[LLVMdev] Proposal: function prefix data

Tyler Hardin tghardin1 at catamount.wcu.edu
Thu Jul 18 12:23:19 PDT 2013

As much as I like this idea for it's use in languages with type systems
like Haskell and Scheme, this proposal would limit LLVM to non-Harvard
architectures. That's generally a really small minority of all processors,
but it would mean there could never be a clang-avr.

An alternative you could use is, instead of using the function pointer as
the variable where you are referring to a function, you could have the
variable be a pointer to a static struct with the data and the actual
function pointer. Basically, it's like how static class variables as
handled in C++.

I don't know LLVM IR, so I'll use C to explain.

Instead of this:

void func(void){}

int main(){
   return 0;

You could do this:

void func(void){}

/* You have to initialize this at compile time. */
struct {
char* data;
int len;
void (*ptr)(void) = func;
} func_data;

int main(){
   return 0;
 On Jul 18, 2013 12:47 PM, "Jevin Sweval" <jevinsweval at gmail.com> wrote:

> On Wed, Jul 17, 2013 at 9:06 PM, Peter Collingbourne <peter at pcc.me.uk>
> wrote:
> >
> > 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.
> What if the prefix data was stored before the start of the function
> code? The function's symbol will point to the code just as before,
> eliminating the need to have instructions that skip the prefix data.
> It would look something like:
> | Prefix Data ... (variable length) | Prefix Data Length (fixed length
> [32 bits?]) | Function code .... |
>             ^ function symbol points here (function code)
> I hope the simple ASCII art makes it through my mail client.
> To access the data, you do
> prefix_data = function_ptr - sizeof(prefix_length) - prefix_length
> Cheers,
> Jevin
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