Here we were using pow2align as the right operand of <<. But for invalid alignments pow2align can be -1 which makes the shifting invalid. Fixed by moving the checking before using pow2align.
Bootstrapped/regtested on x86_64-linux, ok for trunk? 2018-03-01 Marek Polacek <pola...@redhat.com> PR c++/84639 * c-attribs.c (common_handle_aligned_attribute): Don't use invalid alignment in computation. diff --git gcc/c-family/c-attribs.c gcc/c-family/c-attribs.c index 0261a45ec98..3ebb2d6000c 100644 --- gcc/c-family/c-attribs.c +++ gcc/c-family/c-attribs.c @@ -1817,6 +1817,12 @@ common_handle_aligned_attribute (tree *node, tree name, tree args, int flags, /* Log2 of specified alignment. */ int pow2align = check_user_alignment (align_expr, true); + if (pow2align == -1 + || !check_cxx_fundamental_alignment_constraints (*node, pow2align, flags)) + { + *no_add_attrs = true; + return NULL_TREE; + } /* The alignment in bits corresponding to the specified alignment. */ unsigned bitalign = (1U << pow2align) * BITS_PER_UNIT; @@ -1826,10 +1832,7 @@ common_handle_aligned_attribute (tree *node, tree name, tree args, int flags, unsigned curalign = 0; unsigned lastalign = 0; - if (pow2align == -1 - || !check_cxx_fundamental_alignment_constraints (*node, pow2align, flags)) - *no_add_attrs = true; - else if (is_type) + if (is_type) { if ((flags & (int) ATTR_FLAG_TYPE_IN_PLACE)) /* OK, modify the type in place. */; Marek