llvm dev - Jun 2004 - [LLVMdev] removnig instructions with uses

How do you remove an instruction with uses before replacing said uses?  Opt
exits immediately if you do this, it seems like it should wait until your
pass finishes running.  The only way I can get around it now is to work
recursively post order removing the use before the def, which is slower than
if I could just remove them in linear order!  Oh well.

~Patrick

On Tue, 15 Jun 2004, Patrick Meredith wrote:
> How do you remove an instruction with uses before replacing said uses?  Opt
> exits immediately if you do this, it seems like it should wait until your
> pass finishes running.  The only way I can get around it now is to work
> recursively post order removing the use before the def, which is slower
than
> if I could just remove them in linear order!  Oh well.
I'm not sure exactly what you're trying to do, but you have to get rid
of
all uses of a value before you can delete it (otherwise you have dangling
pointers and you get an assertion).

What exactly is the problem that you're having?  Why is it slower to do it
at one time than another?

-Chris

-- 
http://llvm.cs.uiuc.edu/
http://www.nondot.org/~sabre/Projects/

On Tue, Jun 15, 2004 at 03:07:18PM -0500, Patrick Meredith
wrote:
> How do you remove an instruction with uses before replacing said uses?
You cannot, here's why:

  a = add b, c
  d = add a, b

Now if we delete the first instruction that defines a, what do we get?

  d = add <invalid>, b

Now try to iterate over the values of this instruction and read their
properties.... You can't, because the instruction is invalid as it's
pointing to deallocated memory.  Hence, you must delete uses before
deleting the instruction itself.

This is not to make life difficult for you, it's to keep code
well-formed at all times.
> Opt exits immediately if you do this, it seems like it should wait
> until your pass finishes running.  The only way I can get around it
> now is to work recursively post order removing the use before the def,
> which is slower than if I could just remove them in linear order!  Oh
> well.
Perhaps you should consider what exactly you are trying to do if this
instruction really does have so many uses...  If you are deleting a temporary
instruction, it probably doesn't have such a "deep" tree of uses
below it.

-- 
Misha Brukman :: http://misha.brukman.net :: http://llvm.cs.uiuc.edu