[LLVMdev] [RFC] "noclone" function attribute

Krzysztof Parzyszek kparzysz at codeaurora.org
Sat Dec 1 09:19:24 PST 2012

On 12/1/2012 10:02 AM, James Molloy wrote:
> I'm proposing a new function attribute, "noclone", with the semantics that "calls to functions marked "noclone" cannot be cloned or duplicated into the same function.". That is, it is illegal to call J = I->clone() then attach J to the same basic block as I if I is marked "noclone".

The class Loop has something similar in it:

/// isSafeToClone - Return true if the loop body is safe to clone in 
/// Routines that reform the loop CFG and split edges often fail on 
bool Loop::isSafeToClone() const {
   // Return false if any loop blocks contain indirectbrs.
   for (Loop::block_iterator I = block_begin(), E = block_end(); I != E; 
++I) {
     if (isa<IndirectBrInst>((*I)->getTerminator()))
       return false;
   return true;

Maybe a similar interface could be added to Instruction, and an 
instruction would declare itself unsafe to clone if it was a call to a 
function with the attribute that you are proposing.

I could imagine that this may not the only example of an instruction 
that should not be cloned.

I'd only suggest that the attribute is called something like 
"noclonecalls", or (preferably) something shorter that clarifies that 
it's the calls that shouldn't be cloned.  :)


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