https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70424
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |ASSIGNED
Last reconfirmed| |2016-03-29
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
Target Milestone|--- |4.9.4
Ever confirmed|0 |1
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is because of
get_object_alignment_2 (...)
...
known_alignment
= get_pointer_alignment_1 (addr, &ptr_align, &ptr_bitpos);
...
/* When EXP is an actual memory reference then we can use
TYPE_ALIGN of a pointer indirection to derive alignment.
Do so only if get_pointer_alignment_1 did not reveal absolute
alignment knowledge and if using that alignment would
improve the situation. */
if (!addr_p && !known_alignment
&& TYPE_ALIGN (TREE_TYPE (exp)) > align)
align = TYPE_ALIGN (TREE_TYPE (exp));
and get_pointer_alignment_1 returning true as we hit
else if (TREE_CODE (exp) == SSA_NAME
&& POINTER_TYPE_P (TREE_TYPE (exp)))
{
unsigned int ptr_align, ptr_misalign;
struct ptr_info_def *pi = SSA_NAME_PTR_INFO (exp);
if (pi && get_ptr_info_alignment (pi, &ptr_align, &ptr_misalign))
{
*bitposp = ptr_misalign * BITS_PER_UNIT;
*alignp = ptr_align * BITS_PER_UNIT;
/* Make sure to return a sensible alignment when the multiplication
by BITS_PER_UNIT overflowed. */
if (*alignp == 0)
*alignp = 1u << (HOST_BITS_PER_INT - 1);
/* We cannot really tell whether this result is an approximation. */
return true;
thus whenever flow-sensitive propagation computed an alignment value.
I agree (I think we have duplicates for this bug) that this is somewhat
odd behavior and we can unconditionally factor in TYPE_ALIGN.
Note that the current behavior was implemented to catch
char a[4];
int main()
{
return *(int *)a;
}
where GCC _knowns_ that 'a' is not aligned according to 'int'.
So the appropriate fix might be to just do
Index: gcc/builtins.c
===================================================================
--- gcc/builtins.c (revision 234453)
+++ gcc/builtins.c (working copy)
@@ -463,7 +463,7 @@ get_pointer_alignment_1 (tree exp, unsig
if (*alignp == 0)
*alignp = 1u << (HOST_BITS_PER_INT - 1);
/* We cannot really tell whether this result is an approximation. */
- return true;
+ return false;
}
else
{
but it will likely pessimize ipa-prop because it cares for the return value
for some reason. So we'd need sth like
Index: gcc/ipa-prop.c
===================================================================
--- gcc/ipa-prop.c (revision 234453)
+++ gcc/ipa-prop.c (working copy)
@@ -1640,10 +1640,10 @@ ipa_compute_jump_functions_for_edge (str
unsigned align;
if (get_pointer_alignment_1 (arg, &align, &hwi_bitpos)
+ && align > BITS_PER_UNIT
&& align % BITS_PER_UNIT == 0
&& hwi_bitpos % BITS_PER_UNIT == 0)
{
- gcc_checking_assert (align != 0);
jfunc->alignment.known = true;
jfunc->alignment.align = align / BITS_PER_UNIT;
jfunc->alignment.misalign = hwi_bitpos / BITS_PER_UNIT;
as well.
