Do not remove empty lifetime.start/lifetime.end ranges

Summary:
Asan stack-use-after-scope check should poison alloca even if there is
no access between start and end.

This is possible for code like this:
for (int i = 0; i < 3; i++) {
  int x;
  p = &x;
}

"Loop Invariant Code Motion" will move "p = &x;" out of the loop, making
start/end range empty.

PR27453

Reviewers: eugenis

Differential Revision: https://reviews.llvm.org/D22842

llvm-svn: 277072

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -2243,6 +2243,11 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
     break;
   }
   case Intrinsic::lifetime_start:
+    // Asan needs to poison memory to detect invalid access which is possible
+    // even for empty lifetime range.
+    if (II->getFunction()->hasFnAttribute(Attribute::SanitizeAddress))
+      break;
+
     if (removeTriviallyEmptyRange(*II, Intrinsic::lifetime_start,
                                   Intrinsic::lifetime_end, *this))
       return nullptr;

llvm/test/Transforms/InstCombine/lifetime-asan.ll

@@ -0,0 +1,35 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+declare void @llvm.lifetime.start(i64, i8* nocapture)
+declare void @llvm.lifetime.end(i64, i8* nocapture)
+declare void @foo(i8* nocapture)
+
+define void @asan() sanitize_address {
+entry:
+  ; CHECK-LABEL: @asan(
+  %text = alloca i8, align 1
+
+  call void @llvm.lifetime.start(i64 1, i8* %text)
+  call void @llvm.lifetime.end(i64 1, i8* %text)
+  ; CHECK: call void @llvm.lifetime.start
+  ; CHECK-NEXT: call void @llvm.lifetime.end
+
+  call void @foo(i8* %text) ; Keep alloca alive
+
+  ret void
+}
+
+
+define void @no_asan() {
+entry:
+  ; CHECK-LABEL: @no_asan(
+  %text = alloca i8, align 1
+
+  call void @llvm.lifetime.start(i64 1, i8* %text)
+  call void @llvm.lifetime.end(i64 1, i8* %text)
+  ; CHECK-NO: call void @llvm.lifetime
+
+  call void @foo(i8* %text) ; Keep alloca alive
+
+  ret void
+}