This backport contains 1 patch from gcc main stream tree.
The commit id of these patchs list as following in the order of time.

c69325a5db450dbac198f76f1162734af05a1061
0001-sccvn-Fix-up-push_partial_def-little-endian-bitfield.patch

diff -urpN a/gcc/testsuite/gcc.c-torture/execute/pr97764.c b/gcc/testsuite/gcc.c-torture/execute/pr97764.c
--- a/gcc/testsuite/gcc.c-torture/execute/pr97764.c	1969-12-31 19:00:00.000000000 -0500
+++ b/gcc/testsuite/gcc.c-torture/execute/pr97764.c	2020-12-07 03:42:13.404000000 -0500
@@ -0,0 +1,14 @@
+/* PR tree-optimization/97764 */
+/* { dg-require-effective-target int32plus } */
+
+struct S { int b : 3; int c : 28; int d : 1; };
+
+int
+main ()
+{
+  struct S e = {};
+  e.c = -1;
+  if (e.d)
+    __builtin_abort ();
+  return 0;
+}
diff -urpN a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
--- a/gcc/tree-ssa-sccvn.c	2020-12-07 03:43:37.792000000 -0500
+++ b/gcc/tree-ssa-sccvn.c	2020-12-07 03:42:13.404000000 -0500
@@ -2013,12 +2013,12 @@ vn_walk_cb_data::push_partial_def (const
 	}
       else
 	{
-	  size = MIN (size, (HOST_WIDE_INT) needed_len * BITS_PER_UNIT);
 	  if (pd.offset >= 0)
 	    {
 	      /* LSB of this_buffer[0] byte should be at pd.offset bits
 		 in buffer.  */
 	      unsigned int msk;
+	      size = MIN (size, (HOST_WIDE_INT) needed_len * BITS_PER_UNIT);
 	      amnt = pd.offset % BITS_PER_UNIT;
 	      if (amnt)
 		shift_bytes_in_array_left (this_buffer, len + 1, amnt);
@@ -2046,6 +2046,9 @@ vn_walk_cb_data::push_partial_def (const
 	    {
 	      amnt = (unsigned HOST_WIDE_INT) pd.offset % BITS_PER_UNIT;
 	      if (amnt)
+		size -= BITS_PER_UNIT - amnt;
+	      size = MIN (size, (HOST_WIDE_INT) needed_len * BITS_PER_UNIT);
+	      if (amnt)
 		shift_bytes_in_array_left (this_buffer, len + 1, amnt);
 	    }
 	  memcpy (p, this_buffer + (amnt != 0), size / BITS_PER_UNIT);