[Lint] Permit aliasing noalias and readnone arguments

If an argument is readnone we know that it isn't dereferenced. Then
it should be OK if that argument alias with a noalias argument.

Differential Revision: https://reviews.llvm.org/D157737

llvm/lib/Analysis/Lint.cpp

@@ -235,6 +235,10 @@ void Lint::visitCallBase(CallBase &I) {
             // If both arguments are readonly, they have no dependence.
             if (Formal->onlyReadsMemory() && I.onlyReadsMemory(ArgNo))
               continue;
+            // Skip readnone arguments since those are guaranteed not to be
+            // dereferenced anyway.
+            if (I.doesNotAccessMemory(ArgNo))
+              continue;
             if (AI != BI && (*BI)->getType()->isPointerTy()) {
               AliasResult Result = AA->alias(*AI, *BI);
               Check(Result != AliasResult::MustAlias &&

llvm/test/Analysis/Lint/noalias-byval.ll

@@ -8,7 +8,7 @@ declare void @llvm.memcpy.p0.p0.i32(ptr nocapture writeonly, ptr nocapture reado
 ; Function Attrs: argmemonly nounwind
 declare void @llvm.memset.p0.i32(ptr nocapture writeonly, i8, i32, i1) #0
 
-declare void @f1(ptr noalias nocapture sret(%s), ptr nocapture readnone)
+declare void @f1(ptr noalias nocapture sret(%s), ptr nocapture)
 
 define void @f2() {
 entry:

llvm/test/Analysis/Lint/noalias-readnone.ll

@@ -0,0 +1,14 @@
+; RUN: opt < %s -passes=lint -disable-output 2>&1 | FileCheck --allow-empty %s
+
+declare void @foo1(ptr noalias, ptr readnone)
+
+define void @test1(ptr %a) {
+entry:
+  call void @foo1(ptr %a, ptr %a)
+  ret void
+}
+
+; Lint should not complain about passing %a to both arguments even if one is
+; noalias, since the second argument is readnone.
+; CHECK-NOT: Unusual: noalias argument aliases another argument
+; CHECK-NOT: call void @foo1(ptr %a, ptr %a)