[LLVMdev] Replacing uses within a function

John Criswell criswell at illinois.edu
Fri Oct 21 09:38:29 PDT 2011

On 10/21/11 11:15 AM, Carlos Sánchez de La Lama wrote:
> Hi all,
> I am trying to replace all uses of a value1 *inside of a given function*
> to use value2. My strategy was to iterate all the basic blocks, again
> iterating all the instructions in each basic block, and using
> replaceUsesOfWith(value1, value2) for every instruction.
> This used to work all right, but now I am finding some problems. There
> are instructions like this:
> store i32 0, i32* getelementptr inbounds ([3 x i32]* @value1, i32 0, i32
> 0)
> So the store itself is not a user of value1, and the usage does not get
> replaced.

The issue here is that the store instruction is using a constant 
expression (ConstExpr) which is using the value that you wish to 
replace.  The GEP that you see here is not a GEP instruction but a GEP 
constant expression (http://llvm.org/docs/LangRef.html#constantexprs).

> I had a similar problem with PHI nodes recently
> (http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-October/043954.html)
> and I was suggested using value1->replaceAllUsesWith. But again I cannot
> cause I want only to replace uses within a given function.
> I could iterate value1->use_iterator and replace those which are inside
> of the desired function, but with cases as the mentioned one it is not
> so easy to find whether a given value is within a function (the GEP in
> to store is not inside the function, but used by an instruction that
> is).
> Is there an easy way to achieve this? I find it would be better to force
> all instructions (like the GEP above) to be directly place on the
> function, and not as operands of other instructions, unless there is a
> good&  known reason not to.

There's a few options.

First, you could use something like the BreakConstantGEP pass from 
SAFECode to convert constant expression GEPs into GEP instructions.  
Then your old approach will work properly.  However, the speed of the 
code may suffer because GEP constant expressions are replaced with 
constant values during code generation while GEP instructions most 
likely will not.  You could, of course, modify the code to selectively 
convert GEP constant expressions used within the function that you care 

A second approach is to simply search for constant expressions that are 
used within the function you care about and replace them with a new 
constant expression that uses your new value.  I think constants are 
unique in LLVM, so you can't just call 
ConstantExpr::replaceUsesOfWith(); you'll need to create a new constant 
expression that is identical to the old one but using the new value, and 
then replace the specific use of the old constant expression with the 
new one that you've created.

-- John T.

> BR
> Carlos
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