[LoopUnroll] Don't verify domtree by default with +Asserts.

[efriedma]

Summary

domtree verification is linear in the size of the function, so it can cause a quadratic compile-time explosion in a function with many loops to unroll.

[davide]

LGTM. Given this is under EXPENSIVE_CHECK, we might consider a more thorough verification?

[kuhar]

lib/Transforms/Utils/LoopUnrollPeel.cpp

617 ↗

(On Diff #174699)

It would be interesting to see if replacing this with the DomTreeUpdater affects performance much.

[NutshellySima]

LGTM.

llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp

 static cl::opt<bool>
 UnrollRuntimeEpilog("unroll-runtime-epilog", cl::init(false), cl::Hidden,
                     cl::desc("Allow runtime unrolled loops to be unrolled "
                              "with epilog instead of prolog."));
 
 static cl::opt<bool>
 UnrollVerifyDomtree("unroll-verify-domtree", cl::Hidden,
                     cl::desc("Verify domtree after unrolling"),
-#ifdef NDEBUG
-    cl::init(false)
-#else
+#ifdef EXPENSIVE_CHECKS
     cl::init(true)
+#else
+    cl::init(false)
 #endif
                     );
 
 /// Convert the instruction operands from referencing the current values into
 /// those specified by VMap.
 void llvm::remapInstruction(Instruction *I, ValueToValueMapTy &VMap) {
   for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
     Value *Op = I->getOperand(op);

llvm/trunk/lib/Transforms/Utils/LoopUnrollPeel.cpp

for (unsigned Iter = 0; Iter < PeelCount; ++Iter) {
     remapInstructionsInBlocks(NewBlocks, VMap);
 
     if (DT) {
       // Latches of the cloned loops dominate over the loop exit, so idom of the
       // latter is the first cloned loop body, as original PreHeader dominates
       // the original loop body.
       if (Iter == 0)
         DT->changeImmediateDominator(Exit, cast<BasicBlock>(LVMap[Latch]));
+#ifdef EXPENSIVE_CHECKS
       assert(DT->verify(DominatorTree::VerificationLevel::Fast));
+#endif
     }
 
     auto *LatchBRCopy = cast<BranchInst>(VMap[LatchBR]);
     updateBranchWeights(InsertBot, LatchBRCopy, Iter,
                         PeelCount, ExitWeight);
     // Remove Loop metadata from the latch branch instruction
     // because it is not the Loop's latch branch anymore.
     LatchBRCopy->setMetadata(LLVMContext::MD_loop, nullptr);