[LLVMdev] IR extension proposal: bitset constants

[Jim Grosbach]

Hi Peter,

Please forgive if this is an obvious question, but how reasonable is it for this approach to work when not all translation units are available to be rebuilt with the new information?

I'm very interested in what you're proposing here.

Jim

Sent from my iPhone

> On Jan 27, 2015, at 1:07 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
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