Add SimplifyTypeTests pass.

[pcc]

This pass figures out whether inlining has exposed a constant address to
a lowered type test, and remove the test if so and the address is known
to pass the test. Unfortunately this pass ends up needing to reverse
engineer what LowerTypeTests did; this is currently inherent to the design
of ThinLTO importing where LowerTypeTests needs to run at the start.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Peter Collingbourne (pcc)

Changes

This pass figures out whether inlining has exposed a constant address to
a lowered type test, and remove the test if so and the address is known
to pass the test. Unfortunately this pass ends up needing to reverse
engineer what LowerTypeTests did; this is currently inherent to the design
of ThinLTO importing where LowerTypeTests needs to run at the start.

TODO: Add tests.

---

Full diff: https://github.com/llvm/llvm-project/pull/141327.diff

4 Files Affected:

• (modified) llvm/include/llvm/Transforms/IPO/LowerTypeTests.h (+5)
• (modified) llvm/lib/Passes/PassBuilderPipelines.cpp (+2)
• (modified) llvm/lib/Passes/PassRegistry.def (+1)
• (modified) llvm/lib/Transforms/IPO/LowerTypeTests.cpp (+61)

diff --git a/llvm/include/llvm/Transforms/IPO/LowerTypeTests.h b/llvm/include/llvm/Transforms/IPO/LowerTypeTests.h
index 02adcd8bfd45d..40f2d2c4fadf7 100644
--- a/llvm/include/llvm/Transforms/IPO/LowerTypeTests.h
+++ b/llvm/include/llvm/Transforms/IPO/LowerTypeTests.h
@@ -223,6 +223,11 @@ class LowerTypeTestsPass : public PassInfoMixin<LowerTypeTestsPass> {
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
 };
 
+class SimplifyTypeTestsPass : public PassInfoMixin<SimplifyTypeTestsPass> {
+public:
+  PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
+};
+
 } // end namespace llvm
 
 #endif // LLVM_TRANSFORMS_IPO_LOWERTYPETESTS_H
diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index f3654600c5abb..e3adceed7d70a 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1464,6 +1464,8 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   if (!LTOPreLink)
     MPM.addPass(EliminateAvailableExternallyPass());
 
+  MPM.addPass(SimplifyTypeTestsPass());
+
   // Do RPO function attribute inference across the module to forward-propagate
   // attributes where applicable.
   // FIXME: Is this really an optimization rather than a canonicalization?
diff --git a/llvm/lib/Passes/PassRegistry.def b/llvm/lib/Passes/PassRegistry.def
index 94dabe290213d..35ad73738d71d 100644
--- a/llvm/lib/Passes/PassRegistry.def
+++ b/llvm/lib/Passes/PassRegistry.def
@@ -100,6 +100,7 @@ MODULE_PASS("jmc-instrumenter", JMCInstrumenterPass())
 MODULE_PASS("lower-emutls", LowerEmuTLSPass())
 MODULE_PASS("lower-global-dtors", LowerGlobalDtorsPass())
 MODULE_PASS("lower-ifunc", LowerIFuncPass())
+MODULE_PASS("simplify-type-tests", SimplifyTypeTestsPass())
 MODULE_PASS("lowertypetests", LowerTypeTestsPass())
 MODULE_PASS("fatlto-cleanup", FatLtoCleanup())
 MODULE_PASS("pgo-force-function-attrs", PGOForceFunctionAttrsPass(PGOOpt ? PGOOpt->ColdOptType : PGOOptions::ColdFuncOpt::Default))
diff --git a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
index 428c4641a7f56..344ac9f5a8f95 100644
--- a/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
+++ b/llvm/lib/Transforms/IPO/LowerTypeTests.cpp
@@ -2456,3 +2456,64 @@ PreservedAnalyses LowerTypeTestsPass::run(Module &M,
     return PreservedAnalyses::all();
   return PreservedAnalyses::none();
 }
+
+PreservedAnalyses SimplifyTypeTestsPass::run(Module &M,
+                                             ModuleAnalysisManager &AM) {
+  bool Changed = false;
+  // Figure out whether inlining has exposed a constant address to a lowered
+  // type test, and remove the test if so and the address is known to pass the
+  // test. Unfortunately this pass ends up needing to reverse engineer what
+  // LowerTypeTests did; this is currently inherent to the design of ThinLTO
+  // importing where LowerTypeTests needs to run at the start.
+  for (auto &GV : M.globals()) {
+    if (!GV.getName().starts_with("__typeid_") ||
+        !GV.getName().ends_with("_global_addr"))
+      continue;
+    auto *MD = MDString::get(M.getContext(),
+                             GV.getName().substr(9, GV.getName().size() - 21));
+    auto MaySimplify = [&](Value *Op) {
+      auto *PtrAsInt = dyn_cast<ConstantExpr>(Op);
+      if (!PtrAsInt || PtrAsInt->getOpcode() != Instruction::PtrToInt)
+        return false;
+      auto *Ptr = PtrAsInt->getOperand(0);
+      if (auto *GV = dyn_cast<GlobalValue>(Ptr))
+        if (auto *CFIGV = M.getNamedValue((GV->getName() + ".cfi").str()))
+          Ptr = CFIGV;
+      return isKnownTypeIdMember(MD, M.getDataLayout(), Ptr, 0);
+    };
+    for (User *U : GV.users()) {
+      auto *CE = dyn_cast<ConstantExpr>(U);
+      if (!CE || CE->getOpcode() != Instruction::PtrToInt)
+        continue;
+      for (Use &U : make_early_inc_range(CE->uses())) {
+        auto *CE = dyn_cast<ConstantExpr>(U.getUser());
+        if (U.getOperandNo() == 1 && CE &&
+            CE->getOpcode() == Instruction::Sub &&
+            MaySimplify(CE->getOperand(0))) {
+          // This is a computation of PtrOffset as generated by
+          // LowerTypeTestsModule::lowerTypeTestCall above. If
+          // isKnownTypeIdMember passes we just pretend it evaluated to 0. This
+          // should cause later passes to remove the range and alignment checks.
+          // The bitset checks won't be removed but those are uncommon.
+          CE->replaceAllUsesWith(ConstantInt::get(CE->getType(), 0));
+          Changed = true;
+        }
+        auto *CI = dyn_cast<ICmpInst>(U.getUser());
+        if (U.getOperandNo() == 1 && CI &&
+            CI->getPredicate() == CmpInst::ICMP_EQ &&
+            MaySimplify(CI->getOperand(0))) {
+          // This is an equality comparison (TypeTestResolution::Single case in
+          // lowerTypeTestCall). In this case we just replace the comparison
+          // with true.
+          CI->replaceAllUsesWith(ConstantInt::getTrue(M.getContext()));
+          CI->eraseFromParent();
+          Changed = true;
+        }
+      }
+    }
+  }
+
+  if (!Changed)
+    return PreservedAnalyses::all();
+  return PreservedAnalyses::none();
+}

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