[Attributor] Simple casts preserve no-alias property

This is a minimal but important advancement over the existing code. A cast with an operand that is only used in the cast retains the no-alias property of the operand.
llvm · Feb 10, 2020 · 87ddf1f · 87ddf1f
1 parent 1c0ebcc
commit 87ddf1f
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Showing 2 changed files with 53 additions and 2 deletions.
diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -2496,13 +2496,29 @@ struct AANoAliasFloating final : AANoAliasImpl {
   /// See AbstractAttribute::initialize(...).
   void initialize(Attributor &A) override {
     AANoAliasImpl::initialize(A);
-    Value &Val = getAssociatedValue();
+    Value *Val = &getAssociatedValue();
+    do {
+      CastInst *CI = dyn_cast<CastInst>(Val);
+      if (!CI)
+        break;
+      Value *Base = CI->getOperand(0);
+      if (Base->getNumUses() != 1)
+        break;
+      Val = Base;
+    } while (true);
+
     if (isa<AllocaInst>(Val))
       indicateOptimisticFixpoint();
     else if (isa<ConstantPointerNull>(Val) &&
              !NullPointerIsDefined(getAnchorScope(),
-                                   Val.getType()->getPointerAddressSpace()))
+                                   Val->getType()->getPointerAddressSpace()))
       indicateOptimisticFixpoint();
+    else if (Val != &getAssociatedValue()) {
+      const auto &ValNoAliasAA =
+          A.getAAFor<AANoAlias>(*this, IRPosition::value(*Val));
+      if (ValNoAliasAA.isKnownNoAlias())
+        indicateOptimisticFixpoint();
+    }
   }
 
   /// See AbstractAttribute::updateImpl(...).

diff --git a/llvm/test/Transforms/Attributor/noalias.ll b/llvm/test/Transforms/Attributor/noalias.ll
@@ -40,6 +40,13 @@ define i8* @return_noalias_looks_like_capture(){
   ret i8* %1
 }
 
+; CHECK: define noalias i16* @return_noalias_casted()
+define i16* @return_noalias_casted(){
+  %1 = tail call noalias i8* @malloc(i64 4)
+  %c = bitcast i8* %1 to i16*
+  ret i16* %c
+}
+
 declare i8* @alias()
 
 ; TEST 3
@@ -294,3 +301,31 @@ define void @test12_4(){
   tail call void @two_args(i8* %A_0, i8* %B_0)
   ret void
 }
+
+; TEST 13
+define void @use_i8_internal(i8* %a) {
+  call void @use_i8(i8* %a)
+  ret void
+}
+
+define void @test13_use_noalias(){
+  %m1 = tail call noalias i8* @malloc(i64 4)
+  %c1 = bitcast i8* %m1 to i16*
+  %c2 = bitcast i16* %c1 to i8*
+; CHECK: call void @use_i8_internal(i8* noalias nocapture %c2)
+  call void @use_i8_internal(i8* %c2)
+  ret void
+}
+
+define void @test13_use_alias(){
+  %m1 = tail call noalias i8* @malloc(i64 4)
+  %c1 = bitcast i8* %m1 to i16*
+  %c2a = bitcast i16* %c1 to i8*
+  %c2b = bitcast i16* %c1 to i8*
+; CHECK: call void @use_i8_internal(i8* nocapture %c2a)
+; CHECK: call void @use_i8_internal(i8* nocapture %c2b)
+  call void @use_i8_internal(i8* %c2a)
+  call void @use_i8_internal(i8* %c2b)
+  ret void
+}
+