[PATCH] D44666: [XRay] Lazily compute MachineLoopInfo instead of requiring it.

[Michael Zolotukhin via Phabricator via llvm-commits]

mzolotukhin created this revision.
mzolotukhin added reviewers: dberris, kubamracek.
Herald added a subscriber: hiraditya.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D44666

Files:
  llvm/lib/CodeGen/XRayInstrumentation.cpp
  llvm/test/CodeGen/X86/O0-pipeline.ll


Index: llvm/test/CodeGen/X86/O0-pipeline.ll
===================================================================
--- llvm/test/CodeGen/X86/O0-pipeline.ll
+++ llvm/test/CodeGen/X86/O0-pipeline.ll
@@ -55,8 +55,6 @@
 ; 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
Index: llvm/lib/CodeGen/XRayInstrumentation.cpp
===================================================================
--- llvm/lib/CodeGen/XRayInstrumentation.cpp
+++ llvm/lib/CodeGen/XRayInstrumentation.cpp
@@ -52,7 +52,6 @@
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
-    AU.addRequired<MachineLoopInfo>();
     AU.addPreserved<MachineLoopInfo>();
     AU.addPreserved<MachineDominatorTree>();
     MachineFunctionPass::getAnalysisUsage(AU);
@@ -160,11 +159,26 @@
     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.
   }
 


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