[PATCH] D28934: Write a new SSAUpdater

[Daniel Berlin via Phabricator via llvm-commits]

dberlin added a comment.

In https://reviews.llvm.org/D28934#791453, @davide wrote:

> Thanks for your comment, I'll see what I can do given this shows up in some of our internal workload.
>  One thing, I agree we should change the API at some point (and maybe use a O(def + uses) renamer as we do in PredInfo) but, for starters, we should be able to have a drop-in replacement for `RewriteUse`, i.e. the new algorithm will always pick the correct value?
>
>   void SSAUpdater::RewriteUse(Use &U) {
>     Instruction *User = cast<Instruction>(U.getUser());
>  
>     Value *V;
>     if (PHINode *UserPN = dyn_cast<PHINode>(User))
>       V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
>     else
>       V = GetValueInMiddleOfBlock(User->getParent());
>   [...]
>


I believe this should be:

void SSAUpdater::RewriteUse(unsigned Var, Use &U) {
Instruction *User = cast<Instruction>(U.getUser());
BasicBlock *VarBB = User->getParent();
if (auto *PN = dyn_cast<PHINode>(User))

  VarBB = PN->getIncomingBlock(U);

return readVariableAfterDef(Var, VarBB);
}

(Note all these API's, both old and new, assume you are rewriting in program order. It's possible to make it not require that, by storing all the defs in a given block and using ordered instructions to see where the use occurs relative to them)


https://reviews.llvm.org/D28934