[LoopSimplifyCFG] Remove phis whilst removing CFG edges

[dmgreen]

Summary

D54021 adds some extra folding of constant terminators to LoopSimplifyCFG. This fixes up
the phi nodes of successor blocks to those constant folded.

[dmgreen]

lib/Transforms/Scalar/LoopSimplifyCFG.cpp

248

This can remove empty phi's I believe, which I'm not 100% sure about.

I think it's OK as-is but, Max, this may change with your followon patches. Feel free to commandeer if you think that's easier, otherwise let me know what you think.

[mkazantsev]

lib/Transforms/Scalar/LoopSimplifyCFG.cpp

248

Hi, I've done it in a bit more general way (using removePredecessor utility function) when re-enabled the patch after revert, see rL347289. The lack of Phis update caused crash on some lib compilation, it is now fixed and works.

Thanks for your effort, though. :)

[mkazantsev]

This thing is already fixed in a more general way in rL347289, I suggest to abandon this one. You can check in the test if you want, but we have similar tests in this patch.

[dmgreen]

Thanks for the fix!

lib/Transforms/Scalar/LoopSimplifyCFG.cpp

for (BasicBlock *BB : FoldCandidates) {
 
       LLVM_DEBUG(dbgs() << "Replacing terminator of " << BB->getName()
                         << " with an unconditional branch to the block "
                         << TheOnlySucc->getName() << "\n");
 
       SmallPtrSet<BasicBlock *, 2> DeadSuccessors;
       // Remove all BB's successors except for the live one.
       for (auto *Succ : successors(BB))
-        if (Succ != TheOnlySucc)
+        if (Succ != TheOnlySucc) {
           DeadSuccessors.insert(Succ);
+          for (PHINode &Phi : Succ->phis())
+            Phi.removeIncomingValue(BB);
+        }
 
       IRBuilder<> Builder(BB->getContext());
       Instruction *Term = BB->getTerminator();
       Builder.SetInsertPoint(Term);
       Builder.CreateBr(TheOnlySucc);
       Term->eraseFromParent();
 
       for (auto *DeadSucc : DeadSuccessors)

test/Transforms/LoopSimplifyCFG/fold-terminator-phis.ll

+; RUN: opt -loop-simplifycfg -S < %s | FileCheck %s
+
+; CHECK-LABEL: repo
+; CHECK: %inc = add i32 %i, 2
+define void @repo(i32 %size) {
+entry:
+  br label %loop
+
+loop:
+  %i = phi i32 [ 0, %entry ], [ %inc, %loop2 ]
+  br i1 true, label %loop1, label %loop2
+
+loop1:
+  call void @something()
+  br label %loop2
+
+loop2:
+  %phi = phi i32 [ 1, %loop ], [ 2, %loop1 ]
+  %inc = add i32 %i, %phi
+  %cmp = icmp ult i32 %inc, 3900
+  br i1 %cmp, label %loop, label %end
+
+end:
+  ret void
+}
+
+declare void @something()