It doesn't make sense to warn about placement new of over-aligned
types; most occurrences are likely to be doing this in order to
provide properly aligned memory by allocating it first and then using
the standard non-allocating placement new.
Also, the discussion of -Waligned-new on HPPA led me to think that
even though MALLOC_ABI_ALIGNMENT is oddly low on most targets, some
might define it to be larger than max_align_t_align, and we should
take advantage of that when it occurs.
Tested x86_64-pc-linux-gnu, applying to trunk.
commit fb86b2acf865a283c94fea604843308b3b5ba331
Author: Jason Merrill <ja...@redhat.com>
Date: Tue Oct 11 17:24:05 2016 -0400
PR c++/77742 - -Waligned-new and placement new.
* init.c (build_new_1): Don't -Waligned-new about placement new.
(malloc_alignment): New. Consider MALLOC_ABI_ALIGNMENT.
* decl.c (cxx_init_decl_processing): New.
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 8b0442f..88cae04 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5953,6 +5953,7 @@ extern tree build_offset_ref (tree,
tree, bool,
tsubst_flags_t);
extern tree throw_bad_array_new_length (void);
extern bool type_has_new_extended_alignment (tree);
+extern unsigned malloc_alignment (void);
extern tree build_new (vec<tree, va_gc> **, tree,
tree,
vec<tree, va_gc> **, int,
tsubst_flags_t);
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 2d11aef..6240893 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -4082,7 +4082,7 @@ cxx_init_decl_processing (void)
if (aligned_new_threshold == -1)
aligned_new_threshold = (cxx_dialect >= cxx1z) ? 1 : 0;
if (aligned_new_threshold == 1)
- aligned_new_threshold = max_align_t_align () / BITS_PER_UNIT;
+ aligned_new_threshold = malloc_alignment () / BITS_PER_UNIT;
{
tree newattrs, extvisattr;
diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index b4b5e0a..455995a 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -2589,6 +2589,16 @@ type_has_new_extended_alignment (tree t)
&& TYPE_ALIGN_UNIT (t) > (unsigned)aligned_new_threshold);
}
+/* Return the alignment we expect malloc to guarantee. This should just be
+ MALLOC_ABI_ALIGNMENT, but that macro defaults to only BITS_PER_WORD for some
+ reason, so don't let the threshold be smaller than max_align_t_align. */
+
+unsigned
+malloc_alignment ()
+{
+ return MAX (max_align_t_align(), MALLOC_ABI_ALIGNMENT);
+}
+
/* Generate code for a new-expression, including calling the "operator
new" function, initializing the object, and, if an exception occurs
during construction, cleaning up. The arguments are as for
@@ -2974,8 +2984,23 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
tree nelts,
gcc_assert (alloc_fn != NULL_TREE);
+ /* Now, check to see if this function is actually a placement
+ allocation function. This can happen even when PLACEMENT is NULL
+ because we might have something like:
+
+ struct S { void* operator new (size_t, int i = 0); };
+
+ A call to `new S' will get this allocation function, even though
+ there is no explicit placement argument. If there is more than
+ one argument, or there are variable arguments, then this is a
+ placement allocation function. */
+ placement_allocation_fn_p
+ = (type_num_arguments (TREE_TYPE (alloc_fn)) > 1
+ || varargs_function_p (alloc_fn));
+
if (warn_aligned_new
- && TYPE_ALIGN (elt_type) > max_align_t_align ()
+ && !placement_allocation_fn_p
+ && TYPE_ALIGN (elt_type) > malloc_alignment ()
&& (warn_aligned_new > 1
|| CP_DECL_CONTEXT (alloc_fn) == global_namespace)
&& !aligned_allocation_fn_p (alloc_fn))
@@ -3033,20 +3058,6 @@ build_new_1 (vec<tree, va_gc> **placement, tree type,
tree nelts,
while (TREE_CODE (alloc_call) == COMPOUND_EXPR)
alloc_call = TREE_OPERAND (alloc_call, 1);
- /* Now, check to see if this function is actually a placement
- allocation function. This can happen even when PLACEMENT is NULL
- because we might have something like:
-
- struct S { void* operator new (size_t, int i = 0); };
-
- A call to `new S' will get this allocation function, even though
- there is no explicit placement argument. If there is more than
- one argument, or there are variable arguments, then this is a
- placement allocation function. */
- placement_allocation_fn_p
- = (type_num_arguments (TREE_TYPE (alloc_fn)) > 1
- || varargs_function_p (alloc_fn));
-
/* Preevaluate the placement args so that we don't reevaluate them for a
placement delete. */
if (placement_allocation_fn_p)
diff --git a/gcc/testsuite/g++.dg/cpp1z/aligned-new7.C
b/gcc/testsuite/g++.dg/cpp1z/aligned-new7.C
new file mode 100644
index 0000000..e12db9b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/aligned-new7.C
@@ -0,0 +1,15 @@
+// PR c++/77742
+// { dg-options "-Wall -std=c++1z" }
+
+#include <new>
+
+struct X
+{
+ alignas(2*__STDCPP_DEFAULT_NEW_ALIGNMENT__) int i;
+};
+
+int main()
+{
+ alignas(alignof(X)) char buf[sizeof(X)];
+ ::new((void*)buf) X{1};
+}
