[LLVMdev] Proposal: function prefix data

Sean Silva silvas at purdue.edu
Wed Jul 17 19:50:58 PDT 2013

On Wed, Jul 17, 2013 at 6:06 PM, Peter Collingbourne <peter at pcc.me.uk>wrote:

> Hi,
> I would like to propose that we introduce a mechanism in IR to allow
> arbitrary data to be stashed before a function body.  The purpose of
> this would be to allow additional data about a function to be looked
> up via a function pointer.  Two use cases come to mind:
> 1) We'd like to be able to use UBSan to check that the type of the
>    function pointer of an indirect function call matches the type of
>    the function being called.  This can't really be done efficiently
>    without storing type information near the function.

How efficient does it have to be? Have some alternatives already proven to
be "too slow"? (e.g. a binary search into a sorted table)

> 2) Allowing GHC's tables-next-to-code ABI [1] to be implemented.
>    In general, I imagine this feature could be useful for the
>    implementation of languages which require runtime metadata for
>    each function.
> The proposal is that an IR function definition acquires a constant
> operand which contains the data to be emitted immediately before
> the function body (known as the prefix data).  To access the data
> for a given function, a program may bitcast the function pointer to
> a pointer to the constant's type.  This implies that the IR symbol
> points to the start of the prefix data.
> To maintain the semantics of ordinary function calls, the prefix data
> must have a particular format.  Specifically, it must begin with a
> sequence of bytes which decode to a sequence of machine instructions,
> valid for the module's target, which transfer control to the point
> immediately succeeding the prefix data, without performing any other
> visible action.  This allows the inliner and other passes to reason
> about the semantics of the function definition without needing to
> reason about the prefix data.  Obviously this makes the format of the
> prefix data highly target dependent.

I'm not sure that something this target dependent is the right choice. Your
example below suggests that the frontend would then need to know magic to
put "raw" in the instruction stream. Have you considered having the feature
expose just the intent "store this data attached to the function, to be
accessed very quickly", and then have an intrinsic
("llvm.getfuncdata.i{8,16,32,64}"?) which extracts the data in a
target-dependent way? Forcing clients to embed deep
target-specific-machine-code knowledge in their frontends seems like a step
in the wrong direction for LLVM.

> This requirement could be relaxed when combined with my earlier symbol
> offset proposal [2] as applied to functions.  However, this is outside
> the scope of the current proposal.
> Example:
> %0 = type <{ i32, i8* }>
> define void @f() prefix %0 <{ i32 1413876459, i8* bitcast ({ i8*, i8* }*
> @_ZTIFvvE to i8*) }> {
>   ret void
> }
> This is an example of something that UBSan might generate on an
> x86_64 machine.  It consists of a signature of 4 bytes followed by a
> pointer to the RTTI data for the type 'void ()'.  The signature when
> laid out as a little endian 32-bit integer decodes to the instruction
> 'jmp .+0x0c' (which jumps to the instruction immediately succeeding
> the 12-byte prefix) followed by the bytes 'F' and 'T' which identify
> the prefix as a UBSan function type prefix.

Do you know whether OoO CPU's will still attempt to decode the "garbage" in
the instruction stream, even if there is a jump over it? (IIRC they will
decode ahead of the PC and hiccup (but not fault) on garbage). Maybe it
would be better to steganographically encode the value inside the
instruction stream? On x86 you could use 48b8<imm64> which only has 2 bytes
overhead for an i64 (putting a move like that, which moves into a
caller-saved register on entry, would effectively be a noop). This is some
pretty gnarly target-dependent stuff which seems like it would best be
hidden in the backend (e.g. architectures that have "constant island"-like
passes might want to stash the data in there instead).

-- Sean Silva
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