[XRay] Lazily compute MachineLoopInfo instead of requiring it.

[mzolotukhin]

Summary

Currently X-Ray Instrumentation pass has a dependency on MachineLoopInfo
(and thus on MachineDominatorTree as well) and we have to compute them
even if X-Ray is not used. This patch changes it to a lazy computation
to save compile time by avoiding these redundant computations.

[dberris]

LGTM -- thank you!

[mzolotukhin]

Thanks!

llvm/trunk/lib/CodeGen/XRayInstrumentation.cpp

struct XRayInstrumentation : public MachineFunctionPass {
   static char ID;
 
   XRayInstrumentation() : MachineFunctionPass(ID) {
     initializeXRayInstrumentationPass(*PassRegistry::getPassRegistry());
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
-    AU.addRequired<MachineLoopInfo>();
     AU.addPreserved<MachineLoopInfo>();
     AU.addPreserved<MachineDominatorTree>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
 private:
if (!AlwaysInstrument) {
     if (Attr.getValueAsString().getAsInteger(10, XRayThreshold))
       return false; // Invalid value for threshold.
 
     // Count the number of MachineInstr`s in MachineFunction
     int64_t MICount = 0;
     for (const auto &MBB : MF)
       MICount += MBB.size();
 
+    // Get MachineDominatorTree or compute it on the fly if it's unavailable
+    auto *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
+    MachineDominatorTree ComputedMDT;
+    if (!MDT) {
+      ComputedMDT.getBase().recalculate(MF);
+      MDT = &ComputedMDT;
+    }
+
+    // Get MachineLoopInfo or compute it on the fly if it's unavailable
+    auto *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
+    MachineLoopInfo ComputedMLI;
+    if (!MLI) {
+      ComputedMLI.getBase().analyze(MDT->getBase());
+      MLI = &ComputedMLI;
+    }
+
     // Check if we have a loop.
     // FIXME: Maybe make this smarter, and see whether the loops are dependent
     // on inputs or side-effects?
-    MachineLoopInfo &MLI = getAnalysis<MachineLoopInfo>();
-    if (MLI.empty() && MICount < XRayThreshold)
+    if (MLI->empty() && MICount < XRayThreshold)
       return false; // Function is too small and has no loops.
   }
 
   // We look for the first non-empty MachineBasicBlock, so that we can insert
   // the function instrumentation in the appropriate place.
   auto MBI = llvm::find_if(
       MF, [&](const MachineBasicBlock &MBB) { return !MBB.empty(); });
   if (MBI == MF.end())

llvm/trunk/test/CodeGen/X86/O0-pipeline.ll

 ; CHECK-NEXT:       X86 pseudo instruction expansion pass
 ; CHECK-NEXT:       Analyze Machine Code For Garbage Collection
 ; CHECK-NEXT:       X86 Indirect Branch Tracking
 ; CHECK-NEXT:       X86 vzeroupper inserter
 ; CHECK-NEXT:       Contiguously Lay Out Funclets
 ; CHECK-NEXT:       StackMap Liveness Analysis
 ; CHECK-NEXT:       Live DEBUG_VALUE analysis
 ; CHECK-NEXT:       Insert fentry calls
-; CHECK-NEXT:       MachineDominator Tree Construction
-; CHECK-NEXT:       Machine Natural Loop Construction
 ; CHECK-NEXT:       Insert XRay ops
 ; CHECK-NEXT:       Implement the 'patchable-function' attribute
 ; CHECK-NEXT:       X86 Retpoline Thunks
 ; CHECK-NEXT:       Lazy Machine Block Frequency Analysis
 ; CHECK-NEXT:       Machine Optimization Remark Emitter
 ; CHECK-NEXT:       MachineDominator Tree Construction
 ; CHECK-NEXT:       Machine Natural Loop Construction
 ; CHECK-NEXT:       X86 Assembly Printer
 ; CHECK-NEXT:       Free MachineFunction
 
 define void @f() {
   ret void
 }