gimple-fold: Handle bitfields in fold_const_aggregate_ref_1 [PR93121]

When working on __builtin_bit_cast that needs to handle bitfields too,
I've made the following change to handle at least some bitfields in
fold_const_aggregate_ref_1 (those that have integral representative).
It already handles some, but only those that start and end at byte
boundaries.

2020-07-20  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/93121
	* gimple-fold.c (fold_const_aggregate_ref_1): For COMPONENT_REF
	of a bitfield not aligned on byte boundaries try to
	fold_ctor_reference DECL_BIT_FIELD_REPRESENTATIVE if any and
	adjust it depending on endianity.

	* gcc.dg/tree-ssa/pr93121-2.c: New test.

2 files changed, 80 insertions, 2 deletions

diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
index dfda6db..81c77f7 100644
--- a/gcc/gimple-fold.c
+++ b/gcc/gimple-fold.c
@@ -7189,8 +7189,64 @@ fold_const_aggregate_ref_1 (tree t, tree (*valueize) (tree))
       if (maybe_lt (offset, 0))
 	return NULL_TREE;
 
-      return fold_ctor_reference (TREE_TYPE (t), ctor, offset, size,
-				  base);
+      tem = fold_ctor_reference (TREE_TYPE (t), ctor, offset, size, base);
+      if (tem)
+	return tem;
+
+      /* For bit field reads try to read the representative and
+	 adjust.  */
+      if (TREE_CODE (t) == COMPONENT_REF
+	  && DECL_BIT_FIELD (TREE_OPERAND (t, 1))
+	  && DECL_BIT_FIELD_REPRESENTATIVE (TREE_OPERAND (t, 1)))
+	{
+	  HOST_WIDE_INT csize, coffset;
+	  tree field = TREE_OPERAND (t, 1);
+	  tree repr = DECL_BIT_FIELD_REPRESENTATIVE (field);
+	  if (INTEGRAL_TYPE_P (TREE_TYPE (repr))
+	      && size.is_constant (&csize)
+	      && offset.is_constant (&coffset)
+	      && (coffset % BITS_PER_UNIT != 0
+		  || csize % BITS_PER_UNIT != 0)
+	      && !reverse
+	      && BYTES_BIG_ENDIAN == WORDS_BIG_ENDIAN)
+	    {
+	      poly_int64 bitoffset;
+	      poly_uint64 field_offset, repr_offset;
+	      if (poly_int_tree_p (DECL_FIELD_OFFSET (field), &field_offset)
+		  && poly_int_tree_p (DECL_FIELD_OFFSET (repr), &repr_offset))
+		bitoffset = (field_offset - repr_offset) * BITS_PER_UNIT;
+	      else
+		bitoffset = 0;
+	      bitoffset += (tree_to_uhwi (DECL_FIELD_BIT_OFFSET (field))
+			    - tree_to_uhwi (DECL_FIELD_BIT_OFFSET (repr)));
+	      HOST_WIDE_INT bitoff;
+	      int diff = (TYPE_PRECISION (TREE_TYPE (repr))
+			  - TYPE_PRECISION (TREE_TYPE (field)));
+	      if (bitoffset.is_constant (&bitoff)
+		  && bitoff >= 0
+		  && bitoff <= diff)
+		{
+		  offset -= bitoff;
+		  size = tree_to_uhwi (DECL_SIZE (repr));
+
+		  tem = fold_ctor_reference (TREE_TYPE (repr), ctor, offset,
+					     size, base);
+		  if (tem && TREE_CODE (tem) == INTEGER_CST)
+		    {
+		      if (!BYTES_BIG_ENDIAN)
+			tem = wide_int_to_tree (TREE_TYPE (field),
+						wi::lrshift (wi::to_wide (tem),
+							     bitoff));
+		      else
+			tem = wide_int_to_tree (TREE_TYPE (field),
+						wi::lrshift (wi::to_wide (tem),
+							     diff - bitoff));
+		      return tem;
+		    }
+		}
+	    }
+	}
+      break;
 
     case REALPART_EXPR:
     case IMAGPART_EXPR:
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr93121-2.c b/gcc/testsuite/gcc.dg/tree-ssa/pr93121-2.c
new file mode 100644
index 0000000..323dca6
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr93121-2.c
@@ -0,0 +1,22 @@
+/* PR libstdc++/93121 */
+/* { dg-do compile { target { ilp32 || lp64 } } } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+union U { int a[3]; struct S { int d; int a : 3; int b : 24; int c : 5; int e; } b; };
+const union U u = { .a = { 0x7efa3412, 0x5a876543, 0x1eeffeed } };
+int a, b, c;
+
+void
+foo ()
+{
+  a = u.b.a;
+  b = u.b.b;
+  c = u.b.c;
+}
+
+/* { dg-final { scan-tree-dump-times "a = 3;" 1 "optimized" { target le } } } */
+/* { dg-final { scan-tree-dump-times "b = 5303464;" 1 "optimized" { target le } } } */
+/* { dg-final { scan-tree-dump-times "c = 11;" 1 "optimized" { target le } } } */
+/* { dg-final { scan-tree-dump-times "a = 2;" 1 "optimized" { target be } } } */
+/* { dg-final { scan-tree-dump-times "b = -2868438;" 1 "optimized" { target be } } } */
+/* { dg-final { scan-tree-dump-times "c = 3;" 1 "optimized" { target be } } } */