[3] https://godbolt.org/z/0oY39t
Reviewers: kristof.beyls, psmith, danalbert, srhines, MaskRay, ostannard, efriedma
Reviewed By: psmith, danalbert, srhines, MaskRay, efriedma
Subscribers: efriedma, olista01, MaskRay, vhscampos, cfe-commits, llvm-commits, manojgupta, llozano, glider, hctim, eugenis, pcc, peter.smith, srhines
Tags: #clang, #llvm
Differential Revision: https://reviews.llvm.org/D80828
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| .. | ||
| _ocamldoc | ||
| _static | ||
| _templates | ||
| _themes/llvm-theme | ||
| AMDGPU | ||
| CommandGuide | ||
| DependenceGraphs | ||
| Frontend | ||
| GlobalISel | ||
| HistoricalNotes | ||
| PDB | ||
| Proposals | ||
| TableGen | ||
| tutorial | ||
| AddingConstrainedIntrinsics.rst | ||
| AdvancedBuilds.rst | ||
| AliasAnalysis.rst | ||
| AMDGPUDwarfProposalForHeterogeneousDebugging.rst | ||
| AMDGPUInstructionNotation.rst | ||
| AMDGPUInstructionSyntax.rst | ||
| AMDGPUModifierSyntax.rst | ||
| AMDGPUOperandSyntax.rst | ||
| AMDGPUUsage.rst | ||
| ARM-BE-bitcastfail.png | ||
| ARM-BE-bitcastsuccess.png | ||
| ARM-BE-ld1.png | ||
| ARM-BE-ldr.png | ||
| Atomics.rst | ||
| Benchmarking.rst | ||
| BigEndianNEON.rst | ||
| BitCodeFormat.rst | ||
| BlockFrequencyTerminology.rst | ||
| BranchWeightMetadata.rst | ||
| BugLifeCycle.rst | ||
| Bugpoint.rst | ||
| BugpointRedesign.md | ||
| BuildingADistribution.rst | ||
| CFIVerify.rst | ||
| CMake.rst | ||
| CMakeLists.txt | ||
| CMakePrimer.rst | ||
| CodeGenerator.rst | ||
| CodeOfConduct.rst | ||
| CodeReview.rst | ||
| CodingStandards.rst | ||
| CommandLine.rst | ||
| CompileCudaWithLLVM.rst | ||
| CompilerWriterInfo.rst | ||
| conf.py | ||
| Contributing.rst | ||
| Coroutines.rst | ||
| CoverageMappingFormat.rst | ||
| DebuggingJITedCode.rst | ||
| DeveloperPolicy.rst | ||
| Docker.rst | ||
| doxygen-mainpage.dox | ||
| doxygen.cfg.in | ||
| ExceptionHandling.rst | ||
| ExtendedIntegerResults.txt | ||
| ExtendingLLVM.rst | ||
| Extensions.rst | ||
| FAQ.rst | ||
| FaultMaps.rst | ||
| FuzzingLLVM.rst | ||
| GarbageCollection.rst | ||
| gcc-loops.png | ||
| GetElementPtr.rst | ||
| GettingInvolved.rst | ||
| GettingStarted.rst | ||
| GettingStartedTutorials.rst | ||
| GettingStartedVS.rst | ||
| GitBisecting.rst | ||
| GoldPlugin.rst | ||
| GwpAsan.rst | ||
| HowToAddABuilder.rst | ||
| HowToBuildOnARM.rst | ||
| HowToBuildWithPGO.rst | ||
| HowToCrossCompileBuiltinsOnArm.rst | ||
| HowToCrossCompileLLVM.rst | ||
| HowToReleaseLLVM.rst | ||
| HowToSetUpLLVMStyleRTTI.rst | ||
| HowToSubmitABug.rst | ||
| HowToUpdateDebugInfo.rst | ||
| HowToUseAttributes.rst | ||
| HowToUseInstrMappings.rst | ||
| InAlloca.rst | ||
| index.rst | ||
| LangRef.rst | ||
| Lexicon.rst | ||
| LibFuzzer.rst | ||
| LinkTimeOptimization.rst | ||
| linpack-pc.png | ||
| llvm-objdump.1 | ||
| LLVMBuild.rst | ||
| LLVMBuild.txt | ||
| loop-terminology-guarded-loop.png | ||
| loop-terminology-initial-loop.png | ||
| loop-terminology-rotated-loop.png | ||
| LoopTerminology.rst | ||
| make.bat | ||
| Makefile.sphinx | ||
| MarkdownQuickstartTemplate.md | ||
| MarkedUpDisassembly.rst | ||
| MCJIT-creation.png | ||
| MCJIT-dyld-load.png | ||
| MCJIT-engine-builder.png | ||
| MCJIT-load-object.png | ||
| MCJIT-load.png | ||
| MCJIT-resolve-relocations.png | ||
| MCJITDesignAndImplementation.rst | ||
| MeetupGuidelines.rst | ||
| MemorySSA.rst | ||
| MemTagSanitizer.rst | ||
| MergeFunctions.rst | ||
| MIRLangRef.rst | ||
| NVPTXUsage.rst | ||
| OptBisect.rst | ||
| ORCv2.rst | ||
| Packaging.rst | ||
| Passes.rst | ||
| Phabricator.rst | ||
| ProgrammersManual.rst | ||
| Projects.rst | ||
| re_format.7 | ||
| README.txt | ||
| Reference.rst | ||
| ReleaseNotes.rst | ||
| ReleaseProcess.rst | ||
| Remarks.rst | ||
| ReportingGuide.rst | ||
| ScudoHardenedAllocator.rst | ||
| SegmentedStacks.rst | ||
| SourceLevelDebugging.rst | ||
| speculative_load_hardening_microbenchmarks.png | ||
| SpeculativeLoadHardening.md | ||
| SphinxQuickstartTemplate.rst | ||
| StackMaps.rst | ||
| StackSafetyAnalysis.rst | ||
| Statepoints.rst | ||
| SupportLibrary.rst | ||
| SystemLibrary.rst | ||
| TableGenFundamentals.rst | ||
| TestingGuide.rst | ||
| TestSuiteGuide.md | ||
| TestSuiteMakefileGuide.rst | ||
| TransformMetadata.rst | ||
| TypeMetadata.rst | ||
| UserGuides.rst | ||
| Vectorizers.rst | ||
| WritingAnLLVMBackend.rst | ||
| WritingAnLLVMPass.rst | ||
| XRay.rst | ||
| XRayExample.rst | ||
| XRayFDRFormat.rst | ||
| yaml2obj.rst | ||
| YamlIO.rst | ||
LLVM Documentation
==================
LLVM's documentation is written in reStructuredText, a lightweight
plaintext markup language (file extension `.rst`). While the
reStructuredText documentation should be quite readable in source form, it
is mostly meant to be processed by the Sphinx documentation generation
system to create HTML pages which are hosted on <https://llvm.org/docs/> and
updated after every commit. Manpage output is also supported, see below.
If you instead would like to generate and view the HTML locally, install
Sphinx <http://sphinx-doc.org/> and then do:
cd <build-dir>
cmake -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_HTML=true <src-dir>
make -j3 docs-llvm-html
$BROWSER <build-dir>/docs//html/index.html
The mapping between reStructuredText files and generated documentation is
`docs/Foo.rst` <-> `<build-dir>/docs//html/Foo.html` <-> `https://llvm.org/docs/Foo.html`.
If you are interested in writing new documentation, you will want to read
`SphinxQuickstartTemplate.rst` which will get you writing documentation
very fast and includes examples of the most important reStructuredText
markup syntax.
Manpage Output
===============
Building the manpages is similar to building the HTML documentation. The
primary difference is to use the `man` makefile target, instead of the
default (which is `html`). Sphinx then produces the man pages in the
directory `<build-dir>/docs/man/`.
cd <build-dir>
cmake -DLLVM_ENABLE_SPHINX=true -DSPHINX_OUTPUT_MAN=true <src-dir>
make -j3 docs-llvm-man
man -l >build-dir>/docs/man/FileCheck.1
The correspondence between .rst files and man pages is
`docs/CommandGuide/Foo.rst` <-> `<build-dir>/docs//man/Foo.1`.
These .rst files are also included during HTML generation so they are also
viewable online (as noted above) at e.g.
`https://llvm.org/docs/CommandGuide/Foo.html`.
Checking links
==============
The reachability of external links in the documentation can be checked by
running:
cd docs/
make -f Makefile.sphinx linkcheck
Doxygen page Output
==============
Install doxygen <http://www.stack.nl/~dimitri/doxygen/download.html> and dot2tex <https://dot2tex.readthedocs.io/en/latest>.
cd <build-dir>
cmake -DLLVM_ENABLE_DOXYGEN=On <llvm-top-src-dir>
make doxygen-llvm # for LLVM docs
make doxygen-clang # for clang docs
It will generate html in
<build-dir>/docs/doxygen/html # for LLVM docs
<build-dir>/tools/clang/docs/doxygen/html # for clang docs