[ubsan] Don't check alignment if the alignment is 1

[vsk]

Summary

If a pointer is 1-byte aligned, there's no use in checking its
alignment. Somewhat surprisingly, ubsan can spend a significant amount
of time doing just that!

This loosely depends on D30283.

Testing: check-clang, check-ubsan, and a stage2 ubsan build.

I also compiled X86FastISel.cpp with -fsanitize=alignment using
patched/unpatched clangs based on r295686 with D30283 applied. Here are
the number of alignment checks emitted:

Setup	# of alignment checks
unpatched + D30283, -O0	14307
patched + D30283, -O0	12515

[filcab]

LGTM

lib/CodeGen/CGExpr.cpp

void CodeGenFunction::EmitTypeCheck(TypeCheckKind TCK, SourceLocation Loc,
 
   if (SanOpts.has(SanitizerKind::Alignment) &&
       !SkippedChecks.has(SanitizerKind::Alignment)) {
     AlignVal = Alignment.getQuantity();
     if (!Ty->isIncompleteType() && !AlignVal)
       AlignVal = getContext().getTypeAlignInChars(Ty).getQuantity();
 
     // The glvalue must be suitably aligned.
-    if (AlignVal) {
+    if (AlignVal > 1) {
       llvm::Value *Align =
           Builder.CreateAnd(Builder.CreatePtrToInt(Ptr, IntPtrTy),
                             llvm::ConstantInt::get(IntPtrTy, AlignVal - 1));
       llvm::Value *Aligned =
         Builder.CreateICmpEQ(Align, llvm::ConstantInt::get(IntPtrTy, 0));
       Checks.push_back(std::make_pair(Aligned, SanitizerKind::Alignment));
     }
   }

test/CodeGenCXX/ubsan-suppress-checks.cpp

struct B {
   operator A*() const { return nullptr; }
 
   // CHECK-LABEL: define linkonce_odr i32 @_ZN1B11load_memberEv
   static int load_member() {
     // Check &b before converting it to an A*.
     // CHECK: call void @__ubsan_handle_type_mismatch
     //
     // Check the result of the conversion before using it.
-    // CHECK: call void @__ubsan_handle_type_mismatch
+    // NULL: call void @__ubsan_handle_type_mismatch
     //
     // CHECK-NOT: call void @__ubsan_handle_type_mismatch
     B b;
     return static_cast<A *>(b)->load_member();
     // CHECK: ret i32
   }
 };
 

test/CodeGenCXX/ubsan-type-checks.cpp

 // RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -emit-llvm -o - %s -fsanitize=alignment | FileCheck %s -check-prefixes=ALIGN,COMMON
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -emit-llvm -o - %s -fsanitize=null | FileCheck %s -check-prefixes=NULL,COMMON
 // RUN: %clang_cc1 -std=c++11 -triple x86_64-apple-darwin10 -emit-llvm -o - %s -fsanitize=object-size | FileCheck %s -check-prefixes=OBJSIZE,COMMON
 
 struct A {
   // COMMON-LABEL: define linkonce_odr void @_ZN1A10do_nothingEv
   void do_nothing() {
-    // ALIGN: ptrtoint %struct.A* %{{.*}} to i64, !nosanitize
-    // ALIGN: and i64 %{{.*}}, 0, !nosanitize
+    // ALIGN-NOT: ptrtoint %struct.A* %{{.*}} to i64, !nosanitize
  
     // NULL: icmp ne %struct.A* %{{.*}}, null, !nosanitize
  
     // OBJSIZE-NOT: call i64 @llvm.objectsize
   }
 };
 
+struct B {
+  int x;
+
+  // COMMON-LABEL: define linkonce_odr void @_ZN1B10do_nothingEv
+  void do_nothing() {
+    // ALIGN: ptrtoint %struct.B* %{{.*}} to i64, !nosanitize
+    // ALIGN: and i64 %{{.*}}, 3, !nosanitize
+
+    // NULL: icmp ne %struct.B* %{{.*}}, null, !nosanitize
+
+    // OBJSIZE-NOT: call i64 @llvm.objectsize
+  }
+};
+
 void force_irgen() {
   A a;
   a.do_nothing();
+
+  B b;
+  b.do_nothing();
 }