llvm dev - Dec 2010 - [LLVMdev] indirectbr/blockaddress question

In my llvm jit project I needed to lookup BB addresses at execution time and
then jump to the corresponding BB. A C++ routine called at runtime from IR
finds the right BB, gets its BlockAddress and returns it as an i8*. The IR
does an indirectbr on this value...
Well, not really. The routine returns the address of a BlockAddress node. Is
there any way to get the real runtime code address for the indirect jump at
runtime?

Thanks for any advice.
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On Sat, Dec 11, 2010 at 1:19 PM, Maurice Marks <maurice.marks at
gmail.com> wrote:
> In my llvm jit project I needed to lookup BB addresses at execution time
and
> then jump to the corresponding BB. A C++ routine called at runtime from IR
> finds the right BB, gets its BlockAddress and returns it as an i8*. The IR
> does an indirectbr on this value...
> Well, not really. The routine returns the address of a BlockAddress node.
Is
> there any way to get the real runtime code address for the indirect jump at
> runtime?
If you're having trouble with the correctness of your program, don't
use indirectbr; the semantics are tricky and it's usually unnecessary.
 The only advantage over a "switch" instruction is a slight
performance boost for certain kinds of loops.

-Eli

On Sat, Dec 11, 2010 at 4:19 PM, Maurice Marks <maurice.marks at
gmail.com> wrote:
> In my llvm jit project I needed to lookup BB addresses at execution time
and
> then jump to the corresponding BB. A C++ routine called at runtime from IR
> finds the right BB, gets its BlockAddress and returns it as an i8*. The IR
> does an indirectbr on this value...
> Well, not really. The routine returns the address of a BlockAddress node.
Is
> there any way to get the real runtime code address for the indirect jump at
> runtime?
IIRC indirectbr isn't supported in the JIT:
http://llvm.org/bugs/show_bug.cgi?id=6744

Our first cut for implementing Python generators was to simply put a
switch in the entry block and number the re-entry points of the
generator.  On exit, we update the next entrance to use.

Reid

Thanks very to all of you much for your advice. I think I can use a switch
to do what I want, at least for now. I do like the idea of creating virtual
entry points to a function with a switch. That solves a problem for us.

thanks

On Sun, Dec 12, 2010 at 10:25 AM, Reid Kleckner <reid.kleckner at
gmail.com>wrote:
> On Sat, Dec 11, 2010 at 4:19 PM, Maurice Marks <maurice.marks at
gmail.com>
> wrote:
> > In my llvm jit project I needed to lookup BB addresses at execution
time
> and
> > then jump to the corresponding BB. A C++ routine called at runtime
from
> IR
> > finds the right BB, gets its BlockAddress and returns it as an i8*.
The
> IR
> > does an indirectbr on this value...
> > Well, not really. The routine returns the address of a BlockAddress
node.
> Is
> > there any way to get the real runtime code address for the indirect
jump
> at
> > runtime?
>
> IIRC indirectbr isn't supported in the JIT:
> http://llvm.org/bugs/show_bug.cgi?id=6744
>
> Our first cut for implementing Python generators was to simply put a
> switch in the entry block and number the re-entry points of the
> generator.  On exit, we update the next entrance to use.
>
> Reid
>


-- 
Not sent from my Blackberry, Raspberry or Gooseberry!
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