From 9481cf55aea0f853011ea977d9b336d61c2e51b4 Mon Sep 17 00:00:00 2001
From: Geoff Berry <gberry@codeaurora.org>
Date: Fri, 29 Apr 2016 17:18:28 +0000
Subject: [PATCH] [BasicAA] Treat llvm.assume as not accessing memory in
 getModRefBehavior(Function)

Reviewers: dberlin, chandlerc, hfinkel, reames, sanjoy

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19730

llvm-svn: 268068
---
 lib/Analysis/BasicAliasAnalysis.cpp      |  6 ++++++
 test/Transforms/Util/MemorySSA/assume.ll | 17 +++++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 test/Transforms/Util/MemorySSA/assume.ll

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 4c6f00132dc..14d92d633b9 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -575,6 +575,12 @@ FunctionModRefBehavior BasicAAResult::getModRefBehavior(const Function *F) {
   if (F->doesNotAccessMemory())
     return FMRB_DoesNotAccessMemory;
 
+  // While the assume intrinsic is marked as arbitrarily writing so that
+  // proper control dependencies will be maintained, it never aliases any
+  // particular memory location.
+  if (F->getIntrinsicID() == Intrinsic::assume)
+    return FMRB_DoesNotAccessMemory;
+
   FunctionModRefBehavior Min = FMRB_UnknownModRefBehavior;
 
   // If the function declares it only reads memory, go with that.
diff --git a/test/Transforms/Util/MemorySSA/assume.ll b/test/Transforms/Util/MemorySSA/assume.ll
new file mode 100644
index 00000000000..bdd10fc28b4
--- /dev/null
+++ b/test/Transforms/Util/MemorySSA/assume.ll
@@ -0,0 +1,17 @@
+; RUN: opt -basicaa -print-memoryssa -verify-memoryssa -analyze < %s 2>&1 | FileCheck %s
+;
+; Ensures that assumes are treated as not reading or writing memory.
+
+declare void @llvm.assume(i1)
+
+define i32 @foo(i32* %a, i32* %b, i1 %c) {
+; CHECK: 1 = MemoryDef(liveOnEntry)
+; CHECK-NEXT: store i32 4
+  store i32 4, i32* %a, align 4
+; CHECK: call void @llvm.assume
+  call void @llvm.assume(i1 %c)
+; CHECK: MemoryUse(1)
+; CHECK-NEXT: %1 = load i32
+  %1 = load i32, i32* %a, align 4
+  ret i32 %1
+}