Since 4.9 we're quite "strict" about "less alignment" that we might
possibly know about when seeing dereferences. But that implementation
is quite inconsistent as treating 1-byte alignment as never
"possibly known" (unless we see a decl).
So the following patch makes us only consider the case where we
see a decl directly as "known" and override the alignment used
on the access.
This makes
int f(long a)
{
int *p=(int*)(a<<1);
return *p;
}
use alignof(int) for the access *p rather than 2-byte alignment
as derived from the left-shift by 1.
This drops a few "legacy" cases where a pointer derived from a decl
is no longer detected. But if we figure we still need to care about
this we should add a more proper machinery to detect it.
Bootstrapped and tested on x86_64-unknown-linux-gnu, applied but
not planning to backport.
Richard.
2016-03-29 Richard Biener <rguent...@suse.de>
PR middle-end/70424
* ipa-prop.c (ipa_compute_jump_functions_for_edge): Always
use alignment returned by get_pointer_alignment_1 if it is
bigger than BITS_PER_UNIT.
* builtins.c (get_pointer_alignment_1): Do not return true
for alignment extracted from SSA info.
Index: gcc/ipa-prop.c
===================================================================
*** gcc/ipa-prop.c (revision 234453)
--- gcc/ipa-prop.c (working copy)
*************** ipa_compute_jump_functions_for_edge (str
*** 1639,1649 ****
unsigned HOST_WIDE_INT hwi_bitpos;
unsigned align;
! if (get_pointer_alignment_1 (arg, &align, &hwi_bitpos)
&& 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;
--- 1639,1649 ----
unsigned HOST_WIDE_INT hwi_bitpos;
unsigned align;
! get_pointer_alignment_1 (arg, &align, &hwi_bitpos);
! if (align > BITS_PER_UNIT
&& align % BITS_PER_UNIT == 0
&& hwi_bitpos % BITS_PER_UNIT == 0)
{
jfunc->alignment.known = true;
jfunc->alignment.align = align / BITS_PER_UNIT;
jfunc->alignment.misalign = hwi_bitpos / BITS_PER_UNIT;
Index: gcc/builtins.c
===================================================================
*** gcc/builtins.c (revision 234453)
--- gcc/builtins.c (working copy)
*************** get_pointer_alignment_1 (tree exp, unsig
*** 463,469 ****
if (*alignp == 0)
*alignp = 1u << (HOST_BITS_PER_INT - 1);
/* We cannot really tell whether this result is an approximation. */
! return true;
}
else
{
--- 463,469 ----
if (*alignp == 0)
*alignp = 1u << (HOST_BITS_PER_INT - 1);
/* We cannot really tell whether this result is an approximation. */
! return false;
}
else
{
