[cfe-dev] [llvm-dev] RFC: Adding a code size analysis tool

Zachary Turner via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 1 15:09:10 PDT 2018


Will this only be strictly for binary size, or can we use it for memory
size too?

One thing I implemented in llvm-pdbutil kind of as a side-exercise to see
if it found anything useful was a padding detector.  It turns out it's
really annoyingly difficult to reconstruct an exact class layout from debug
info, but I think it's about 85% correct now (although for now it only
works on Windows until the native high-level PDB access api is complete --
currently only the native low level api is complete).  It will allow you to
sort all classes by amount of padding or percentage of class size
attributable to padding.  We shaved a couple of percent off of V8's memory
usage with this tool.

Granted, it's better to have the compiler detect this if possible, but
we're talking about a tool that can be run on an arbitrary executable not
necessarily built with a compiler we control.

BTW, even though DWARF doesn't describe types which parameterize templates,
it does give you mangled names, so you should be able to reconstruct those
types from the mangled names.

On Mon, Oct 1, 2018 at 2:25 PM Vedant Kumar via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi Ben,
>
> On Oct 1, 2018, at 12:48 PM, Ben Craig <ben.craig at ni.com> wrote:
>
> Something that I’ve been looking to do for a while now is to do this at
> the .o level, and have something to combine the per .o results as well.
> I’ve wanted to do that to figure out where I can speed up builds that are
> overly slow because of redundant template instantiations.
>
> You might also consider a view that goes across templates and across
> namespaces.  It can be useful to see that X% of your code is in std::map
> instantiations (for example).  This seems similar to how you have
> inheritance covered.  I’ve also wanted to find ways to visualize the
> opposite… where I have class Foo and I want to see its total cost,
> including the size of std::vector<Foo>.
>
>
> These are great ideas. DWARF might not provide enough information to
> generate these views (it doesn’t explicitly describe the types which
> parameterize classes, or the names of un-specialized templates). But it
> should be possible to piece some of this together by parsing type names.
>
> vedant
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Jake
> Ehrlich via llvm-dev
> *Sent:* Friday, September 28, 2018 3:51 PM
> *To:* Vedant Kumar <vsk at apple.com>
> *Cc:* Apple Inc. <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] RFC: Adding a code size analysis tool
>
> Fantastic! I have been looking at creating a tool that a) only spits out
> actionable size reductions (preferably with a specific action should be
> specified) and b) only analyzes the size of allocated sections. The other
> deficiency I've seen with bloaty is speed and scaling. It's very hard to
> get bloaty to analyze across a large system of interdependent shared
> libraries. You can add me as a reviewer to any changes as I would very much
> like to see such a tool exist.
>
> > Unlike bloaty, this tool focuses exclusively on the text segment.
>
> I'd like to see support for everything within PT_LOAD segments, not just
> the executable parts. Everything else you've said is basically what I
> wanted.
>
> On Wed, Sep 26, 2018 at 12:03 PM Vedant Kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hello,
>
> I worked on a code size analysis tool for a 'week of code' project and
> think
> that it might be useful enough to upstream.
>
> The tool is inspired by bloaty (https://github.com/google/bloaty
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_google_bloaty&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=TUijEuSCtxtUJpZmMT25zPk7FzJ8WtWuvS23OJjbOfc&s=rUbgj05XdrQ_kilEakWhwzGJ8YFX-fhMe18O4iypAfA&e=>),
> but tries to
> do more to attribute code size in actionable ways.
>
> For example, it can calculate how many bytes inlined instances of a
> function
> added to a binary. In its diff mode, it can show how much more
> aggressively a
> function was inlined compared to a baseline. This can be useful when
> you're,
> say, trying to figure out why firmware compiled by a new compiler is just
> a few
> bytes over the size limit imposed by your embedded device :). In this case,
> extra information about inlining can help inform a decision to either
> tweak the
> inliner's cost model or to judiciously add a few `noinline` attributes.
> (Note
> that if you're willing to recompile & write a few SQL queries, optimization
> remarks can give you similar information, albeit at the IR level.)
>
> As another example, this code size tool can attribute code size to
> semantically
> interesting groups of code, like C++/Swift classes, or files. In the diff
> mode,
> you can see how the code size of a class/file grew compared to a baseline.
> The
> tool understands inheritance, so you can also see interesting high-level
> trends.
> E.g `clang::Sema` grew more than `llvm::Pass` between clang-6 and clang-7.
>
> Unlike bloaty, this tool focuses exclusively on the text segment. Also
> unlike
> bloaty, it uses LLVM's DWARF parser instead of rolling its own. The tool is
> currently implemented as a sub-tool of llvm-dwarfdump.
>
> To get size information about a program, you do:
>
>   llvm-dwarfdump size-info -baseline <object> -stats-dir <dir>
>
> This emits four *.stats files into <dir>, each containing a distinct
> 'view' into
> the code groups in <object>. There's a file view, a function view, a class
> view,
> and an inlining view. Each view is sorted by code size, so you can see the
> largest functions/classes/etc immediately.
>
> The *.stats files are just human-readable text files. As it happens, they
> use
> the flamegraph format (http://brendangregg.com/flamegraphs.html
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__brendangregg.com_flamegraphs.html&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=TUijEuSCtxtUJpZmMT25zPk7FzJ8WtWuvS23OJjbOfc&s=m4EVbw6_KOkrA5-3qUMtnY1DQIjeDLvCl7hNKSWmuhU&e=>).
> This makes it
> easy to visualize any view as a flamegraph. (If you haven't seen one
> before,
> it's a hierarchical visualization where the width of each entry
> corresponds to
> its frequency (or in this case size).)
>
> To look at code growth between two programs, you'd do:
>
>   llvm-dwarfdump size-info -baseline <object> -target <object> -stats-dir
> <dir>
>
> Similarly, this emits four 'view' files into <dir>, but with a *.diffstats
> suffix. The format is the same.
>
> Pending Work
> ------------
>
> I think the main piece of work the tool needs is better testing. Currently
> there's just a single end-to-end test in clang. It might be better to
> check in
> a few binaries so we can check that the tool reports sizes correctly.
>
> Also, it may turn out that folks are interested in different ways of
> visualizing
> size data. While the textual format of flamegraphs is really convenient for
> humans to read, the graphs themselves do make more sense when the
> underlying
> data have a frequentist interpretation. If there's enough interest I can
> explore
> using an alternative format for visualization, e.g:
>
>   http://neugierig.org/software/chromium/bloat/
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__neugierig.org_software_chromium_bloat_&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=TUijEuSCtxtUJpZmMT25zPk7FzJ8WtWuvS23OJjbOfc&s=2Upr9RUNU0l11KRmg-q55MYe1-LiLeO8I1-Uu1-2_6E&e=>
>   https://github.com/evmar/webtreemap
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_evmar_webtreemap&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=TUijEuSCtxtUJpZmMT25zPk7FzJ8WtWuvS23OJjbOfc&s=VlUC_1511ebsevn6Q87mtH-pSC_vX7uoRSGlJCjTVsQ&e=>
>
> (Thanks JF for pointing these out!)
>
> Here's a link to the source code:
>
>   https://github.com/vedantk/llvm-project/tree/sizeinfo
> <https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_vedantk_llvm-2Dproject_tree_sizeinfo&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=TUijEuSCtxtUJpZmMT25zPk7FzJ8WtWuvS23OJjbOfc&s=nVZKEg23-nVbh0QtuYgPJibiDRIiWQ113rHCuNdfPRQ&e=>
>
>
> Selected Examples
> -----------------
>
> Here are a few interesting snippets from a comparison of clang-6 vs.
> clang-7.
>
> First, let's take a look at the function view diffstat. Here are the 10
> functions which grew in size the most. On the left hand side, you'll see
> the
> demangled function name. The *change* in code size in bytes is reported on
> the
> right hand side (only positive changes are reported).
>
>   clang::Sema::CheckHexagonBuiltinCpu([snip]) [function] 170316
>   ProcessDeclAttribute([snip]) [function] 125893
>   llvm::AArch64InstPrinter::printAliasInstr([snip]) [function] 105133
>   llvm::AArch64AppleInstPrinter::printAliasInstr([snip]) [function] 105133
>   ParseCodeGenArgs([snip]) [function] 64692
>   unswitchNontrivialInvariants([snip]) [function] 40180
>   getAttrKind([snip]) [function] 35811
>   clang::DumpCompilerOptionsAction::ExecuteAction() [function] 32417
>   llvm::UpgradeIntrinsicCall([snip]) [function] 30239
>   bool llvm::InstructionSelector::executeMatchTable<(anonymous
> namespace)::ARMInstructionSelector const, [snip]) const [function] 29352
>
>
> Next, let's look at the file view diffstat. This can be useful because it
> goes
> beyond simply identifying the files which grew the most. It actually
> describes
> which *functions* grew the most in those files, creating more opportunites
> to
> do something about the code growth.
>
>   lib/Target/X86/X86ISelLowering.cpp [file];combineX86ShuffleChain([snip])
> [function] 24864
>   lib/Target/X86/X86ISelLowering.cpp [file];combineMul([snip]) [function]
> 14907
>   lib/Target/X86/X86ISelLowering.cpp [file];combineStore([snip])
> [function] 12220
>   ...
>   tools/clang/lib/Sema/SemaExpr.cpp
> [file];clang::Sema::CheckCompareOperands([snip]) [function] 16024
>   tools/clang/lib/Sema/SemaExpr.cpp
> [file];diagnoseTautologicalComparison([snip]) [function] 1740
>   tools/clang/lib/Sema/SemaExpr.cpp
> [file];clang::Sema::ActOnNumericConstant([snip]) [function] 1436
>   tools/clang/lib/Sema/SemaExpr.cpp
> [file];checkThreeWayNarrowingConversion([snip]) [function] 1356
>   tools/clang/lib/Sema/SemaExpr.cpp
> [file];CheckIdentityFieldAssignment([snip]) [function] 1280
>
>
> The class view diffstat is a bit different because it has more levels of
> nesting than the other views, due to inheritance. This might help give a
> sense
> for the high-level changes in a program, but may also be less actionable.
>
>   clang::Sema [class];clang::Sema::CheckHexagonBuiltinCpu([snip])
> [function] 170316
>   clang::Sema [class];clang::Sema::CheckHexagonBuiltinArgument([snip])
> [function] 24156
>   clang::Sema [class];clang::Sema::ActOnTag([snip]) [function] 22373
>   ...
>   llvm::AArch64InstPrinter [class];llvm::AArch64AppleInstPrinter
> [class];llvm::AArch64AppleInstPrinter::printAliasInstr([snip]) [function]
> 105133
>   llvm::AArch64InstPrinter [class];llvm::AArch64AppleInstPrinter
> [class];llvm::AArch64AppleInstPrinter::printInstruction([snip]) [function]
> 5824
>   ...
>   llvm::Pass [class];llvm::FunctionPass [class];llvm::MachineFunctionPass
> [class];(anon)::X86SpeculativeLoadHardeningPass [class];(anonymous
> namespace)::X86SpeculativeLoadHardeningPass::checkAllLoads(llvm::MachineFunction&)
> [function] 19287
>   ...
>   llvm::Pass [class];llvm::FunctionPass [class];llvm::MachineFunctionPass
> [class];(anon)::MachineLICMBase [class];(anonymous
> namespace)::MachineLICMBase::runOnMachineFunction(llvm::MachineFunction&)
> [function] 20343
>
> Here's a link to a flamegraph of the class view diffstat (warning: it's
> big):
>
>
> http://net.vedantk.com/static/llvm/swift-clang-4.2-vs-5.0.class-view.diffstats.svg
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__net.vedantk.com_static_llvm_swift-2Dclang-2D4.2-2Dvs-2D5.0.class-2Dview.diffstats.svg&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8mub81SfUi-UCZRX0Vl1g&m=TUijEuSCtxtUJpZmMT25zPk7FzJ8WtWuvS23OJjbOfc&s=zA8Dt4NET3Uksk2Rhs5I0AsRMdr3xRuKBd4eyFc2prk&e=>
>
> Finally, here are a few interesting entries from the inlining view
> diffstat. As
> with all of the other views, the right hand side still shows code growth in
> bytes. For a given inlining target, this size is computed by diffing the
> sum of
> PC range lengths from all DW_TAG_inlined_subroutines referring to that
> target.
> This allows the size tool to attribute code size to an inlining target even
> when the inlined code is not contiguous in the caller.
>
>   llvm::raw_ostream::operator<<(char const*) [inlining-target] 66720
>   llvm::MCRegisterClass::contains(unsigned int) const [inlining-target]
> 64161
>   llvm::StringRef::StringRef(char const*) [inlining-target] 39262
>   llvm::MCInst::getOperand(unsigned int) const [inlining-target] 33268
>   clang::CodeCompletionResult::~CodeCompletionResult() [inlining-target]
> 25763
>   llvm::operator+(llvm::Twine const&, llvm::Twine const&)
> [inlining-target] 25525
>   clang::ASTImporter::Import(clang::SourceLocation) [inlining-target] 21096
>   clang::Sema::Diag(clang::SourceLocation, unsigned int) [inlining-target]
> 20898
>
> Feedback & questions welcome!
>
> thanks,
> vedant
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