[Lint] Permit aliasing noalias readonly arguments

[cuviper]

Summary

If two arguments are both readonly, then they have no memory dependency
that would violate noalias, even if they do actually overlap.

[cuviper]

Ping for review, please.

[efriedma]

LGTM. It seems easy to trigger false positives with this lint, so I'm not sure how useful it is in general, but this is clearly an improvement.

llvm/lib/Analysis/Lint.cpp

for (; AI != AE; ++AI) {
                &I);
 
         // Check that noalias arguments don't alias other arguments. This is
         // not fully precise because we don't know the sizes of the dereferenced
         // memory regions.
         if (Formal->hasNoAliasAttr() && Actual->getType()->isPointerTy()) {
           AttributeList PAL = CS.getAttributes();
           unsigned ArgNo = 0;
-          for (CallSite::arg_iterator BI = CS.arg_begin(); BI != AE; ++BI) {
+          for (CallSite::arg_iterator BI = CS.arg_begin(); BI != AE;
+               ++BI, ++ArgNo) {
             // Skip ByVal arguments since they will be memcpy'd to the callee's
             // stack so we're not really passing the pointer anyway.
-            if (PAL.hasParamAttribute(ArgNo++, Attribute::ByVal))
+            if (PAL.hasParamAttribute(ArgNo, Attribute::ByVal))
+              continue;
+            // If both arguments are readonly, they have no dependence.
+            if (Formal->onlyReadsMemory() && CS.onlyReadsMemory(ArgNo))
               continue;
             if (AI != BI && (*BI)->getType()->isPointerTy()) {
               AliasResult Result = AA->alias(*AI, *BI);
               Assert(Result != MustAlias && Result != PartialAlias,
                      "Unusual: noalias argument aliases another argument", &I);
             }
           }
         }

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

+; RUN: opt < %s -lint -disable-output 2>&1 | FileCheck %s
+
+declare void @f1(i8* noalias readonly, i8*)
+
+define void @f2(i8* %a) {
+entry:
+  call void @f1(i8* %a, i8* %a)
+  ret void
+}
+
+; Lint should complain about us passing %a to both arguments, since the noalias
+; argument may depend on writes to the other.
+; CHECK: Unusual: noalias argument aliases another argument
+; CHECK-NEXT: call void @f1(i8* %a, i8* %a)
+
+declare void @f3(i8* noalias, i8* readonly)
+
+define void @f4(i8* %a) {
+entry:
+  call void @f3(i8* %a, i8* %a)
+  ret void
+}
+
+; Lint should complain about us passing %a to both arguments, since writes to
+; the noalias argument may cause a dependency for the other.
+; CHECK: Unusual: noalias argument aliases another argument
+; CHECK-NEXT: call void @f3(i8* %a, i8* %a)
+
+declare void @f5(i8* noalias readonly, i8* readonly)
+
+define void @f6(i8* %a) {
+entry:
+  call void @f5(i8* %a, i8* %a)
+  ret void
+}
+
+; Lint should not complain about passing %a to both arguments even if one is
+; noalias, since they are both readonly and thus have no dependence.
+; CHECK-NOT: Unusual: noalias argument aliases another argument
+; CHECK-NOT: call void @f5(i8* %a, i8* %a)