Simplify recursive launder.invariant.group and strip

Summary:
This patch is crucial for proving equality laundered/stripped
pointers. eg:

  bool foo(A *a) {
    return a == std::launder(a);
  }

Clang with -fstrict-vtable-pointers will emit something like:

    define dso_local zeroext i1 @_Z3fooP1A(%struct.A* %a) {
    entry:
      %c = bitcast %struct.A* %a to i8*
      %call = tail call i8* @llvm.launder.invariant.group.p0i8(i8* %c)
      %0 = bitcast %struct.A* %a to i8*
      %1 = tail call i8* @llvm.strip.invariant.group.p0i8(i8* %0)
      %2 = tail call i8* @llvm.strip.invariant.group.p0i8(i8* %call)
      %cmp = icmp eq i8* %1, %2
      ret i1 %cmp
    }

and because %2 can be replaced with @llvm.strip.invariant.group(%0)
and that %2 and %1 will produce the same value (because strip is readnone)
we can replace compare with true.

Reviewers: rsmith, hfinkel, majnemer, amharc, kuhar

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D47423

llvm-svn: 336963

1 files changed, 39 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index cdf5746bb97..af79851ad36 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1266,6 +1266,40 @@ static Instruction *simplifyMaskedGather(IntrinsicInst &II, InstCombiner &IC) {
   return nullptr;
 }
 
+/// This function transforms launder.invariant.group and strip.invariant.group
+/// like:
+/// launder(launder(%x)) -> launder(%x)       (the result is not the argument)
+/// launder(strip(%x)) -> launder(%x)
+/// strip(strip(%x)) -> strip(%x)             (the result is not the argument)
+/// strip(launder(%x)) -> strip(%x)
+/// This is legal because it preserves the most recent information about
+/// the presence or absence of invariant.group.
+static Instruction *simplifyInvariantGroupIntrinsic(IntrinsicInst &II,
+                                                    InstCombiner &IC) {
+  auto *Arg = II.getArgOperand(0);
+  auto *StrippedArg = Arg->stripPointerCasts();
+  auto *StrippedInvariantGroupsArg = Arg->stripPointerCastsAndInvariantGroups();
+  if (StrippedArg == StrippedInvariantGroupsArg)
+    return nullptr; // No launders/strips to remove.
+
+  Value *Result = nullptr;
+
+  if (II.getIntrinsicID() == Intrinsic::launder_invariant_group)
+    Result = IC.Builder.CreateLaunderInvariantGroup(StrippedInvariantGroupsArg);
+  else if (II.getIntrinsicID() == Intrinsic::strip_invariant_group)
+    Result = IC.Builder.CreateStripInvariantGroup(StrippedInvariantGroupsArg);
+  else
+    llvm_unreachable(
+        "simplifyInvariantGroupIntrinsic only handles launder and strip");
+  if (Result->getType()->getPointerAddressSpace() !=
+      II.getType()->getPointerAddressSpace())
+    Result = IC.Builder.CreateAddrSpaceCast(Result, II.getType());
+  if (Result->getType() != II.getType())
+    Result = IC.Builder.CreateBitCast(Result, II.getType());
+
+  return cast<Instruction>(Result);
+}
+
 static Instruction *simplifyMaskedScatter(IntrinsicInst &II, InstCombiner &IC) {
   // If the mask is all zeros, a scatter does nothing.
   auto *ConstMask = dyn_cast<Constant>(II.getArgOperand(3));
@@ -1940,7 +1974,11 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
     return simplifyMaskedGather(*II, *this);
   case Intrinsic::masked_scatter:
     return simplifyMaskedScatter(*II, *this);
-
+  case Intrinsic::launder_invariant_group:
+  case Intrinsic::strip_invariant_group:
+    if (auto *SkippedBarrier = simplifyInvariantGroupIntrinsic(*II, *this))
+      return replaceInstUsesWith(*II, SkippedBarrier);
+    break;
   case Intrinsic::powi:
     if (ConstantInt *Power = dyn_cast<ConstantInt>(II->getArgOperand(1))) {
       // 0 and 1 are handled in instsimplify