From 07bbdf45645b1b87713931d851c7547a47d834b7 Mon Sep 17 00:00:00 2001
From: Josh Stone <jistone@redhat.com>
Date: Tue, 23 Apr 2019 23:43:47 +0000
Subject: [PATCH] [Lint] Permit aliasing noalias readonly arguments

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

Reviewers: hfinkel, efriedma

Reviewed By: efriedma

Subscribers: efriedma, hiraditya, llvm-commits, tstellar

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60239

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@359047 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/Lint.cpp                  |  8 ++++--
 test/Analysis/Lint/noalias-readonly.ll | 40 ++++++++++++++++++++++++++
 2 files changed, 46 insertions(+), 2 deletions(-)
 create mode 100644 test/Analysis/Lint/noalias-readonly.ll

diff --git a/lib/Analysis/Lint.cpp b/lib/Analysis/Lint.cpp
index 37ce56d268b..d28b8a189d4 100644
--- a/lib/Analysis/Lint.cpp
+++ b/lib/Analysis/Lint.cpp
@@ -267,10 +267,14 @@ void Lint::visitCallSite(CallSite CS) {
         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);
diff --git a/test/Analysis/Lint/noalias-readonly.ll b/test/Analysis/Lint/noalias-readonly.ll
new file mode 100644
index 00000000000..29b288a9bba
--- /dev/null
+++ b/test/Analysis/Lint/noalias-readonly.ll
@@ -0,0 +1,40 @@
+; 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)
-- 
2.50.1