[PATCH] D31302: [CodeGen] Compute DT/LI lazily in SafeStackLegacyPass. NFC.

[Ahmed Bougacha via Phabricator via llvm-commits]

ab created this revision.
Herald added a subscriber: sanjoy.

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.

This updates a test I added in my local branch, that checks the entire O0 llc x86 pipeline.  It's a little overzealous, but hopefully that makes changes to the O0 pipeline obvious; it's easy to require expensive passes, and this helps make informed decisions.


https://reviews.llvm.org/D31302

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


Index: test/CodeGen/X86/O0-pipeline.ll
===================================================================
--- test/CodeGen/X86/O0-pipeline.ll
+++ test/CodeGen/X86/O0-pipeline.ll
@@ -27,9 +27,6 @@
 ; CHECK-NEXT:     FunctionPass Manager
 ; CHECK-NEXT:       Dominator Tree Construction
 ; CHECK-NEXT:       Exception handling preparation
-; CHECK-NEXT:       Dominator Tree Construction
-; CHECK-NEXT:       Natural Loop Information
-; CHECK-NEXT:       Scalar Evolution Analysis
 ; CHECK-NEXT:       Safe Stack instrumentation pass
 ; CHECK-NEXT:       Insert stack protectors
 ; CHECK-NEXT:       Module Verifier
Index: lib/CodeGen/SafeStack.cpp
===================================================================
--- lib/CodeGen/SafeStack.cpp
+++ lib/CodeGen/SafeStack.cpp
@@ -774,7 +774,8 @@
   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 @@
       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();
   }


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