increase alignment of global structs in increase_alignment pass
Prathamesh Kulkarni
prathamesh.kulkarni@linaro.org
Wed May 4 11:17:00 GMT 2016
On 23 February 2016 at 21:49, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
> On 23 February 2016 at 17:31, Richard Biener <rguenther@suse.de> wrote:
>> On Tue, 23 Feb 2016, Prathamesh Kulkarni wrote:
>>
>>> On 22 February 2016 at 17:36, Richard Biener <rguenther@suse.de> wrote:
>>> > On Mon, 22 Feb 2016, Prathamesh Kulkarni wrote:
>>> >
>>> >> Hi Richard,
>>> >> As discussed in private mail, this version of patch attempts to
>>> >> increase alignment
>>> >> of global struct decl if it contains an an array field(s) and array's
>>> >> offset is a multiple of the alignment of vector type corresponding to
>>> >> it's scalar type and recursively checks for nested structs.
>>> >> eg:
>>> >> static struct
>>> >> {
>>> >> int a, b, c, d;
>>> >> int k[4];
>>> >> float f[10];
>>> >> };
>>> >> k is a candidate array since it's offset is 16 and alignment of
>>> >> "vector (4) int" is 8.
>>> >> Similarly for f.
>>> >>
>>> >> I haven't been able to create a test-case where there are
>>> >> multiple candidate arrays and vector alignment of arrays are different.
>>> >> I suppose in this case we will have to increase alignment
>>> >> of the struct by the max alignment ?
>>> >> eg:
>>> >> static struct
>>> >> {
>>> >> <fields>
>>> >> T1 k[S1]
>>> >> <fields>
>>> >> T2 f[S2]
>>> >> <fields>
>>> >> };
>>> >>
>>> >> if V1 is vector type corresponding to T1, and V2 corresponding vector
>>> >> type to T2,
>>> >> offset (k) % align(V1) == 0 and offset (f) % align(V2) == 0
>>> >> and align (V1) > align(V2) then we will increase alignment of struct
>>> >> by align(V1).
>>> >>
>>> >> Testing showed FAIL for g++.dg/torture/pr31863.C due to program timeout.
>>> >> Initially it appeared to me, it went in infinite loop. However
>>> >> on second thoughts I think it's probably not an infinite loop, rather
>>> >> taking (extraordinarily) large amount of time
>>> >> to compile the test-case with the patch.
>>> >> The test-case builds quickly for only 2 instantiations of ClassSpec
>>> >> (ClassSpec<Key, A001, 1>,
>>> >> ClassSpec<Key, A002, 2>)
>>> >> Building with 22 instantiations (upto ClassSpec<Key, A0023, 22>) takes up
>>> >> to ~1m to compile.
>>> >> with:
>>> >> 23 instantiations: ~2m
>>> >> 24 instantiations: ~5m
>>> >> For 30 instantiations I terminated cc1plus after 13m (by SIGKILL).
>>> >>
>>> >> I guess it shouldn't go in an infinite loop because:
>>> >> a) structs cannot have circular references.
>>> >> b) works for lower number of instantiations
>>> >> However I have no sound evidence that it cannot be in infinite loop.
>>> >> I don't understand why a decl node is getting visited more than once
>>> >> for that test-case.
>>> >>
>>> >> Using a hash_map to store alignments of decl's so that decl node gets visited
>>> >> only once prevents the issue.
>>> >
>>> > Maybe aliases. Try not walking vnode->alias == true vars.
>>> Hi,
>>> I have a hypothesis why decl node gets visited multiple times.
>>>
>>> Consider the test-case:
>>> template <typename T, unsigned N>
>>> struct X
>>> {
>>> T a;
>>> virtual int foo() { return N; }
>>> };
>>>
>>> typedef struct X<int, 1> x_1;
>>> typedef struct X<int ,2> x_2;
>>>
>>> static x_1 obj1 __attribute__((used));
>>> static x_2 obj2 __attribute__((used));
>>>
>>> Two additional structs are created by C++FE, c++filt shows:
>>> _ZTI1XIiLj1EE -> typeinfo for X<int, 1u>
>>> _ZTI1XIiLj2EE -> typeinfo for X<int, 2u>
>>>
>>> Both of these structs have only one field D.2991 and it appears it's
>>> *shared* between them:
>>> struct D.2991;
>>> const void * D.2980;
>>> const char * D.2981;
>>>
>>> Hence the decl node D.2991 and it's fields (D.2890, D.2981) get visited twice:
>>> once when walking _ZTI1XIiLj1EE and 2nd time when walking _ZTI1XIiLj2EE
>>>
>>> Dump of walking over the global structs for above test-case:
>>> http://pastebin.com/R5SABW0c
>>>
>>> So it appears to me to me a DAG (interior node == struct decl, leaf ==
>>> non-struct field,
>>> edge from node1 -> node2 if node2 is field of node1) is getting
>>> created when struct decl
>>> is a type-info object.
>>>
>>> I am not really clear on how we should proceed:
>>> a) Keep using hash_map to store alignments so that every decl gets
>>> visited only once.
>>> b) Skip walking artificial record decls.
>>> I am not sure if skipping all artificial struct decls would be a good
>>> idea, but I don't
>>> think it's possible to identify if a struct-decl is typeinfo struct at
>>> middle-end ?
>>
>> You shouldn't end up walking those when walking the type of
>> global decls. That is, don't walk typeinfo decls - yes, practically
>> that means just not walking DECL_ARTIFICIAL things.
> Hi,
> I have done the changes in the patch (attached) and cross-tested
> on arm*-*-* and aarch64*-*-* without failures.
> Is it OK for stage-1 ?
Hi,
Is the attached patch OK for trunk ?
Bootstrapped and tested on aarch64-linux-gnu, ppc64le-linux-gnu.
Cross-tested on arm*-*-* and aarch64*-*-*.
Thanks,
Prathamesh
>
> Thanks,
> Prathamesh
>>
>> Richard.
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diff --git a/gcc/testsuite/gcc.dg/vect/section-anchors-vect-70.c b/gcc/testsuite/gcc.dg/vect/section-anchors-vect-70.c
new file mode 100644
index 0000000..7010a52
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/section-anchors-vect-70.c
@@ -0,0 +1,33 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target section_anchors } */
+/* { dg-require-effective-target vect_int } */
+
+#define N 32
+
+/* Increase alignment of struct if an array's offset is multiple of alignment of
+ vector type corresponding to it's scalar type.
+ For the below test-case:
+ offsetof(e) == 8 bytes.
+ i) For arm: let x = alignment of vector type corresponding to int,
+ x == 8 bytes.
+ Since offsetof(e) % x == 0, set DECL_ALIGN(a, b, c) to x.
+ ii) For aarch64, ppc: x == 16 bytes.
+ Since offsetof(e) % x != 0, don't increase alignment of a, b, c.
+*/
+
+static struct A {
+ int p1, p2;
+ int e[N];
+} a, b, c;
+
+int foo(void)
+{
+ for (int i = 0; i < N; i++)
+ a.e[i] = b.e[i] + c.e[i];
+
+ return a.e[0];
+}
+
+/* { dg-final { scan-ipa-dump-times "Increasing alignment of decl" 0 "increase_alignment" { target aarch64*-*-* } } } */
+/* { dg-final { scan-ipa-dump-times "Increasing alignment of decl" 0 "increase_alignment" { target powerpc64*-*-* } } } */
+/* { dg-final { scan-ipa-dump-times "Increasing alignment of decl" 3 "increase_alignment" { target arm*-*-* } } } */
diff --git a/gcc/tree-vectorizer.c b/gcc/tree-vectorizer.c
index 2b25b45..0dd0d24 100644
--- a/gcc/tree-vectorizer.c
+++ b/gcc/tree-vectorizer.c
@@ -794,6 +794,88 @@ make_pass_slp_vectorize (gcc::context *ctxt)
This should involve global alignment analysis and in the future also
array padding. */
+static unsigned get_vec_alignment_for_decl (tree);
+
+/* Return alignment of array's vector type corresponding to scalar type.
+ 0 if no vector type exists. */
+static unsigned
+get_vec_alignment_for_array_decl (tree array_decl)
+{
+ tree type = TREE_TYPE (array_decl);
+ gcc_assert (TREE_CODE (type) == ARRAY_TYPE);
+
+ tree vectype = get_vectype_for_scalar_type (strip_array_types (type));
+ return (vectype) ? TYPE_ALIGN (vectype) : 0;
+}
+
+/* Return alignment of field having maximum alignment of vector type
+ corresponding to it's scalar type. For now, we only consider fields whose
+ offset is a multiple of it's vector alignment.
+ 0 if no suitable field is found. */
+static unsigned
+get_vec_alignment_for_record_decl (tree record_decl)
+{
+ tree type = TREE_TYPE (record_decl);
+ gcc_assert (TREE_CODE (type) == RECORD_TYPE);
+ unsigned max_align = 0, alignment;
+ HOST_WIDE_INT offset;
+
+ /* Skip artificial decls like typeinfo decls or if
+ record is packed. */
+ if (DECL_ARTIFICIAL (record_decl) || TYPE_PACKED (type))
+ return 0;
+
+ for (tree field = first_field (type);
+ field != NULL_TREE;
+ field = DECL_CHAIN (field))
+ {
+ /* C++FE puts node "._0" of code TYPE_DECL. skip that. */
+ if (TREE_CODE (field) != FIELD_DECL)
+ continue;
+
+ offset = int_byte_position (field);
+ alignment = get_vec_alignment_for_decl (field);
+
+ if (alignment
+ && (offset % (alignment / BITS_PER_UNIT) == 0)
+ && (alignment > max_align))
+ max_align = alignment;
+ }
+
+ return max_align;
+}
+
+/* Return alignment of vector type corresponding to decl's scalar type
+ or 0 if it doesn't exist or the vector alignment is lesser than
+ decl's alignment. */
+static unsigned
+get_vec_alignment_for_decl (tree decl)
+{
+ if (decl == NULL_TREE)
+ return 0;
+
+ gcc_assert (DECL_P (decl));
+
+ static unsigned alignment = 0;
+ tree type = TREE_TYPE (decl);
+
+ switch (TREE_CODE (type))
+ {
+ case ARRAY_TYPE:
+ alignment = get_vec_alignment_for_array_decl (decl);
+ break;
+ case RECORD_TYPE:
+ alignment = get_vec_alignment_for_record_decl (decl);
+ break;
+ default:
+ alignment = 0;
+ break;
+ }
+
+ return (alignment > DECL_ALIGN (decl)) ? alignment : 0;
+}
+
+/* Entry point to increase_alignment pass. */
static unsigned int
increase_alignment (void)
{
@@ -804,23 +886,14 @@ increase_alignment (void)
/* Increase the alignment of all global arrays for vectorization. */
FOR_EACH_DEFINED_VARIABLE (vnode)
{
- tree vectype, decl = vnode->decl;
- tree t;
+ tree decl = vnode->decl;
unsigned int alignment;
- t = TREE_TYPE (decl);
- if (TREE_CODE (t) != ARRAY_TYPE)
- continue;
- vectype = get_vectype_for_scalar_type (strip_array_types (t));
- if (!vectype)
- continue;
- alignment = TYPE_ALIGN (vectype);
- if (DECL_ALIGN (decl) >= alignment)
- continue;
-
- if (vect_can_force_dr_alignment_p (decl, alignment))
+ alignment = get_vec_alignment_for_decl (decl);
+
+ if (alignment && vect_can_force_dr_alignment_p (decl, alignment))
{
- vnode->increase_alignment (TYPE_ALIGN (vectype));
+ vnode->increase_alignment (alignment);
dump_printf (MSG_NOTE, "Increasing alignment of decl: ");
dump_generic_expr (MSG_NOTE, TDF_SLIM, decl);
dump_printf (MSG_NOTE, "\n");
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