expansion: Sign or zero extend on MEM_REF stores into SUBREG with SUBREG_PROMOTED_VAR_P [PR98190]

Some targets decide to promote certain scalar variables to wider mode,
so their DECL_RTL is a SUBREG with SUBREG_PROMOTED_VAR_P.
When storing to such vars, store_expr takes care of sign or zero extending,
but if we store e.g. through MEM_REF into them, no sign or zero extension
happens and that leads to wrong-code e.g. on the following testcase on
aarch64-linux.

The following patch uses store_expr if we overwrite all the bits and it is
not reversed storage order, i.e. something that store_expr handles normally,
and otherwise (if the most significant bit is (or for pdp11 might be, but
pdp11 doesn't promote) being modified), the code extends manually.

2020-12-11  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/98190
	* expr.c (expand_assignment): If to_rtx is a promoted SUBREG,
	ensure sign or zero extension either through use of store_expr
	or by extending manually.

	* gcc.dg/pr98190.c: New test.

1 files changed, 24 insertions, 0 deletions

diff --git a/gcc/expr.c b/gcc/expr.c
index 798285e..a56594e 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -5451,6 +5451,30 @@ expand_assignment (tree to, tree from, bool nontemporal)
 					       mode1, to_rtx, to, from,
 					       reversep))
 	    result = NULL;
+	  else if (SUBREG_P (to_rtx)
+		   && SUBREG_PROMOTED_VAR_P (to_rtx))
+	    {
+	      /* If to_rtx is a promoted subreg, we need to zero or sign
+		 extend the value afterwards.  */
+	      if (TREE_CODE (to) == MEM_REF
+		  && !REF_REVERSE_STORAGE_ORDER (to)
+		  && known_eq (bitpos, 0)
+		  && known_eq (bitsize, GET_MODE_BITSIZE (GET_MODE (to_rtx))))
+		result = store_expr (from, to_rtx, 0, nontemporal, false);
+	      else
+		{
+		  rtx to_rtx1
+		    = lowpart_subreg (subreg_unpromoted_mode (to_rtx),
+				      SUBREG_REG (to_rtx),
+				      subreg_promoted_mode (to_rtx));
+		  result = store_field (to_rtx1, bitsize, bitpos,
+					bitregion_start, bitregion_end,
+					mode1, from, get_alias_set (to),
+					nontemporal, reversep);
+		  convert_move (SUBREG_REG (to_rtx), to_rtx1,
+				SUBREG_PROMOTED_SIGN (to_rtx));
+		}
+	    }
 	  else
 	    result = store_field (to_rtx, bitsize, bitpos,
 				  bitregion_start, bitregion_end,