[Attributor] Handle `null` differently in capture and alias logic

Summary:
`null` in the default address space (=AS 0) cannot be captured nor can
it alias anything. We make this clear now as it can be important for
callbacks and other cases later on. In addition, this patch improves the
debug output for noalias deduction.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68624

llvm-svn: 374280

1 files changed, 21 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
index 4943721c469..cf45c23a6b8 100644
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -1907,7 +1907,11 @@ struct AANoAliasFloating final : AANoAliasImpl {
   /// See AbstractAttribute::initialize(...).
   void initialize(Attributor &A) override {
     AANoAliasImpl::initialize(A);
-    if (isa<AllocaInst>(getAnchorValue()))
+    Value &Val = getAssociatedValue();
+    if (isa<AllocaInst>(Val))
+      indicateOptimisticFixpoint();
+    if (isa<ConstantPointerNull>(Val) &&
+        Val.getType()->getPointerAddressSpace() == 0)
       indicateOptimisticFixpoint();
   }
 
@@ -1971,8 +1975,12 @@ struct AANoAliasCallSiteArgument final : AANoAliasImpl {
     //             check only uses possibly executed before this callsite.
 
     auto &NoCaptureAA = A.getAAFor<AANoCapture>(*this, IRP);
-    if (!NoCaptureAA.isAssumedNoCaptureMaybeReturned())
+    if (!NoCaptureAA.isAssumedNoCaptureMaybeReturned()) {
+      LLVM_DEBUG(
+          dbgs() << "[Attributor][AANoAliasCSArg] " << V
+                 << " cannot be noalias as it is potentially captured\n");
       return indicatePessimisticFixpoint();
+    }
 
     // (iii) Check there is no other pointer argument which could alias with the
     // value.
@@ -1986,13 +1994,15 @@ struct AANoAliasCallSiteArgument final : AANoAliasImpl {
 
       if (const Function *F = getAnchorScope()) {
         if (AAResults *AAR = A.getInfoCache().getAAResultsForFunction(*F)) {
+          bool IsAliasing = AAR->isNoAlias(&getAssociatedValue(), ArgOp);
           LLVM_DEBUG(dbgs()
                      << "[Attributor][NoAliasCSArg] Check alias between "
                         "callsite arguments "
                      << AAR->isNoAlias(&getAssociatedValue(), ArgOp) << " "
-                     << getAssociatedValue() << " " << *ArgOp << "\n");
+                     << getAssociatedValue() << " " << *ArgOp << " => "
+                     << (IsAliasing ? "" : "no-") << "alias \n");
 
-          if (AAR->isNoAlias(&getAssociatedValue(), ArgOp))
+          if (IsAliasing)
             continue;
         }
       }
@@ -2881,6 +2891,13 @@ struct AANoCaptureImpl : public AANoCapture {
   void initialize(Attributor &A) override {
     AANoCapture::initialize(A);
 
+    // You cannot "capture" null in the default address space.
+    if (isa<ConstantPointerNull>(getAssociatedValue()) &&
+        getAssociatedValue().getType()->getPointerAddressSpace() == 0) {
+      indicateOptimisticFixpoint();
+      return;
+    }
+
     const IRPosition &IRP = getIRPosition();
     const Function *F =
         getArgNo() >= 0 ? IRP.getAssociatedFunction() : IRP.getAnchorScope();