[llvm-branch-commits] [llvm] Add SimplifyTypeTests pass. (PR #141327)

[Florian Mayer via llvm-branch-commits]

================
@@ -2478,3 +2479,76 @@ 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 MaySimplifyPtr = [&](Value *Ptr) {
+      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);
+    };
+    auto MaySimplifyInt = [&](Value *Op) {
+      auto *PtrAsInt = dyn_cast<ConstantExpr>(Op);
+      if (!PtrAsInt || PtrAsInt->getOpcode() != Instruction::PtrToInt)
+        return false;
+      return MaySimplifyPtr(PtrAsInt->getOperand(0));
+    };
+    for (User *U : make_early_inc_range(GV.users())) {
+      if (auto *CI = dyn_cast<ICmpInst>(U)) {
+        if (CI->getPredicate() == CmpInst::ICMP_EQ &&
+            MaySimplifyPtr(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;
+        }
+      }
+      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 &&
+            MaySimplifyInt(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 &&
+            MaySimplifyInt(CI->getOperand(0))) {
+          // This is an equality comparison. Unlike in the case above it
+          // remained as an integer compare.
+          CI->replaceAllUsesWith(ConstantInt::getTrue(M.getContext()));
+          CI->eraseFromParent();
+          Changed = true;
+        }
+      }
+    }
+  }
+
+  if (!Changed)
+    return PreservedAnalyses::all();
+  return PreservedAnalyses::none();
----------------
fmayer wrote:

Is that true? E.g. the CFG doesn't change, right? Could we do what we do in HWASan: https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L485?

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