llvm dev - May 2015 - [LLVMdev] global variable uses

Hi,

 

I'm trying to change all uses of a global variable so they'll use a
different global variable.

For instance, I have:

@a = global [100 x [100 x i64]] zeroinitializer, align 16

@b = global [100 x [100 x i64]] zeroinitializer, align 16

 

And I want to change the use

%arrayidx = getelementptr inbounds [100 x [100 x i64]]* @a, i32 0, i64 %3

to

%arrayidx = getelementptr inbounds [100 x [100 x i64]]* @b, i32 0, i64 %3 

 

But, when I'm iterating the uses with the use_iterator of the Global
variable, I get the Global variable's definition/allocation as the use(s)
and not the "real" uses (the GEP). 

Why? Isn't the meaning of use here is where @a is used?

 

The relevant code (var is a GlobalVariable *):

var->dump();  // prints: @a = global [100 x [100 x i64]] zeroinitializer,
align 16

for (GlobalVariable::use_iterator U = var->use_begin(); U !=
var->use_end();
U++) {

                Constant * CE = dyn_cast<Constant>(*U); // this is  the
only
cast that works here

                CE->dump(); // prints: @a = global [100 x [100 x i64]]
zeroinitializer, align 16

}

 

Thanks for your help,

Tehila.

-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150525/50aac2a8/attachment.html>

Hi Tehila,

On 25 May 2015 at 02:38, Tehila Mayzels <tehila at cs.technion.ac.il>
wrote:
> Why? Isn't the meaning of use here is where @a is used?
Each element you're iterating through is an llvm::Use, which records
both what is being used and the user. Possibly confusingly, what you
get when you simply dereference it is the thing being used. You should
either call U->getUser() or iterate using user_begin/user_end instead.

Though there's actually a function Value::ReplaceAllUsesWith which
might mean you don't need the loop at all, depending on how complex
your change is.

Cheers.

Tim.

Thanks a lot for the explanation, Tim!
Tehila.

-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com] 
Sent: Monday, May 25, 2015 6:42 PM
To: Tehila Mayzels
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] global variable uses

Hi Tehila,

On 25 May 2015 at 02:38, Tehila Mayzels <tehila at cs.technion.ac.il>
wrote:
> Why? Isn't the meaning of use here is where @a is used?
Each element you're iterating through is an llvm::Use, which records both
what is being used and the user. Possibly confusingly, what you get when you
simply dereference it is the thing being used. You should either call
U->getUser() or iterate using user_begin/user_end instead.

Though there's actually a function Value::ReplaceAllUsesWith which might
mean you don't need the loop at all, depending on how complex your change
is.

Cheers.

Tim.