[llvm-dev] Proposal: virtual constant propagation

[Peter Collingbourne via llvm-dev]

Hi all,

I'd like to make the following proposal to implement an optimization called
virtual constant propagation.

==Introduction==
After enabling control flow integrity protection in Chromium, we have
observed an unacceptable performance regression in certain critical layout
microbenchmarks. Profiling [0] revealed that the cause of the regression was
a large number of virtual calls, each requiring the overhead of a control
flow integrity check.

We took a closer look at the hottest virtual call sites, and found that they
were mostly type checking functions of the following form:

struct Base {
  enum Type { kBase, kDerived };
  virtual bool isDerived() const { return false; } // or:
  virtual bool isOfType(Type t) const { return t == kBase; }
};
struct Derived : Base {
  virtual bool isDerived() const { return true; }
  virtual bool isOfType(Type t) const { return t == kDerived; }
};

bool f(Base *b) {
  return b->isDerived();
}
bool g(Base *b) {
  return b->isOfType(Base::kDerived);
}

We can make the observation that if the type hierarchy is known to be closed
and the definition of each virtual function and each virtual call is visible
to the compiler, calls to either of these functions can be implemented more
efficiently by loading directly from the vtable (we refer to this as virtual
constant propagation):

struct Base_VTable {
  bool isDerived : 1, isOfTypeBase : 1, isOfTypeDerived : 1;
};
Base_VTable base_vtable{false, true, false};
Derived_VTable derived_vtable{true, false, true};
bool f(Base *b) {
  return b->vtable->isDerived;
}
bool g(Base *b) {
  return b->vtable->isOfTypeDerived;
}

Another advantage of implementing the calls this way is that because there
is no indirect call taking place, no control flow integrity check is necessary.

I implemented a prototype of this idea that specifically targeted the hottest
virtual function in one of Chromium’s layout benchmarks, and observed a
performance improvement of median 1.62%, min -4.72%, max 204.52% for the
layout benchmark suite. I’d like to emphasise that these numbers are from
a regular LTO build without CFI, and that we expect to see better performance
from a general implementation that targets all virtual functions.

==User interface==
To instruct the compiler to assume that all type hierarchies are closed, a
user can pass the -fwhole-program-vtables flag. The -fwhole-program-vtables
flag requires the -flto flag to also be specified.

Of course, there may be some type hierarchies that are not entirely closed, but
the underlying assumption is that most hierarchies will not be. To support open
hierarchies the user can also specify the path to a blacklist that lists the
names of the open class hierarchies using a -fwhole-program-vtables-blacklist=
flag. By default, the compiler will assume that hierarchies in the std
namespace are open.

==High level design==
Devirtualization will take place at LTO time. Candidates for virtual constant
propagation will be found by searching for call sites with all-constant
arguments to virtual functions which in all derived classes are readnone,
which means that it is a pure function of its arguments. For each (virtual
function, arguments) candidate pair, the compiler will compute a offset in
bits (positive or negative) from the class’s vtable address point in which
to store the function result, extend the size of any vtable objects to cover
any such offsets, evaluate the function to compute a result and store it
at the correct offsets. To avoid breaking ABI, the vtable layout will not
be changed in any way. This includes pointers to optimized functions in the
vtable, and (for Itanium ABI) the distance between address points within a
single vtable symbol.

For example, the vtable for Base and Derived will look like this:

struct Base_VTable {
  bool isDerived : 1, isOfTypeBase : 1, isOfTypeDerived : 1;
  size_t padding : 61;
  size_t offset_to_top;
  size_t rtti;
  bool (*isDerived)(const Base *this);
  bool (*isOfType)(const Base *this, Type t);
};
Base_VTable base_vtable{
  false, true, false, 0, 0, 0, &Base::isDerived, &Base::IsOfType
};
Base_VTable derived_vtable{
  True, false, true, 0, 0, 0, &Derived::isDerived, &Derived::IsOfType
};

The symbol _ZTV4Base, representing the start of Base’s vtable set, will
be defined as base_vtable+8; similarly for _ZTV7Derived and derived_vtable+8.

To avoid bloating virtual tables by too much in degenerate cases, there will
be a limit for each virtual function on the number of bits that the virtual
function may contribute to the size of the vtable, which we may fix at (say)
2 machine words.

To give an idea of how the generated code would look, here is an asm snippet
from the hottest virtual call from Chromium’s layout benchmarks before
the transformation:

 255c9ae:       48 8b 03                mov    (%rbx),%rax
 255c9b1:       be 30 00 00 00          mov    $0x30,%esi
 255c9b6:       48 89 df                mov    %rbx,%rdi
 255c9b9:       ff 90 e0 02 00 00       callq  *0x2e0(%rax)
 255c9bf:       84 c0                   test   %al,%al
 255c9c1:       74 04                   je     255c9c7
 255c9c3:       31 c0                   xor    %eax,%eax
 255c9c5:       5b                      pop    %rbx
 255c9c6:       c3                      retq   

And after:

 255a8ee:       48 8b 03                mov    (%rbx),%rax
 255a8f1:       f6 40 e6 01             testb  $0x1,-0x1a(%rax)
 255a8f5:       74 04                   je     255a8fb
 255a8f7:       31 c0                   xor    %eax,%eax
 255a8f9:       5b                      pop    %rbx
 255a8fa:       c3                      retq   

==LLVM IR-level design==
Given a class name, how can we determine the closed set of derived classes and
possible function pointers for a call site? As it turns out the IR already
has a way of expressing this information: bitsets [1], which are already being
used to implement CFI. We can encode the information for devirtualization
by combining the @llvm.bitset.test and @llvm.assume intrinsics.

%vtable = load i8**, i8*** %obj
%p = call i1 @llvm.bitset.test(%vtable, “Base”)
call void @llvm.assume(i1 %p) ; %vtable is assumed to point to
                              ; _ZTV4Base+16 or _ZTV7Derived+16.
%fptrptr = getelementptr i8* %vtable, 1
%fptr = load i8*, i8** %fptrptr ; %fptr must point to Base::isOfType
                                ; or Derived::isOfType.
%fptr_casted = bitcast i8% %fptr to i1 (i8***, i32)
%result = call i1 %fptr_casted(i8*** %obj, i32 0)

This gives the optimizer all the information it needs to implement the
optimization: the addresses of the virtual tables, the addresses of the
function pointers (within the virtual table initializers) and the constant
arguments. Note that the compiler does not need to do any of the work
described in the above link to lay out globals or create bitsets if all
calls to llvm.bitset.test are passed to llvm.assume.

If virtual constant propagation succeeds, the transformed IR will look
like this:

%vtable = load i8*, i8** %obj
%vtable_bit_addr = getelementptr i8* %vtable, i64 -17
%vtable_bit = load i8 %vtable_bit_addr
%vtable_bit_and = and i8 %vtable_bit, 1
%result = icmp ne %vtable_bit_and, 0

The pass that applies this transformation will be placed early in the LTO
pipeline, before most of the regular optimization passes. The pass could later
be extended to do general devirtualization based on the bitset information:

 - The pass can implement single-deriver (i.e. only one derived class with a
   non-pure virtual function definition) and single-implementation
   (i.e. multiple derived classes all sharing a virtual function definition
   from a base class) devirtualization by checking that each of the possible
   values loaded from the vtables in the bitset are either identical or
   __cxa_pure_virtual (calls to pure virtuals are UB, so we’d be fine
   disregarding them), and propagating the identical address into the
   function callee.
 - The pass could also potentially implement speculative devirtualization
   (i.e. testing the vtable entry and guarding direct calls on it) by
   generating a comparison against the known address point. The pass will
   know exactly how many possible classes there will be for each call site,
   so we could have logic to speculatively devirtualize if that number is
   sufficiently small.

The design when CFI is enabled is slightly different, as we need to give
the compiler the ability to eliminate the unnecessary llvm.bitset.test
call. Recall that the IR looks like this:

%vtable = load i8**, i8*** %obj
%p = call i1 @llvm.bitset.test(%vtable, “Base”)
br i1 %p, label %call, label %abort

call:
%fptr = load i8*, i8** %vtable
%fptr_casted = bitcast i8% %fptr to i1 (i8***, i32)
call i1 %fptr_casted(i8*** %obj, i32 0)

abort:
call void @llvm.trap()

We cannot simply have our optimization pass transform this IR to eliminate the
llvm.bitset.test call, as the transformation would not be semantics-preserving
(we already have a CFI setting that emits diagnostics in the abort branch,
which would break under such a transformation). Instead, we can introduce
a new intrinsic, @llvm.bitset.check, to be used only in production mode
(i.e. with diagnostics disabled):

{i8*, i1} @llvm.bitset.check(i8* %ptr, metadata %name)

If %ptr is in the bitset identified by %name, the function returns {%ptr,
true}. But if %ptr is not in %name, the behaviour is undefined, modulo that
if the second element of the result is non-zero, the program may load and
call a function pointer from an address derived from the first element of the
result without causing an indirect function call to any function other than
one potentially loaded from one of the constant globals of which %name is
a member (i.e. one of the valid vtables for %name). A CFI-protected virtual
call would then look like this (eliding bitcasts for brevity):

%vtable = load i8**, i8*** %obj
%pair = call {i8*, i1} @llvm.bitset.check(i8* %vtable, “Base”)
%checked_vtable = extractelement %pair, 0
%p = extractelement %pair, 1
br i1 %p, label %call, label %abort

call:
%fptr = load i8*, i8** %checked_vtable
%result = call i1 %fptr(i8*** %obj, i32 0)

abort:
call void @llvm.trap()

Applying virtual constant propagation to @llvm.bitset.check would be very
similar to applying it to @llvm.bitset.test, but if the constant propagation
succeeds, the function will return true as the second element:

%vtable = load i8**, i8*** %obj
br i1 true, label %call, label %abort

call:
%vtable_bit_addr = getelementptr i8* %vtable, i64 -17
%vtable_bit = load i8 %vtable_bit_addr
%vtable_bit_and = and i8 %vtable_bit, 1
%result = icmp ne %vtable_bit_and, 0

abort:
call void @llvm.trap()

Now the existing SimplifyCFG pass can easily eliminate the unnecessary branching.

In order to allow for regular constant folding through the llvm.bitset.check
intrinsic, the code in lib/Analysis/ConstantFolding.cpp would be taught to
look through the intrinsic’s argument.

-- 
Peter

[0] https://code.google.com/p/chromium/issues/detail?id=580389
[1] http://llvm.org/docs/BitSets.html