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Yonghong Song 18796da7b0 [BPF] add new intrinsics preserve_{array,union,struct}_access_index
For background of BPF CO-RE project, please refer to
  http://vger.kernel.org/bpfconf2019.html
In summary, BPF CO-RE intends to compile bpf programs
adjustable on struct/union layout change so the same
program can run on multiple kernels with adjustment
before loading based on native kernel structures.

In order to do this, we need keep track of GEP(getelementptr)
instruction base and result debuginfo types, so we
can adjust on the host based on kernel BTF info.
Capturing such information as an IR optimization is hard
as various optimization may have tweaked GEP and also
union is replaced by structure it is impossible to track
fieldindex for union member accesses.

Three intrinsic functions, preserve_{array,union,struct}_access_index,
are introducted.
  addr = preserve_array_access_index(base, index, dimension)
  addr = preserve_union_access_index(base, di_index)
  addr = preserve_struct_access_index(base, gep_index, di_index)
here,
  base: the base pointer for the array/union/struct access.
  index: the last access index for array, the same for IR/DebugInfo layout.
  dimension: the array dimension.
  gep_index: the access index based on IR layout.
  di_index: the access index based on user/debuginfo types.

For example, for the following example,
  $ cat test.c
  struct sk_buff {
     int i;
     int b1:1;
     int b2:2;
     union {
       struct {
         int o1;
         int o2;
       } o;
       struct {
         char flags;
         char dev_id;
       } dev;
       int netid;
     } u[10];
  };

  static int (*bpf_probe_read)(void *dst, int size, const void *unsafe_ptr)
      = (void *) 4;

  #define _(x) (__builtin_preserve_access_index(x))

  int bpf_prog(struct sk_buff *ctx) {
    char dev_id;
    bpf_probe_read(&dev_id, sizeof(char), _(&ctx->u[5].dev.dev_id));
    return dev_id;
  }
  $ clang -target bpf -O2 -g -emit-llvm -S -mllvm -print-before-all \
    test.c >& log

The generated IR looks like below:

  ...
  define dso_local i32 @bpf_prog(%struct.sk_buff*) #0 !dbg !15 {
    %2 = alloca %struct.sk_buff*, align 8
    %3 = alloca i8, align 1
    store %struct.sk_buff* %0, %struct.sk_buff** %2, align 8, !tbaa !45
    call void @llvm.dbg.declare(metadata %struct.sk_buff** %2, metadata !43, metadata !DIExpression()), !dbg !49
    call void @llvm.lifetime.start.p0i8(i64 1, i8* %3) #4, !dbg !50
    call void @llvm.dbg.declare(metadata i8* %3, metadata !44, metadata !DIExpression()), !dbg !51
    %4 = load i32 (i8*, i32, i8*)*, i32 (i8*, i32, i8*)** @bpf_probe_read, align 8, !dbg !52, !tbaa !45
    %5 = load %struct.sk_buff*, %struct.sk_buff** %2, align 8, !dbg !53, !tbaa !45
    %6 = call [10 x %union.anon]* @llvm.preserve.struct.access.index.p0a10s_union.anons.p0s_struct.sk_buffs(
         %struct.sk_buff* %5, i32 2, i32 3), !dbg !53, !llvm.preserve.access.index !19
    %7 = call %union.anon* @llvm.preserve.array.access.index.p0s_union.anons.p0a10s_union.anons(
         [10 x %union.anon]* %6, i32 1, i32 5), !dbg !53
    %8 = call %union.anon* @llvm.preserve.union.access.index.p0s_union.anons.p0s_union.anons(
         %union.anon* %7, i32 1), !dbg !53, !llvm.preserve.access.index !26
    %9 = bitcast %union.anon* %8 to %struct.anon.0*, !dbg !53
    %10 = call i8* @llvm.preserve.struct.access.index.p0i8.p0s_struct.anon.0s(
         %struct.anon.0* %9, i32 1, i32 1), !dbg !53, !llvm.preserve.access.index !34
    %11 = call i32 %4(i8* %3, i32 1, i8* %10), !dbg !52
    %12 = load i8, i8* %3, align 1, !dbg !54, !tbaa !55
    %13 = sext i8 %12 to i32, !dbg !54
    call void @llvm.lifetime.end.p0i8(i64 1, i8* %3) #4, !dbg !56
    ret i32 %13, !dbg !57
  }

  !19 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "sk_buff", file: !3, line: 1, size: 704, elements: !20)
  !26 = distinct !DICompositeType(tag: DW_TAG_union_type, scope: !19, file: !3, line: 5, size: 64, elements: !27)
  !34 = distinct !DICompositeType(tag: DW_TAG_structure_type, scope: !26, file: !3, line: 10, size: 16, elements: !35)

Note that @llvm.preserve.{struct,union}.access.index calls have metadata llvm.preserve.access.index
attached to instructions to provide struct/union debuginfo type information.

For &ctx->u[5].dev.dev_id,
  . The "%6 = ..." represents struct member "u" with index 2 for IR layout and index 3 for DI layout.
  . The "%7 = ..." represents array subscript "5".
  . The "%8 = ..." represents union member "dev" with index 1 for DI layout.
  . The "%10 = ..." represents struct member "dev_id" with index 1 for both IR and DI layout.

Basically, traversing the use-def chain recursively for the 3rd argument of bpf_probe_read() and
examining all preserve_*_access_index calls, the debuginfo struct/union/array access index
can be achieved.

The intrinsics also contain enough information to regenerate codes for IR layout.
For array and structure intrinsics, the proper GEP can be constructed.
For union intrinsics, replacing all uses of "addr" with "base" should be enough.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D61810

llvm-svn: 365352
2019-07-08 17:08:28 +00:00
..
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AMDGPU [AMDGPU][MC][DOC] Updated AMD GPU assembler syntax description. 2019-07-08 16:50:11 +00:00
CommandGuide [docs][llvm-readobj][llvm-readelf] Improve wording 2019-07-08 15:46:26 +00:00
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AMDGPUInstructionNotation.rst [AMDGPU][MC][DOC] Updated AMD GPU assembler description. 2018-12-28 11:48:23 +00:00
AMDGPUInstructionSyntax.rst [AMDGPU][MC][DOC] Updated AMD GPU assembler syntax description. 2019-07-08 16:50:11 +00:00
AMDGPUModifierSyntax.rst [AMDGPU][MC][DOC] Updated AMD GPU assembler syntax description. 2019-07-08 16:50:11 +00:00
AMDGPUOperandSyntax.rst [AMDGPU][MC][DOC] Updated AMD GPU assembler syntax description. 2019-07-08 16:50:11 +00:00
AMDGPUUsage.rst [AMDGPU][MC][DOC] Updated AMD GPU assembler syntax description. 2019-07-08 16:50:11 +00:00
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Atomics.rst [docs][AtomicExpandPass] Document the alternate lowering strategy for part-word atomicrmw/cmpxchg 2018-11-30 09:23:24 +00:00
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CMakePrimer.rst [docs] In the CMake primer, correct the description of the ARGV/ARGN variables. 2017-08-24 05:38:39 +00:00
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Coroutines.rst [coroutines] Add support for llvm.coro.noop intrinsics 2018-04-02 16:55:12 +00:00
CoverageMappingFormat.rst [Coverage] Use gap regions to select better line exec counts 2017-09-18 23:37:28 +00:00
DebuggingJITedCode.rst [Docs] Modernize references to macOS 2019-05-30 16:46:22 +00:00
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ExceptionHandling.rst [NFC] fix trivial typos in documents and comments 2018-04-12 05:53:20 +00:00
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Extensions.rst Add IR support, ELF section and user documentation for partitioning feature. 2019-05-29 03:29:01 +00:00
FAQ.rst Remove some autoconf references from docs and comments 2019-07-03 09:57:59 +00:00
FaultMaps.rst Fix the docs build 2017-02-07 20:36:03 +00:00
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GlobalISel.rst [globalisel] Update the legalizer documentation 2019-05-01 16:52:29 +00:00
GoldPlugin.rst [docs][gold] Fix a typo 2018-08-21 21:00:54 +00:00
HowToAddABuilder.rst Added more info on silent master to the doc. 2017-07-05 20:45:44 +00:00
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HowToBuildWithPGO.rst HowToBuildWithPGO.rst: Fix a few details in the manual steps 2018-12-05 08:35:30 +00:00
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HowToSetUpLLVMStyleRTTI.rst
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HowToUseAttributes.rst Fix RST docs AttributeList heading underline 2017-03-21 17:05:00 +00:00
HowToUseInstrMappings.rst [docs] Fixing Sphinx warnings to unclog the buildbot 2016-07-20 12:16:38 +00:00
InAlloca.rst [docs] Fixing Sphinx warnings to unclog the buildbot 2016-07-20 12:16:38 +00:00
index.rst [docs] Add new document on building distributions 2019-05-21 16:29:31 +00:00
LangRef.rst [BPF] add new intrinsics preserve_{array,union,struct}_access_index 2019-07-08 17:08:28 +00:00
Lexicon.rst Update the coding standards and developer policy documentation surrounding whitespace. 2018-08-10 17:26:07 +00:00
LibFuzzer.rst [libFuzzer] document -len_control 2019-03-18 22:20:47 +00:00
LinkTimeOptimization.rst [docs] Typos and whitespace fixed in LTO docs. 2016-11-29 18:00:31 +00:00
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Statepoints.rst [docs][statepoints] Reformulate open issues list 2018-11-09 17:09:16 +00:00
SupportLibrary.rst [Docs] [Support] Correct a missed reference and use up to date .inc examples. 2018-09-21 01:53:51 +00:00
SystemLibrary.rst [Docs] [Support] System Library to Support Library transition along with minor corrections to reflect it. 2018-09-18 18:05:38 +00:00
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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 <http://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` <-> `http://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.
`http://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