[llvm] r374280 - [Attributor] Handle `null` differently in capture and alias logic

[Johannes Doerfert via llvm-commits]

Author: jdoerfert
Date: Wed Oct  9 22:33:21 2019
New Revision: 374280

URL: http://llvm.org/viewvc/llvm-project?rev=374280&view=rev
Log:
[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

Modified:
    llvm/trunk/lib/Transforms/IPO/Attributor.cpp
    llvm/trunk/test/Transforms/FunctionAttrs/callbacks.ll
    llvm/trunk/test/Transforms/FunctionAttrs/nocapture.ll

Modified: llvm/trunk/lib/Transforms/IPO/Attributor.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/Attributor.cpp?rev=374280&r1=374279&r2=374280&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/IPO/Attributor.cpp (original)
+++ llvm/trunk/lib/Transforms/IPO/Attributor.cpp Wed Oct  9 22:33:21 2019
@@ -1907,7 +1907,11 @@ struct AANoAliasFloating final : AANoAli
   /// 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 :
     //             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 :
 
       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 AANoCapt
   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();

Modified: llvm/trunk/test/Transforms/FunctionAttrs/callbacks.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionAttrs/callbacks.ll?rev=374280&r1=374279&r2=374280&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/FunctionAttrs/callbacks.ll (original)
+++ llvm/trunk/test/Transforms/FunctionAttrs/callbacks.ll Wed Oct  9 22:33:21 2019
@@ -24,7 +24,7 @@ define void @t0_caller(i32* %a) {
 ; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i32* [[B]] to i8*
 ; CHECK-NEXT:    store i32 42, i32* [[B]], align 32
 ; CHECK-NEXT:    store i32* [[B]], i32** [[C]], align 64
-; CHECK-NEXT:    call void (i32*, i32*, void (i32*, i32*, ...)*, ...) @t0_callback_broker(i32* null, i32* nonnull align 128 dereferenceable(4) [[PTR]], void (i32*, i32*, ...)* nonnull bitcast (void (i32*, i32*, i32*, i64, i32**)* @t0_callback_callee to void (i32*, i32*, ...)*), i32* [[A:%.*]], i64 99, i32** nonnull align 64 dereferenceable(8) [[C]])
+; CHECK-NEXT:    call void (i32*, i32*, void (i32*, i32*, ...)*, ...) @t0_callback_broker(i32* noalias null, i32* nonnull align 128 dereferenceable(4) [[PTR]], void (i32*, i32*, ...)* nonnull bitcast (void (i32*, i32*, i32*, i64, i32**)* @t0_callback_callee to void (i32*, i32*, ...)*), i32* [[A:%.*]], i64 99, i32** nonnull align 64 dereferenceable(8) [[C]])
 ; CHECK-NEXT:    ret void
 ;
 entry:

Modified: llvm/trunk/test/Transforms/FunctionAttrs/nocapture.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/FunctionAttrs/nocapture.ll?rev=374280&r1=374279&r2=374280&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/FunctionAttrs/nocapture.ll (original)
+++ llvm/trunk/test/Transforms/FunctionAttrs/nocapture.ll Wed Oct  9 22:33:21 2019
@@ -320,5 +320,14 @@ define i1 @captureDereferenceableOrNullI
   ret i1 %2
 }
 
+declare void @unknown(i8*)
+define void @test_callsite() {
+entry:
+; We know that 'null' in AS 0 does not alias anything and cannot be captured
+; CHECK: call void @unknown(i8* noalias nocapture null)
+  call void @unknown(i8* null)
+  ret void
+}
+
 declare i8* @llvm.launder.invariant.group.p0i8(i8*)
 declare i8* @llvm.strip.invariant.group.p0i8(i8*)