[CodeGen] Use ABI alignment for C++ new expressions

In case of placement new, if we do not know the alignment of the
operand, we can't assume it has the preferred alignment. It might be
e.g. a pointer to a struct member which follows ABI alignment rules.

This makes UBSAN no longer report "constructor call on misaligned
address" when constructing a double into a struct field of type double
on i686. The psABI specifies an alignment of 4 bytes, but the preferred
alignment used by Clang is 8 bytes.

We now use ABI alignment for allocating new as well, as the preferred
alignment should be used for over-aligning e.g. local variables, which
isn't relevant for ABI code dealing with operator new. AFAICT there
wouldn't be problems either way though.

Fixes llvm#54845.

Differential Revision: https://reviews.llvm.org/D124736

Author: BertalanD

clang/lib/CodeGen/CGExprCXX.cpp

@@ -1573,7 +1573,7 @@ llvm::Value *CodeGenFunction::EmitCXXNewExpr(const CXXNewExpr *E) {
   llvm::Value *allocSize =
     EmitCXXNewAllocSize(*this, E, minElements, numElements,
                         allocSizeWithoutCookie);
-  CharUnits allocAlign = getContext().getPreferredTypeAlignInChars(allocType);
+  CharUnits allocAlign = getContext().getTypeAlignInChars(allocType);
 
   // Emit the allocation call.  If the allocator is a global placement
   // operator, just "inline" it directly.

clang/test/CodeGenCXX/pr54845.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple i686-linux-gnu -emit-llvm %s -o - | FileCheck %s
+// https://github.com/llvm/llvm-project/issues/54845
+
+void *operator new(unsigned int, void *);
+
+void test(double *d) {
+  // This store used to have an alignment of 8, which was incorrect as
+  // the i386 psABI only guarantees a 4-byte alignment for doubles.
+
+  // CHECK: store double 0.000000e+00, {{.*}}, align 4
+  new (d) double(0);
+}