llvm dev - Jul 2007 - [LLVMdev] Implementing sizeof

Hi folks,

Assuming that I'm writing a pass and that for bizarre reasons I need to 
programmatically do the equivalent of a C/C++ sizeof on a Value (or a 
Type, it doesn't matter which really), yielding a result in bytes, what 
is the known-safe way to do this? I notice that doing something like

struct thingy
{
    ... some stuff ...
};

...
    printf("Size = %d", sizeof(thingy));
...

turns into a constant at the bitcode disassembly level, which is fair 
enough but I can't rely on the source passing down information like that 
-- I have to be able to work purely at the opt pass level. One of the 
transformations I need to do is turn load and store instructions into 
model-checker-specific intrinsics so I can track memory modifications 
efficiently, but just having the address isn't enough, I really need the 
size too. Or are reads and writes always guaranteed to be simple types, 
so the bit width (as given by the Type::getPrimitiveSizeInBits() member 
function) divided by 8 and rounded up if necessary  is enough? (The 
latter works well for me too, I just don't want to mess myself up by 
making assumptions).

As usual, thank you in  advance.

Sarah

Check out http://nondot.org/sabre/LLVMNotes

-Chris

http://nondot.org/sabre
http://llvm.org

On Jul 27, 2007, at 12:00 PM, Sarah Thompson <thompson at email.arc.nasa.gov 
 > wrote:
> Hi folks,
>
> Assuming that I'm writing a pass and that for bizarre reasons I need  
> to
> programmatically do the equivalent of a C/C++ sizeof on a Value (or a
> Type, it doesn't matter which really), yielding a result in bytes,  
> what
> is the known-safe way to do this? I notice that doing something like
>
> struct thingy
> {
>    ... some stuff ...
> };
>
> ...
>    printf("Size = %d", sizeof(thingy));
> ...
>
> turns into a constant at the bitcode disassembly level, which is fair
> enough but I can't rely on the source passing down information like  
> that
> -- I have to be able to work purely at the opt pass level. One of the 
> transformations I need to do is turn load and store instructions into
> model-checker-specific intrinsics so I can track memory modifications
> efficiently, but just having the address isn't enough, I really need  
> the
> size too. Or are reads and writes always guaranteed to be simple  
> types,
> so the bit width (as given by the Type::getPrimitiveSizeInBits()  
> member
> function) divided by 8 and rounded up if necessary  is enough? (The
> latter works well for me too, I just don't want to mess myself up by
> making assumptions).
>
> As usual, thank you in  advance.
>
> Sarah
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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

On 2007-07-27, at 15:00, Sarah Thompson wrote:
> Are reads and writes always guaranteed to be simple types, so the  
> bit width (as given by the Type::getPrimitiveSizeInBits() member  
> function) divided by 8 and rounded up if necessary  is enough? (The  
> latter works well for me too, I just don't want to mess myself up  
> by making assumptions).
Sarah,

Indeed, loads and stores must be of first class type: iN, float,  
double, pointers, or vectors.

http://llvm.org/docs/LangRef.html#i_load
http://llvm.org/docs/LangRef.html#t_primitive

— Gordon

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