[ubsan] Skip null checks if they are constant-folded away

The IR builder can constant-fold null checks if the pointer operand
points to a constant. If the "is-non-null" check is folded away to
"true", don't emit the null check + branch.

Testing: check-clang, check-ubsan.

This slightly 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 |            25251 |
  | patched, -O0   |            23925 | (-5.3%)
  -------------------------------------

llvm-svn: 300509

clang/lib/CodeGen/CGExpr.cpp

@@ -568,15 +568,23 @@ void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
     // The glvalue must not be an empty glvalue.
     llvm::Value *IsNonNull = Builder.CreateIsNotNull(Ptr);
 
-    if (AllowNullPointers) {
-      // When performing pointer casts, it's OK if the value is null.
-      // Skip the remaining checks in that case.
-      Done = createBasicBlock("null");
-      llvm::BasicBlock *Rest = createBasicBlock("not.null");
-      Builder.CreateCondBr(IsNonNull, Rest, Done);
-      EmitBlock(Rest);
-    } else {
-      Checks.push_back(std::make_pair(IsNonNull, SanitizerKind::Null));
+    // The IR builder can constant-fold the null check if the pointer points to
+    // a constant.
+    bool PtrIsNonNull =
+        IsNonNull == llvm::ConstantInt::getTrue(getLLVMContext());
+
+    // Skip the null check if the pointer is known to be non-null.
+    if (!PtrIsNonNull) {
+      if (AllowNullPointers) {
+        // When performing pointer casts, it's OK if the value is null.
+        // Skip the remaining checks in that case.
+        Done = createBasicBlock("null");
+        llvm::BasicBlock *Rest = createBasicBlock("not.null");
+        Builder.CreateCondBr(IsNonNull, Rest, Done);
+        EmitBlock(Rest);
+      } else {
+        Checks.push_back(std::make_pair(IsNonNull, SanitizerKind::Null));
+      }
     }
   }
 

clang/test/CodeGenCXX/ubsan-suppress-checks.cpp

@@ -10,6 +10,8 @@ void load_non_null_pointers() {
   int arr[1];
   arr[0] = arr[0];
 
+  char c = "foo"[0];
+
   // CHECK-NOT: icmp ne {{.*}}, null, !nosanitize
   // CHECK: ret void
 }