increase alignment of global structs in increase_alignment pass

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

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.
The patch is cross-tested on aarch64*-*-* and arm*-*-* and passes with
using hash_map
workaround.

Thanks,
Prathamesh

Attachment: patch.diff
Description: Text document

Attachment: ChangeLog
Description: Binary data