[llvm-dev] RFC [ThinLTO]: An embedded summary encoding to support CFI and vtable opt

Peter Collingbourne via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 7 10:16:33 PDT 2016

On Mon, Jun 6, 2016 at 9:09 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:

> Hi,
> On May 4, 2016, at 3:02 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> Hi all,
> I wanted to make this proposal to extend ThinLTO to allow a bitcode module
> to embed another bitcode module containing summary information. The purpose
> of doing so is to support CFI and  whole-program devirtualization
> optimizations under ThinLTO.
> Overview
> The CFI and whole-program devirtualization optimizations work by
> transforming vtables according to the class hierarchy. For example, if a
> class A has two derived classes B and C, CFI will lay out the vtables for
> A, B and C consecutively, so that clients can check that a vtable refers to
> a derived class of A by performing arithmetic on the virtual function
> pointer. For more details, see [1].
> Both CFI and vtable opt rely on bitset metadata [2] in order to know where
> the address points for the vtables are located. This is currently encoded
> using module-level metadata.
> In order to lay out the vtables correctly, all vtables need to be visible
> at once. This is the only part of the process that requires full LTO. The
> rest of the process can just rely on a set of summary metadata that
> contains information about how to perform CFI checks for a particular
> class, or how to devirtualize a particular virtual call. This information
> could be made part of the ThinLTO summary.
> Implementation
> The idea is to allow bitcode to contain embedded summary blobs. For
> example, in our scenario, the summary bitcode would contain a section with
> an embedded blob consisting of a bitcode file containing definitions of the
> vtables defined by that translation unit and the bitset metadata for CFI
> and vtable opt, and the "top-level" bitcode would contain everything else.
> The mechanism for merging summaries would be to link the embedded summary
> bitcode files into a single module using the IRMover, with a mechanism very
> similar to regular LTO. This would move all the necessary vtables and
> metadata into a single module where they can be processed using the
> existing LowerBitSets and WholeProgramDevirt passes, which would be
> extended to export summary metadata. This summary metadata would be copied
> into the regular summary information, where it can be used by individual
> ThinLTO backends.
> It is not clear to me how this would play with our current way of handling
> ThinLTO importing. You are mentioning that the existing WholeProgramDevirt
> is supposed to handled a module that would contains only the Vtables and
> the metadata: it seems to me that currently it relies on seeing the
> call-sites.
> I'd expect that we have available the devirtualization information as
> "first class" in the summary-based call graph to be able to perform the
> devirtualization without touching any IR and in a way that can be used to
> drive accurate importing decisions.

Yes, one thing I did not cover was what the summary information would look
like in the individual modules, and what it would look like in the combined

The summary information in the individual modules would be stored in the
FunctionSummary. For CFI this would consist of the set of type identifiers
that are used to check pointers at call sites, and for devirtualization it
would be the set of (type identifier, offset of virtual function from
address point) pairs used at virtual call sites. In the latter case I would
use the routines I moved into lib/Analysis/BitSetUtils.cpp (to be renamed
TypeMetadataUtils.cpp) to summarize the function.

The combined summary would look very similar, except that instead of sets
we would have maps from either identifiers or (identifier, offset) pairs to
the "resolution" for that key (e.g. for a successful single implementation
devirtualization this would name the single possible callee). If we
successfully do single-implementation devirtualization, we would add an
edge to the call graph for the associated FunctionSummary.


> --
> Mehdi
> In the future, we could also consider representing importing summaries as
> metadata. That would also make the summary loading process very
> straightforward.
> Alternatives
> 1) We could use a native object file, with one section named ".llvmbc"
> containing the summary module with the vtables and CFI metadata, and
> another section ".llvmbc.thin" containing "everything else". This would be
> my preferred option, as it would make things even simpler. For example, the
> linker could handle the top-level sections as it reads them, and it would
> allow the individual sections to be extracted (e.g. using objcopy) and
> inspected by normal tools, such as llvm-as and llvm-dis. The native object
> format could also be the container for native code; see my earlier proposal
> [3].
> The implementation in lld is very simple (about 10 lines in my prototype),
> but I can accept that it may be more difficult in other linkers, so those
> linkers may want to use bitcode as the top-level format. In that case, we
> would probably want to go with what I described in "Implementation".
> 2) We could emit the vtables and CFI metadata directly into the top-level
> bitcode. However, this would create a need for a mechanism to distinguish
> vtables from non-vtables for when we link the LTO parts of the module. In
> order to do this, we could add a new bitcode record type for bitset
> metadata that could also act as an index for vtables in a similar way to
> how ThinLTO importing summaries already work. However, this would add even
> more complexity to the bitcode format, when I feel that we should really be
> going the other way with a simpler bitcode format.
> Thanks,
> --
> --
> Peter
> [1] http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html
> [2] http://llvm.org/docs/BitSets.html
> [3] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098081.html

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