[LLVMdev] IR extension proposal: bitset constants

Peter Collingbourne
Wed, 28 Jan 2015 18:32:11 -0000

Hi Sean,

I agree that we should first try to accomplish this without changing every
part of the toolchain. This is why I proposed to first introduce only the IR
support and bitset lowering pass. This would provide a working implementation
that is confined to the compiler, but does require LTO.

In the long run, however, I would like to drop the dependency on LTO, in
order to permit fast incremental builds.

Once the initial approach has proven to be effective, we can think about
dropping the LTO dependency by implementing linker support. I only included
the linker implementation details to show that the design can work without
requiring LTO.

Peter

On Wed, Jan 28, 2015 at 10:57:21AM +0000, Sean Silva
wrote:
> Is there any way to accomplish this that doesn't require changes in
every
> part of the toolchain?
>
> What you describe here sounds really efficient, but for a change this
> pervasive throughout the toolchain, I would at least like to see an attempt
> done that does not require changing the toolchain. Then we can circle back
> to changing the toolchain if that proves inadequate and with concrete
> experience informing what really is going to require a toolchain change
> (Maybe you've already done this? Please share.).
>
> -- Sean Silva
>
>
> On Tue, Jan 27, 2015 at 9:07 PM, Peter Collingbourne <peter at
pcc.me.uk>
> wrote:
>
> > Hi all,
> >
> > I would like to propose a mechanism that allows IR modules to
> > co-operatively
> > build a pointer set corresponding to addresses within a given set of
> > globals. The specific use case I have in mind is to provide a
mechanism
> > for a C++ program to efficiently verify (at each call site) that a
vtable
> > pointer is in the set of valid vtable pointers for the class or its
derived
> > classes. One way of doing this is for a toolchain component to build,
for
> > each
> > class, a bit set that maps to the memory region allocated for the
vtables,
> > such that each 1 bit in the bit set maps to a valid vtable for that
class,
> > and lay out the vtables next to each other, to minimize the total size
of
> > the bit sets. Call sites would perform a range check followed by a
load of
> > the appropriate bit from the bit set.
> >
> > To give a concrete example, suppose we have the following classes:
> >
> > struct A { virtual void f(); };
> > struct B : A { virtual void f(), g(); };
> > struct C : A { virtual void f(), h(); };
> >
> > with the following vtables:
> >
> > _ZTV1A = { &A::rtti, &A::f };
> > _ZTV1B = { &B::rtti, &B::f, &B::g };
> > _ZTV1C = { &C::rtti, &C::f, &C::h };
> >
> > The set of valid vtables for static class A is {&_ZTV1A[1],
&_ZTV1B[1],
> > &_ZTV1C[1]}, for B is {&_ZTV1B[1]} and for C is
{&_ZTV1C[1]}. The toolchain
> > would lay out _ZTV1A, _ZTV1B and _ZTV1C consecutively, and generate
bit
> > sets
> > like this:
> >
> > A = {0,1,0,1,0,0,1,0}
> > B = {0,0,0,1,0,0,0,0}
> > C = {0,0,0,0,0,0,1,0}
> >
> > A call site (say the static type of the call is A) will check whether
the
> > object's vtable falls in the range [_ZTV1A, _ZTV1C], and is
sizeof(void
> > *)-aligned. It would then load the (vtable-_ZTV1A)/sizeof(void
*)'th entry
> > in A's bitset, which will be 1 if the vtable is valid for A.
> >
> > We are now faced with a number of implementation questions: how do we
> > represent
> > these (potentially partial) bit sets in LLVM, and how do we arrange to
lay
> > out the globals and build the complete bit sets? Remember that classes
B
> > and C may be in different translation units, so we do not necessarily
have
> > a complete picture of A's bit set at A's compile time.
> >
> > What I propose is that the parts of the bit sets be represented as
arrays
> > of pointers, which will be resolved through some mechanism (either at
> > LTO time, or at regular (non-LTO) link time) to bit sets corresponding
to
> > those addresses. This mechanism would also be responsible for laying
out
> > the referenced globals (i.e. the vtables) as close together as
possible. We
> > introduce a new flag on globals, the so-called "bitset"
flag, which causes
> > the global to be transformed using this mechanism. Such a global
cannot be
> > accessed in the normal way. It may only be accessed using an intrinsic
that
> > tests whether a given pointer is in the set.
> >
> > To return to our concrete example, a translation unit defining the
vtable
> > for A may also define the following global:
> >
> > @_ZBS1A = appending bitset constant [1 x i8*] [gep(@_ZTV1A, 0, 1)]
> >
> > A translation unit defining B would define:
> >
> > @_ZBS1A = appending bitset constant [1 x i8*] [gep(@_ZTV1B, 0, 1)]
> > @_ZBS1B = appending bitset constant [1 x i8*] [gep(@_ZTV1B, 0, 1)]
> >
> > And C:
> >
> > @_ZBS1A = appending bitset constant [1 x i8*] [gep(@_ZTV1C, 0, 1)]
> > @_ZBS1C = appending bitset constant [1 x i8*] [gep(@_ZTV1C, 0, 1)]
> >
> > A later mechanism would combine the bitset globals, lay out the
referenced
> > globals and build the bit sets. In the LTO case, the IR linker can
already
> > do the combination step if the globals are given appending linkage. A
> > later compiler pass (which we will call the "bitset lowering
pass") would
> > lay out
> > the globals (by combining them into a single large global, and RAUWing
the
> > original globals with aliases that gep into the large global) and
build the
> > bit sets. In the non-LTO case, we could also invent an
object-file-specific
> > mechanism for the backend to tell the linker to combine and build the
bit
> > sets,
> > for example by placing the bitset globals in a specifically named
section,
> > and place the referenced globals in individual sections so that the
linker
> > can rearrange them.
> >
> > As previously mentioned, an intrinsic would be used to test entries in
the
> > bit set at call sites. For example, the relevant IR for the following
C++
> > code:
> >
> > A *some_object = ...;
> > some_object->f();
> >
> > might look like this:
> >
> > %vtable = load i8** %some_object
> > %p = call i1 @llvm.bitset.test(i8* %vtable, i8* @_ZBS1A) readnone
> > br i1 %p, label %continue, label %trap
> >
> > In the LTO case, such intrinsic calls would be lowered by the bitset
> > lowering pass
> > into the appropriate range/bit set check code, which might look like
this:
> > (pseudocode for 64-bit platform)
> >
> > if %vtable & 7 != 0 || %vtable < global_start_addr(_ZBS1A) ||
%vtable >
> > global_end_addr(_ZBS1A) {
> >   %p = 0
> > } else {
> >   %p = bitset(_ZBS1A)[(%vtable - global_start_addr(_ZBS1A)) >>
3]
> > }
> >
> > In the non-LTO case, we would generate similar code, but with
appropriate
> > relocations so that the required constants (global start/end address,
> > bitset
> > address, mask, shift amount) would receive appropriate
linker-determined
> > values.
> >
> > The first step I propose to make in this direction is a patch that
adds
> > support for the bitset flag on globals, and introduces the bitset
lowering
> > pass. A
> > later step would involve building backend support, and linker support
in
> > lld.
> >
> > Comments appreciated.
> >
> > Thanks,
> > --
> > Peter
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >

--
Peter