r300508 - [ubsan] Skip null checks on pointers to the start of an alloca

[Vedant Kumar via cfe-commits]

Author: vedantk
Date: Mon Apr 17 17:26:07 2017
New Revision: 300508

URL: http://llvm.org/viewvc/llvm-project?rev=300508&view=rev
Log:
[ubsan] Skip null checks on pointers to the start of an alloca

Pointers to the start of an alloca are non-null, so we don't need to
emit runtime null checks for them.

Testing: check-clang, check-ubsan.

This significantly reduces the amount of null checks we emit when
compiling X86ISelLowering.cpp. Here are the numbers from patched /
unpatched clangs based on r300371.

  -------------------------------------
  | Setup          | # of null checks |
  -------------------------------------
  | unpatched, -O0 |            45439 |
  | patched, -O0   |            25251 | (-44.4%)
  -------------------------------------

Modified:
    cfe/trunk/lib/CodeGen/CGExpr.cpp
    cfe/trunk/test/CodeGenCXX/ubsan-suppress-checks.cpp

Modified: cfe/trunk/lib/CodeGen/CGExpr.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGExpr.cpp?rev=300508&r1=300507&r2=300508&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGExpr.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGExpr.cpp Mon Apr 17 17:26:07 2017
@@ -533,6 +533,15 @@ bool CodeGenFunction::sanitizePerformTyp
          SanOpts.has(SanitizerKind::Vptr);
 }
 
+/// Check if a runtime null check for \p Ptr can be omitted.
+static bool canOmitPointerNullCheck(llvm::Value *Ptr) {
+  // Note: do not perform any constant-folding in this function. That is best
+  // left to the IR builder.
+
+  // Pointers to alloca'd memory are non-null.
+  return isa<llvm::AllocaInst>(Ptr->stripPointerCastsNoFollowAliases());
+}
+
 void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
                                     llvm::Value *Ptr, QualType Ty,
                                     CharUnits Alignment,
@@ -554,7 +563,8 @@ void CodeGenFunction::EmitTypeCheck(Type
   bool AllowNullPointers = TCK == TCK_DowncastPointer || TCK == TCK_Upcast ||
                            TCK == TCK_UpcastToVirtualBase;
   if ((SanOpts.has(SanitizerKind::Null) || AllowNullPointers) &&
-      !SkippedChecks.has(SanitizerKind::Null)) {
+      !SkippedChecks.has(SanitizerKind::Null) &&
+      !canOmitPointerNullCheck(Ptr)) {
     // The glvalue must not be an empty glvalue.
     llvm::Value *IsNonNull = Builder.CreateIsNotNull(Ptr);
 

Modified: cfe/trunk/test/CodeGenCXX/ubsan-suppress-checks.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/ubsan-suppress-checks.cpp?rev=300508&r1=300507&r2=300508&view=diff
==============================================================================
--- cfe/trunk/test/CodeGenCXX/ubsan-suppress-checks.cpp (original)
+++ cfe/trunk/test/CodeGenCXX/ubsan-suppress-checks.cpp Mon Apr 17 17:26:07 2017
@@ -2,6 +2,18 @@
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -emit-llvm -o - %s -fsanitize=null | FileCheck %s --check-prefixes=CHECK,NULL
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -emit-llvm -o - %s -fsanitize=alignment,null -DCHECK_LAMBDA | FileCheck %s --check-prefixes=LAMBDA
 
+// CHECK-LABEL: define void @_Z22load_non_null_pointersv
+void load_non_null_pointers() {
+  int var;
+  var = *&var;
+
+  int arr[1];
+  arr[0] = arr[0];
+
+  // CHECK-NOT: icmp ne {{.*}}, null, !nosanitize
+  // CHECK: ret void
+}
+
 struct A {
   int foo;
 
@@ -29,8 +41,7 @@ struct A {
     };
     f();
 
-    // LAMBDA: icmp ne %class.anon* %[[FUNCVAR:.*]], null, !nosanitize
-    // LAMBDA: %[[LAMBDAINT:[0-9]+]] = ptrtoint %class.anon* %[[FUNCVAR]] to i64, !nosanitize
+    // LAMBDA: %[[LAMBDAINT:[0-9]+]] = ptrtoint %class.anon* %[[FUNCVAR:.*]] to i64, !nosanitize
     // LAMBDA: and i64 %[[LAMBDAINT]], 7, !nosanitize
     // LAMBDA: call void @__ubsan_handle_type_mismatch
 
@@ -127,8 +138,8 @@ struct A {
 struct B {
   operator A*() const { return nullptr; }
 
-  // CHECK-LABEL: define linkonce_odr i32 @_ZN1B11load_memberEv
-  static int load_member() {
+  // CHECK-LABEL: define linkonce_odr i32 @_ZN1B11load_memberEPS_
+  static int load_member(B *bp) {
     // Check &b before converting it to an A*.
     // CHECK: call void @__ubsan_handle_type_mismatch
     //
@@ -136,8 +147,7 @@ struct B {
     // NULL: call void @__ubsan_handle_type_mismatch
     //
     // CHECK-NOT: call void @__ubsan_handle_type_mismatch
-    B b;
-    return static_cast<A *>(b)->load_member();
+    return static_cast<A *>(*bp)->load_member();
     // CHECK: ret i32
   }
 };
@@ -210,7 +220,7 @@ void force_irgen() {
   A::call_through_reference(*a);
   A::call_through_pointer(a);
 
-  B::load_member();
+  B::load_member(nullptr);
 
   Base *b = new Derived;
   b->load_member_1();
@@ -218,4 +228,6 @@ void force_irgen() {
   Derived *d;
   d->load_member_2();
   d->load_member_3();
+
+  load_non_null_pointers();
 }