[CodeGen] Compute DT/LI lazily in SafeStackLegacyPass. NFC.

We currently require SCEV, which requires DT/LI.  Those are expensive to
compute, but the pass only runs for functions that have the safestack
attribute.

Compute DT/LI to build SCEV lazily, only when the pass is actually going
to transform the function.

Differential Revision: https://reviews.llvm.org/D31302

llvm-svn: 302610

1 files changed, 15 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SafeStack.cpp b/llvm/lib/CodeGen/SafeStack.cpp
index f0663ebde82..08b3d345f68 100644
--- a/llvm/lib/CodeGen/SafeStack.cpp
+++ b/llvm/lib/CodeGen/SafeStack.cpp
@@ -774,7 +774,8 @@ public:
   SafeStackLegacyPass() : SafeStackLegacyPass(nullptr) {}
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
-    AU.addRequired<ScalarEvolutionWrapperPass>();
+    AU.addRequired<TargetLibraryInfoWrapperPass>();
+    AU.addRequired<AssumptionCacheTracker>();
   }
 
   bool runOnFunction(Function &F) override {
@@ -799,7 +800,19 @@ public:
       report_fatal_error("TargetLowering instance is required");
 
     auto *DL = &F.getParent()->getDataLayout();
-    auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+    auto &TLI = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
+    auto &ACT = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
+
+    // Compute DT and LI only for functions that have the attribute.
+    // This is only useful because the legacy pass manager doesn't let us
+    // compute analyzes lazily.
+    // In the backend pipeline, nothing preserves DT before SafeStack, so we
+    // would otherwise always compute it wastefully, even if there is no
+    // function with the safestack attribute.
+    DominatorTree DT(F);
+    LoopInfo LI(DT);
+
+    ScalarEvolution SE(F, TLI, ACT, DT, LI);
 
     return SafeStack(F, *TL, *DL, SE).run();
   }