[SimpleLoopUnswitch][NFC] Add option to always drop make.implicit metadata in non-trivial unswitching and save compile time

[mkazantsev]

Summary

We might want this if we find out that using of MustExecute analysis is too expensive.
By default we do the analysis because its complexity does not exceed the complexity
of whole loop copying in unswitching. Follow-up for D84925.

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

 static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
     "unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden,
     cl::desc("Number of unswitch candidates that are ignored when calculating "
              "cost multiplier."));
 static cl::opt<bool> UnswitchGuards(
     "simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
     cl::desc("If enabled, simple loop unswitching will also consider "
              "llvm.experimental.guard intrinsics as unswitch candidates."));
+static cl::opt<bool> DropNonTrivialImplicitNullChecks(
+    "simple-loop-unswitch-drop-non-trivial-implicit-null-checks",
+    cl::init(false), cl::Hidden,
+    cl::desc("If enabled, drop make.implicit metadata in unswitched implicit "
+             "null checks to save time analyzing if we can keep it."));
 
 /// Collect all of the loop invariant input values transitively used by the
 /// homogeneous instruction graph from a given root.
 ///
 /// This essentially walks from a root recursively through loop variant operands
 /// which have the exact same opcode and finds all inputs which are loop
 /// invariant. For some operations these can be re-associated and unswitched out
 /// of the loop entirely.
for (auto *SuccBB : UnswitchedSuccBBs) {
     ClonedPHs[SuccBB] = buildClonedLoopBlocks(
         L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
         DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU);
   }
 
   // Drop metadata if we may break its semantics by moving this instr into the
   // split block.
   if (TI.getMetadata(LLVMContext::MD_make_implicit)) {
-    // It is only legal to preserve make.implicit metadata if we are guaranteed
-    // to reach implicit null check block after following this branch.
+    if (DropNonTrivialImplicitNullChecks)
+      // Do not spend time trying to understand if we can keep it, just drop it
+      // to save compile time.
+      TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
+    else {
+      // It is only legal to preserve make.implicit metadata if we are
+      // guaranteed no reach implicit null check after following this branch.
       ICFLoopSafetyInfo SafetyInfo;
       SafetyInfo.computeLoopSafetyInfo(&L);
       if (!SafetyInfo.isGuaranteedToExecute(TI, &DT, &L))
         TI.setMetadata(LLVMContext::MD_make_implicit, nullptr);
     }
+  }
 
   // The stitching of the branched code back together depends on whether we're
   // doing full unswitching or not with the exception that we always want to
   // nuke the initial terminator placed in the split block.
   SplitBB->getTerminator()->eraseFromParent();
   if (FullUnswitch) {
     // Splice the terminator from the original loop and rewrite its
     // successors.