sra: Avoid totally scalarizing overallping field_decls (PR 93667)

[[no_unique_address]] C++ attribute can cause two fields of a
RECORD_TYPE overlap, which currently confuses the totally scalarizing
code into creating invalid access tree.  For GCC 10, I'd like to
simply disable total scalarization of types where this happens.

For GCC 11 I'll write down a TODO item to enable total scalarization
of cases like this where the problematic fields are basically empty -
despite having a non-zero size - i.e. when they are just RECORD_TYPEs
without any data fields.

2020-02-19  Martin Jambor  <mjambor@suse.cz>

	gcc/

	PR tree-optimization/93667
	* tree-sra.c (scalarizable_type_p): Return false if record fields
	do not follow wach other.

	gcc/testsuite/

	PR tree-optimization/93667
	* g++.dg/tree-ssa/pr93667.C: New test.

gcc/ChangeLog

+2020-02-19  Martin Jambor  <mjambor@suse.cz>
+
+	PR tree-optimization/93667
+	* tree-sra.c (scalarizable_type_p): Return false if record fields
+	do not follow wach other.
+
 2020-01-21  Kito Cheng  <kito.cheng@sifive.com>
 
 	* config/riscv/riscv.c (riscv_output_move) Using fmv.x.w/fmv.w.x

gcc/testsuite/ChangeLog

+2020-02-19  Martin Jambor  <mjambor@suse.cz>
+
+	PR tree-optimization/93667
+	* g++.dg/tree-ssa/pr93667.C: New test.
+
 2020-02-19  Hongtao Liu  <hongtao.liu@intel.com>
 
 	* g++.dg/other/i386-2.C: add -mavx512vbmi2

gcc/testsuite/g++.dg/tree-ssa/pr93667.C

+// { dg-do compile }
+// { dg-options "-O2 -std=c++2a" } */
+
+struct a {};
+struct b { [[no_unique_address]] a aq; };
+struct c {
+  int d;
+  [[no_unique_address]] b e;
+};
+c f() {return {};}
+void g() { f(); }

gcc/tree-sra.c

@@ -958,6 +958,9 @@ scalarizable_type_p (tree type, bool const_decl)
   if (type_contains_placeholder_p (type))
     return false;
 
+  bool have_predecessor_field = false;
+  HOST_WIDE_INT prev_pos = 0;
+
   switch (TREE_CODE (type))
   {
   case RECORD_TYPE:
@@ -966,6 +969,17 @@ scalarizable_type_p (tree type, bool const_decl)
 	{
 	  tree ft = TREE_TYPE (fld);
 
+	  if (zerop (DECL_SIZE (fld)))
+	    continue;
+
+	  HOST_WIDE_INT pos = int_bit_position (fld);
+	  if (have_predecessor_field
+	      && pos <= prev_pos)
+	    return false;
+
+	  have_predecessor_field = true;
+	  prev_pos = pos;
+
 	  if (DECL_BIT_FIELD (fld))
 	    return false;