[LLVMdev] difficulty in replicating a sequence of instructions + inserting at a different location -- "instruction doesn't dominate all uses"

[Chuck Zhao]

John, Ether,

Thank you for the reply.

The root of the problem is exactly as what John described --
"there are attempted uses of instruction(s) before its/their respective 
definition(s)", so the LLVM Verify pass complains as "Instruction 
doesn't dominate all uses".

This unveils the problem, but I haven't come up with a proper solution yet.
I will address in a different question.

Thank you for the help.

Chuck

On 5/2/2011 2:29 PM, John Criswell wrote:
> On 5/2/11 11:59 AM, Chuck Zhao wrote:
>> I am having difficulty in replicating a sequence of instructions (2+, 
>> with def-use dependencies within) and inserting them at a different 
>> location.
>>
>> I have tried a few different approaches (IRBuilder, new 
>> Instruction(), I->clone(), insertBefore/insertAfter,etc.), all 
>> leading to the same error msg:
>> "Instruction doesn't dominate all uses"
>
> In LLVM IR, an instruction must dominate all of its uses.  What this 
> means is that an instruction must be inserted into a basic block that 
> dominates the basic blocks of all instructions that use the inserted 
> instruction's result.  In other words, it must be the case that the 
> inserted instruction is always executed once before any instruction 
> that uses its result.
>
> I'm not sure what you are doing wrong that is causing the above 
> problem to occur.  It might be that you're not adjusting the def-use 
> chains after replication or you might be inserting instructions in the 
> wrong order in a basic block (defs must come before uses).
>
> The above error comes from the verifier pass, so you can disable the 
> verifier pass, save the bitcode, and disassemble it to see what IR 
> you're building that is incorrect.  Alternatively, you can have your 
> transform dump the new instructions to stdout or stderr to see what IR 
> it is generating.
>
> -- John T.
>
>>
>> The DevList has a few previous discussions on this, but not much of 
>> hints I can get from there.
>>
>> Could somebody point me a code template or an existing LLVM source 
>> file that does similar things?
>>
>>
>> Thank you very much
>>
>> Chuck
>>
>>
>>
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