[LLVMdev] Replacing uses within a function
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
> So the store itself is not a user of value1, and the usage does not get
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
> 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 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.
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