emit constant expression for new expression array size if it is one

[nlewycky]

Summary

When the new expr's array size is an ICE, emit it as a constant expression.

This bypasses integer sanitization checks which are redundant on the expression since it's been checked by Sema. Fixes a clang codegen assertion on "void test() { new int[0+1]{0}; }" when building with -fsanitize=signed-integer-overflow.

lib/CodeGen/CGExprCXX.cpp

static llvm::Value *EmitCXXNewAllocSize(CodeGenFunction &CGF,
 
   // Figure out the cookie size.
   llvm::APInt cookieSize(sizeWidth,
                          CalculateCookiePadding(CGF, e).getQuantity());
 
   // Emit the array size expression.
   // We multiply the size of all dimensions for NumElements.
   // e.g for 'int[2][3]', ElemType is 'int' and NumElements is 6.
+  numElements = CGF.CGM.EmitConstantExpr(e->getArraySize(),
+                                         CGF.getContext().getSizeType(), &CGF);
+  if (!numElements)
     numElements = CGF.EmitScalarExpr(e->getArraySize());
   assert(isa<llvm::IntegerType>(numElements->getType()));
 
   // The number of elements can be have an arbitrary integer type;
   // essentially, we need to multiply it by a constant factor, add a
   // cookie size, and verify that the result is representable as a
   // size_t.  That's just a gloss, though, and it's wrong in one
   // important way: if the count is negative, it's an error even if
   // the cookie size would bring the total size >= 0.

test/CodeGenCXX/new-array-init.cpp

 // RUN: %clang_cc1 -std=c++11 -triple i386-unknown-unknown %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -std=c++11 -triple i386-unknown-unknown %s -emit-llvm -fsanitize=signed-integer-overflow -o - | FileCheck --check-prefix=SIO %s
 
 // CHECK: @[[ABC4:.*]] = {{.*}} constant [4 x i8] c"abc\00"
 // CHECK: @[[ABC15:.*]] = {{.*}} constant [15 x i8] c"abc\00\00\00\00
 
 // CHECK-LABEL: define void @_Z2fni
 void fn(int n) {
   // CHECK: icmp ult i{{32|64}} %{{[^ ]+}}, 3
   // CHECK: store i32 1
void aggr_sufficient(int n) {
   // CHECK: store i32 0, i32* %[[FIELD]]
   // CHECK: %[[PTR2:.*]] = getelementptr inbounds %[[AGGR]], %[[AGGR]]* %[[PTR1]], i32 1{{$}}
   // CHECK: %[[REMAIN:.*]] = sub i32 {{.*}}, 16
   // CHECK: %[[MEM:.*]] = bitcast %[[AGGR]]* %[[PTR2]] to i8*
   // CHECK: call void @llvm.memset{{.*}}(i8* %[[MEM]], i8 0, i32 %[[REMAIN]],
   struct Aggr { int a, b; };
   new Aggr[n] { 1, 2, 3 };
 }
+
+// SIO-LABEL: define void @_Z14constexpr_testv
+void constexpr_test() {
+  // SIO: call i8* @_Zna{{.}}(i32 4)
+  new int[0+1]{0};
+}