[PATCH] D36351: [lld][ELF] Add profile guided section layout

Rui Ueyama via llvm-commits llvm-commits at lists.llvm.org
Tue Oct 24 21:25:30 PDT 2017 

On Tue, Oct 24, 2017 at 9:23 PM, Michael Spencer <bigcheesegs at gmail.com>
wrote:

> On Tue, Oct 24, 2017 at 8:58 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> Sorry, as you pointed out, I made a mistake when I applied the patches.
>> Now I succeeded to build it. But the output file of llvm-profdata seems too
>> small. Is this correct?
>>
>> This is what I did.
>>
>> $ CC=`which clang` CXX=`which clang++` /ssd/cmake/bin/cmake -GNinja
>> -DCMAKE_BUILD_TYPE=RelWithDebInfo -DLLVM_ENABLE_PROJECTS='clang;lld'
>> -DCMAKE_C{,XX}_FLAGS=-fprofile-instr-generate ../llvm-project/llvm/
>>
>> $ ninja clang lld
>>
>> $ bin/ld.lld <options-to-link-clang>
>>
>> $ bin/llvm-profdata merge default.profraw -o default.profdata
>>
>> $ ls -l default.*
>> -rw-r----- 1 ruiu eng     560 Oct 24 20:55 default.profdata
>> -rw-r----- 1 ruiu eng 2151240 Oct 24 20:55 default.profraw
>>
>> $ bin/llvm-profdata show default.profraw
>> error: default.profraw: Empty raw profile file
>>
>
> It could be that lld is using quick exit and not properly flushing the
> profile data to disk. I'll test out this configuration.
>

Ah, it's likely. I'll try again with `--full-shutdown` option.


> - Michael Spencer
>
>
>>
>> On Mon, Oct 23, 2017 at 7:52 PM, Michael Spencer <bigcheesegs at gmail.com>
>> wrote:
>>
>>> On Mon, Oct 23, 2017 at 6:53 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>> It didn't pass cmake because lld/ELF/CMakeLists.txt lacked
>>>> "CallGraphSort.cpp", so I added that to the CMakeLists.txt.
>>>>
>>>> It still didn't compile because `Config::CallGraphProfile` doesn't
>>>> exist, so I added that boolean variable.
>>>>
>>>> /ssd/llvm-project/lld/ELF/CallGraphSort.cpp:274:29: error: no member
>>>> named 'CallGraphProfile' in 'lld::elf::Configuration'
>>>>   CallGraphSort CGS(Config->CallGraphProfile);
>>>>                       ~~~~~~  ^
>>>>
>>>> Now, I'm seeing the following errors. How can I fix them?
>>>>
>>>> /ssd/llvm-project/lld/ELF/CallGraphSort.cpp:274:17: error: no matching
>>>> constructor for initialization of '(anonymous namespace)::CallGraphSort'
>>>>   CallGraphSort CGS(Config->CallGraphProfile);
>>>>                 ^   ~~~~~~~~~~~~~~~~~~~~~~~~
>>>> /ssd/llvm-project/lld/ELF/CallGraphSort.cpp:43:7: note: candidate
>>>> constructor (the implicit copy constructor) not viable: no known conversion
>>>> from 'bool' to 'const (anonymous namespace)::CallGraphSort' for 1st argument
>>>> class CallGraphSort {
>>>>       ^
>>>> /ssd/llvm-project/lld/ELF/CallGraphSort.cpp:43:7: note: candidate
>>>> constructor (the implicit move constructor) not viable: no known conversion
>>>> from 'bool' to '(anonymous namespace)::CallGraphSort' for 1st argument
>>>> class CallGraphSort {
>>>>       ^
>>>> /ssd/llvm-project/lld/ELF/CallGraphSort.cpp:115:16: note: candidate
>>>> constructor not viable: no known conversion from 'bool' to
>>>> 'DenseMap<std::pair<const SymbolBody *, const SymbolBody *>, uint64_t> &'
>>>> (aka 'DenseMap<pair<const lld::elf::SymbolBody *, const
>>>> lld::elf::SymbolBody *>, unsigned long> &') for 1st argument
>>>> CallGraphSort::CallGraphSort(
>>>>                ^
>>>>
>>>>
>>> Are you sure you applied the patch correctly? I just checked the diff
>>> and it has all that.
>>>
>>> diff --git a/ELF/CMakeLists.txt b/ELF/CMakeLists.txt
>>> index 205702975..3b3c388d2 100644
>>> --- a/ELF/CMakeLists.txt
>>> +++ b/ELF/CMakeLists.txt
>>> @@ -18,6 +18,7 @@ add_lld_library(lldELF
>>>    Arch/SPARCV9.cpp
>>>    Arch/X86.cpp
>>>    Arch/X86_64.cpp
>>> +  CallGraphSort.cpp
>>>    Driver.cpp
>>>    DriverUtils.cpp
>>>    EhFrame.cpp
>>>
>>> It also has the change that adds CallGraphProfile to Config.h.
>>>
>>> - Michael Spencer
>>>
>>>
>>>> On Mon, Oct 23, 2017 at 5:02 PM, Michael Spencer <bigcheesegs at gmail.com
>>>> > wrote:
>>>>
>>>>>
>>>>>
>>>>> - Michael Spencer
>>>>>
>>>>> On Tue, Oct 3, 2017 at 6:43 PM, Rui Ueyama via Phabricator <
>>>>> reviews at reviews.llvm.org> wrote:
>>>>>
>>>>>> ruiu added a comment.
>>>>>>
>>>>>> Could you send me a patch to produce a call graph file so that I can
>>>>>> try this patch on my machine?
>>>>>>
>>>>>
>>>>> Sorry for the delay. I've attached the llvm and lld patches that
>>>>> implement a full testable version of the feature.
>>>>>
>>>>> To use:
>>>>>
>>>>> -1) Have an elf system with working clang instrumentation based
>>>>> profiling
>>>>> 0) Compile clang and lld with the supplied patch
>>>>> 1) Compile the code with `-fprofile-instr-generate` and link with any
>>>>> linker
>>>>> 2) Run the program on a representative sample
>>>>> 3) `$ llvm-profdata merge default.profraw -o default.profdata`
>>>>> 4) Compile the code again with `-fprofile-instr-use=default.profdata
>>>>> -ffunction-sections -fuse-ld=lld`
>>>>>
>>>>> The output of #4 is the program with sections ordered by profile data.
>>>>> You can add -Wl,-no-call-graph-profile-sort to disable sorting to
>>>>> measure the difference.
>>>>>
>>>>> `llvm-readobj -elf-cg-profile` will dump the cg profile section.
>>>>>
>>>>> This also works with LTO.
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:99-101
>>>>>> +  if (To != Other.To)
>>>>>> +    return To < Other.To;
>>>>>> +  return false;
>>>>>> ----------------
>>>>>> You can just return `To < Other.To`.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:127
>>>>>> +  // Create the graph.
>>>>>> +  for (const auto &C : Profile) {
>>>>>> +    if (C.second == 0)
>>>>>> ----------------
>>>>>> This loop is a bit too dense. It cannot be understood without reading
>>>>>> each line carefully as I don't understand the whole picture. Please insert
>>>>>> a blank line between code blocks. Adding more comment would help.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:128
>>>>>> +  for (const auto &C : Profile) {
>>>>>> +    if (C.second == 0)
>>>>>> +      continue;
>>>>>> ----------------
>>>>>> Please define local variables for `C.first.first`, `C.first.second`
>>>>>> and `C.second` so that they are accessed through meaningful names.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:130-135
>>>>>> +    auto FromDR = dyn_cast_or_null<DefinedRegula
>>>>>> r>(Symtab->find(C.first.first));
>>>>>> +    auto ToDR = dyn_cast_or_null<DefinedRegula
>>>>>> r>(Symtab->find(C.first.second));
>>>>>> +    if (!FromDR || !ToDR)
>>>>>> +      continue;
>>>>>> +    auto FromSB = dyn_cast_or_null<const
>>>>>> InputSectionBase>(FromDR->Section);
>>>>>> +    auto ToSB = dyn_cast_or_null<const InputSectionBase>(ToDR->Sectio
>>>>>> n);
>>>>>> ----------------
>>>>>> auto -> auto *
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:147
>>>>>> +      Nodes[To].IncidentEdges.push_back(EI);
>>>>>> +    } else
>>>>>> +      Edges[EI].Weight = SaturatingAdd(Edges[EI].Weight, C.second);
>>>>>> ----------------
>>>>>> nit: add {}
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:153
>>>>>> +
>>>>>> +void CallGraphSort::contractEdge(EdgeIndex CEI) {
>>>>>> +  // Make a copy of the edge as the original will be marked killed
>>>>>> while being
>>>>>> ----------------
>>>>>> Please add a function comment as to what this function is intended to
>>>>>> do. I do not understand this function because I don't get a whole picture.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:158-163
>>>>>> +  // Remove the self edge from From.
>>>>>> +  FE.erase(std::remove(FE.begin(), FE.end(), CEI));
>>>>>> +  std::vector<EdgeIndex> &TE = Nodes[CE.To].IncidentEdges;
>>>>>> +  // Update all edges incident with To to reference From instead.
>>>>>> Then if they
>>>>>> +  // aren't self edges add them to From.
>>>>>> +  for (EdgeIndex EI : TE) {
>>>>>> ----------------
>>>>>> Add blank lines before comments.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:165-166
>>>>>> +    Edge &E = Edges[EI];
>>>>>> +    // E.From = E.From == CE.To ? CE.From : E.From;
>>>>>> +    // E.To = E.To == CE.To ? CE.From : E.To;
>>>>>> +    if (E.From == CE.To)
>>>>>> ----------------
>>>>>> Please remove debug code.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:178-180
>>>>>> +  // Free memory.
>>>>>> +  std::vector<EdgeIndex>().swap(TE);
>>>>>> +
>>>>>> ----------------
>>>>>> This looks odd. Why do you need to do this? I think you can just
>>>>>> leave it alone.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:207-208
>>>>>> +
>>>>>> +// Group InputSections into clusters using the Call-Chain Clustering
>>>>>> heuristic
>>>>>> +// then sort the clusters by density.
>>>>>> +void CallGraphSort::generateClusters() {
>>>>>> ----------------
>>>>>> This might be understood for those who read the paper, but I don't
>>>>>> think that is enough. Please write more comment as to what are clusters,
>>>>>> what is density, and what is the heuristic.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/CallGraphSort.cpp:272-273
>>>>>> +DenseMap<const InputSectionBase *, int>
>>>>>> elf::computeCallGraphProfileOrder() {
>>>>>> +  CallGraphSort CGS(Config->CallGraphProfile);
>>>>>> +  return CGS.run();
>>>>>> +}
>>>>>> ----------------
>>>>>> You can do this in one line without defining a local variable.
>>>>>>
>>>>>>
>>>>>> ================
>>>>>> Comment at: ELF/Writer.cpp:926-928
>>>>>> +    for (BaseCommand *Base : Script->Opt.Commands)
>>>>>> +      if (auto *OS = dyn_cast<OutputSection>(Base))
>>>>>> +        if (OS->Name == ".text") {
>>>>>> ----------------
>>>>>> I'd factor this code out as `OutputSection
>>>>>> *findOutputSection(StringRef Name)`.
>>>>>>
>>>>>>
>>>>>> https://reviews.llvm.org/D36351
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
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