[LLVMdev] Forcing JIT of all functions before execution starts (was: Implementing sizeof)

[Sarah Thompson]

Chris Lattner wrote:
> Check out http://nondot.org/sabre/LLVMNotes
>
>   

  %Size = getelementptr %T* null, int 1
  %SizeI = cast %T* %Size to uint


How incredibly cunning. :-) Thanks for that.


Next stupid question. I've put together a simple coroutine/fibre style 
threading system on top of the Linux setcontext/getcontext stuff, which 
surprisingly enough seems to work *almost* perfectly under the JITted 
environment, with the exception that the JITter doesn't like running in 
anything other than the primary fibre. For example,

#include <stdio.h>
#include "../../mcp/tools/mcp2/mcpapi.h"

void thread1(void *udata)
{
	int i;
	for(i=0; i<10; i++)
	{
		printf("--- thread1: Hello (%d)\n", i);
	}
}

void thread2(void *udata)
{
	int i;
	for(i=0; i<10; i++)
	{
		printf("--- thread2 Hello (%d)\n", i);
	}
}

int main(int argc, char **argv)
{
	printf("--- Running threads serially:\n");
	thread1(0);
	thread2(0);

	printf("--- Running threads in parallel:\n");
	mcp_begin_thread(thread1, 0);
	mcp_begin_thread(thread2, 0);

	printf("--- Waiting for child threads to terminate\n");
	mcp_wait_for_exit();

	printf("--- Bye...\n");
	return 0;
}

works perfectly, because thread1() and thread2() are forced into 
existence before they are referenced by mcp_begin_thread() (i.e. as a 
parameter to makecontext). Commenting out the running threads serially 
section results in a seg fault inside the jitter. Since we have to make 
so many performance compromises in the name of model checking anyway, 
it's no big deal for me to just basically force every function in the 
module to be compiled before execution starts, which should effectively 
mean that the JITter can never execute at run time (and therefore can 
never accidentally be run inside a child thread). What is the 
best/safest way to do this? Is it OK just to enumerate all the functions 
within the module and resolve them to non-lazy function pointers? I 
think in V1.8 just taking the address of a function was enough to cause 
it to be jitted, so something has obviously changed a bit since then.

Thanks,
Sarah