llvm dev - Aug 2014 - [LLVMdev] difference between replaceAllUsesWith and replaceUsesOfWith?

On Sat, Aug 9, 2014 at 6:06 AM, Tim Northover <t.p.northover at gmail.com>
wrote:
> Hi Rob,
>
> On 9 August 2014 02:03, Rob Jansen <jansen at cs.umn.edu> wrote:
> > Why is the first for loop not equivalent to the second?
>
> In the second loop, "*ui" is an llvm::Use object. It's owned
by a
> User, but isn't a subclass of one. To match the first loop, you either
> need to call Value::user_begin instead of use_begin, or do the cast on
> "ui->getUser()".
>
I think this is incorrect since the use_iterator is defined as:

`typedef value_use_iterator<User> use_iterator;`

and indeed `ui->getUser();` is not a callable function on a User. What am I
missing?

-Rob
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To make this clearer, I've reposted the loops below, replacing dyn_cast
with cast. I've also added the assertion, which triggers in the bottom
loop, but not the top.

-Rob

==========================
for (GlobalVariable **i = Globals.begin(), **e = Globals.end(); i != e;
++i) {
  GlobalVariable *GV = *i;

  Constant *GEP = ConstantExpr::getGetElementPtr(...);
  GV->replaceAllUsesWith(GEP);
}
assert(GV->use_empty());
==========================
for (GlobalVariable **i = Globals.begin(), **e = Globals.end(); i != e;
++i) {
  GlobalVariable *GV = *i;

  Constant *GEP = ConstantExpr::getGetElementPtr(...);

  for (Value::use_iterator ui = GV->use_begin(); ui != GV->use_end();
++ui)
{
    User *u = cast < User > (*ui);
    u->replaceUsesOfWith(GV, GEP);
  }
}
assert(GV->use_empty());
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> I think this is incorrect since the use_iterator is defined as:
>
> `typedef value_use_iterator<User> use_iterator;`
>
> and indeed `ui->getUser();` is not a callable function on a User. What
am I
> missing?
Ah, looks like this has changed this year. What I said applies to
trunk; you may be right there about 3.4 or earlier (looks plausible
from the key commit).

Another problem might be that User::replaceUsesOfWith only works in
limited situations if the User is a Constant. But I'd expect that to
fail with an assertion (assuming your LLVM has been built with
assertions enabled, of course).

Do you have (preferably as simple as possible) IR module that shows
the replacement failure?

Cheers.

Tim.

Ahh ha! I believe that `User::replaceUsesOfWith` modifies the use list,
which probably invalidated the use_iterator and caused my code to miss some
of the uses. The following works great! (I'll spare you the details about
what I am actually trying to do here, unless you really want to know more.)

Thanks,
Rob

==========================while(GV->getNumUses() > 0) {
  User* u = GV->use_back();
  u->replaceUsesOfWith(GV, GEP);
}
assert(GV->use_empty());
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