[llvm-commits] SSI patch

[Owen Anderson]

On Jun 25, 2009, at 11:01 AM, Nick Lewycky wrote:
>   // TODO: We do modify the programs code, but we do not know which
>   // Passes we break.
>   void SSI::getAnalysisUsage(AnalysisUsage &AU) const {
>     AU.addRequired<DominanceFrontier> ();
>     AU.addRequired<DominatorTree> ();
>     AU.setPreservesAll();
>   }

Actually, since he doesn't do anything in his runOnFunction(), he  
does, in fact, preserves all passes.  It's the client transformation's  
responsibility to maintain other analyses.  This might also be an  
argument in favor of making this a utility function under lib/ 
Transforms/Utils rather than a separate pass.

Also, he needs to addRequiredTransitively() his dependencies, rather  
than addRequired().  The former guarantees that they will be around  
until his pass is destructed, while the latter only guarantees their  
existence until the end of runOnFunction().

>   /// Test if the BasicBlock BB dominates any use or definition of  
> value.
>   ///
>   bool SSI::dominateAny(BasicBlock * BB, Instruction * value) {
>     Value::use_iterator begin = value->use_begin();
>     Value::use_iterator end = value->use_end();
>     for (; begin != end; ++begin) {
>       Instruction * I = dyn_cast<Instruction> (*begin);

This function can be much simpler.  The only use of an instruction  
that isn't dominated by the def is a phi-use.  So you really only need  
to check the def itself, and any uses that are also phis.

--Owen