This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH] Alias analysis of zero sized arrays
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: sellcey at cavium dot com
- Cc: gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 26 Apr 2017 11:27:04 +0200
- Subject: Re: [PATCH] Alias analysis of zero sized arrays
- Authentication-results: sourceware.org; auth=none
- References: <1493159225.3139.189.camel@cavium.com>
On Wed, Apr 26, 2017 at 12:27 AM, Steve Ellcey <sellcey@cavium.com> wrote:
> This patch changes get_ref_base_and_extent to treat zero sized arrays
> like C99 flexible arrays and assume that references to zero sized
> arrays can also be made beyond the 'end' of the array. C99 flexible
> arrays are recognized by not having an INTEGER_CST limit/size and the
> routine then sets maxsize to -1 for special handling. This patch
> checks for a value of 0 when we do have an INTEGER_CST limit/size and
> handles it like a flexible array.
>
> Tested with a bootstrap on aarch64 and a GCC test run with no
> regressions.
>
> I did not create a testcase because the test I have (attached) is not
> runnable. If you compile this test case for aarch64 with
> '-UFLEX -O2 -fno-strict-aliasing' you will get two loads, then two
> stores in the main loop. In the original large program that this came
> from, that generated bad code. If compiled with -DFLEX instead of
> -UFLEX, you get load, store, load, store and that was working. With
> this patch, both versions (-UFLEX and -DFLEX) generate the load, store,
> load, store sequence.
>
> OK for checkin?
Huh. This doesn't look like the correct fix for anything.
Ah, so the issue is that we prune MEM_EXPR of MEMs off ARRAY_REFs
and end up with a MEM_EXPR of a.0_1->o which has zero size. Then
we run into
/* If MEM_OFFSET or MEM_SIZE are unknown what we got from MEM_EXPR
is conservative, so trust it. */
if (!MEM_OFFSET_KNOWN_P (mem)
|| !MEM_SIZE_KNOWN_P (mem))
return true;
but MEM_SIZE_KNOWN_P is true so we miss to do
/* Refine size and offset we got from analyzing MEM_EXPR by using
MEM_SIZE and MEM_OFFSET. */
ref->offset += MEM_OFFSET (mem) * BITS_PER_UNIT;
ref->size = MEM_SIZE (mem) * BITS_PER_UNIT;
thus a nearly minimal fix would be (looks like some other sanity checks
might better be moved before the early "trust it" out):
Index: gcc/alias.c
===================================================================
--- gcc/alias.c (revision 247273)
+++ gcc/alias.c (working copy)
@@ -322,6 +322,13 @@ ao_ref_from_mem (ao_ref *ref, const_rtx
ref->ref_alias_set = MEM_ALIAS_SET (mem);
+ /* Refine size and offset we got from analyzing MEM_EXPR by using
+ MEM_SIZE and MEM_OFFSET. */
+ if (MEM_OFFSET_KNOWN_P (mem))
+ ref->offset += MEM_OFFSET (mem) * BITS_PER_UNIT;
+ if (MEM_SIZE_KNOWN_P (mem))
+ ref->size = MEM_SIZE (mem) * BITS_PER_UNIT;
+
/* If MEM_OFFSET or MEM_SIZE are unknown what we got from MEM_EXPR
is conservative, so trust it. */
if (!MEM_OFFSET_KNOWN_P (mem)
@@ -336,12 +343,6 @@ ao_ref_from_mem (ao_ref *ref, const_rtx
> ref->max_size)))
ref->ref = NULL_TREE;
- /* Refine size and offset we got from analyzing MEM_EXPR by using
- MEM_SIZE and MEM_OFFSET. */
-
- ref->offset += MEM_OFFSET (mem) * BITS_PER_UNIT;
- ref->size = MEM_SIZE (mem) * BITS_PER_UNIT;
-
/* The MEM may extend into adjacent fields, so adjust max_size if
necessary. */
if (ref->max_size != -1
note that in the end we might want to stop pruning MEM_EXPR so much...
(set_mem_attributes_minus_bitpos). At least dropping component/array
refs in this case isn't conservative as the size zero access doesn't cover
the array ref stripped. So arguably the bug is in
set_mem_attributes_minus_bitpos
itself. Thus
Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c (revision 247273)
+++ gcc/emit-rtl.c (working copy)
@@ -1954,7 +1954,9 @@ set_mem_attributes_minus_bitpos (rtx ref
while (TREE_CODE (t2) == ARRAY_REF);
if (DECL_P (t2)
- || TREE_CODE (t2) == COMPONENT_REF)
+ || (TREE_CODE (t2) == COMPONENT_REF
+ && (! DECL_SIZE (TREE_OPERAND (t2, 1))
+ || ! integer_zerop (DECL_SIZE (TREE_OPERAND (t2, 1))))))
{
attrs.expr = t2;
attrs.offset_known_p = false;
is an alternative / additional fix (OTOH for all trailing arrays this
isn't really a
conservative MEM_EXPR, not just for size zero ones). Thus
Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c (revision 247273)
+++ gcc/emit-rtl.c (working copy)
@@ -1954,7 +1954,10 @@ set_mem_attributes_minus_bitpos (rtx ref
while (TREE_CODE (t2) == ARRAY_REF);
if (DECL_P (t2)
- || TREE_CODE (t2) == COMPONENT_REF)
+ || (TREE_CODE (t2) == COMPONENT_REF
+ /* For trailing arrays t2 doesn't have a size that
+ covers all valid accesses. */
+ && ! array_at_struct_end_p (t, false)))
{
attrs.expr = t2;
attrs.offset_known_p = false;
is probably better.
Richard.
> Steve Ellcey
> sellcey@cavium.com
>
>
> 2017-04-25 Steve Ellcey <sellcey@cavium.com>
>
> * tree-dfa.c (get_ref_base_and_extent): Treat zero size array like
> a C99 flexible array.
>