From cea4f833717e3a05975478d2a7e0b684cf1026fb Mon Sep 17 00:00:00 2001
From: Alexander Potapenko <glider@google.com>
Date: Mon, 31 Dec 2018 09:42:23 +0000
Subject: [PATCH] [MSan] Handle llvm.is.constant intrinsic

MSan used to report false positives in the case the argument of
llvm.is.constant intrinsic was uninitialized.
In fact checking this argument is unnecessary, as the intrinsic is only
used at compile time, and its value doesn't depend on the value of the
argument.

llvm-svn: 350173
---
 .../Instrumentation/MemorySanitizer.cpp       |  6 ++++++
 .../MemorySanitizer/msan_llvm_is_constant.ll  | 21 +++++++++++++++++++
 2 files changed, 27 insertions(+)
 create mode 100644 llvm/test/Instrumentation/MemorySanitizer/msan_llvm_is_constant.ll

diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index 1fad7fc6381c..1bac44c38019 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -3099,6 +3099,12 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       handleVectorComparePackedIntrinsic(I);
       break;
 
+    case Intrinsic::is_constant:
+      // The result of llvm.is.constant() is always defined.
+      setShadow(&I, getCleanShadow(&I));
+      setOrigin(&I, getCleanOrigin());
+      break;
+
     default:
       if (!handleUnknownIntrinsic(I))
         visitInstruction(I);
diff --git a/llvm/test/Instrumentation/MemorySanitizer/msan_llvm_is_constant.ll b/llvm/test/Instrumentation/MemorySanitizer/msan_llvm_is_constant.ll
new file mode 100644
index 000000000000..b7847db06ac2
--- /dev/null
+++ b/llvm/test/Instrumentation/MemorySanitizer/msan_llvm_is_constant.ll
@@ -0,0 +1,21 @@
+; Make sure MSan doesn't insert shadow checks for @llvm.is.constant.* arguments.
+
+; RUN: opt < %s -msan -msan-kernel=1 -S | FileCheck -check-prefixes=CHECK %s
+; RUN: opt < %s -msan -S | FileCheck -check-prefixes=CHECK %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: nounwind readnone uwtable
+define dso_local i32 @bar(i32 %v) local_unnamed_addr sanitize_memory {
+entry:
+  %0 = tail call i1 @llvm.is.constant.i32(i32 %v)
+  %1 = zext i1 %0 to i32
+  ret i32 %1
+}
+
+; CHECK-LABEL: bar
+; CHECK-NOT: call void @__msan_warning
+
+; Function Attrs: nounwind readnone
+declare i1 @llvm.is.constant.i32(i32)