[LLVMdev] copy Value object?

John Criswell criswell at illinois.edu
Mon Sep 19 13:09:44 PDT 2011

On 9/19/11 2:48 PM, ret val wrote:
> Sorry, I'm a bit confused by your reply. I think part of my problem is
> I can only think of this in terms of Passes.
> For instance, my pass looks for assignments and tries to use the same
> pointer operand, before the assignment it finds. Like this:
>          new ICmpInst(*block, CmpInst::ICMP_NE, shadow,
> store->getPointerOperand(), "Shadow check");

Can you clarify what you mean by "an assignment?"  Do you mean an LLVM 
store instruction, or do you mean an SSA assignment of an LLVM virtual 
> So I'm not sure how alloca could help me, seems like it wouldn't be a
> valid Value * for me to pass in at that point. Am I missing something?

Perhaps this would be easier if I knew what your pass is trying to do.  
Can you explain what you are trying to do?

-- John T.

> Thank you
> On Mon, Sep 19, 2011 at 2:02 PM, John Criswell<criswell at illinois.edu>  wrote:
>> On 9/19/11 12:53 PM, ret val wrote:
>>> Is there a easy way to copy a Value object, so it can be used in
>>> multiple instructions without a dominance issue?
>> A value object can be used multiple times with no problem.
>> The dominance problem stems from the fact that the program must be in SSA
>> form.  When the program uses a value, it must have been defined dynamically
>> for all possible executions of the program.  This is why the definition of a
>> value must dominate all of its uses.
>> If you're instrumenting code and having trouble meeting the dominance
>> requirement, what you should do is add the "variable" as an alloca'ed stack
>> variable that is read/written by loads/stores instead of as an SSA value
>> that is defined only once.
>> For example, instead of adding code that does this:
>> %y = add %x, 5
>> ...
>> %z = sub %y, 2
>> ... you add this:
>> entry_block:
>>     %yp = alloca<type of y>
>> ...
>>     %y1 = add %x, 5
>>     store %y1, %yp
>> ...
>>     %y2 = load %yp
>>     %z = sub %y2, 2
>> The second version of the code allows the "variable" stored into the
>> alloca'ed memory to never have been initialized or to be written multiple
>> times.  You are freed from trying to keep %y in valid SSA form at all times.
>> After your pass is run, you can run the -mem2reg pass.  This pass will
>> convert your instrumented code into SSA form (i.e., it will change the
>> second version of the code into the first version of the code).
>> -- John T.
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