[PATCH] i386: Fix split_double_mode with paradoxical subreg [PR100626]

[Uros Bizjak via Gcc-patches]

split_double_mode calls simplify_gen_subreg, which fails for the
high half of the paradoxical subreg.  Return temporary register
instead of NULL RTX in this case.

I was not able to construct a testcase without warnings.

2021-05-18  Uroš Bizjak  <ubiz...@gmail.com>

gcc/
    PR target/100626
    * config/i386/i386-expand.c (split_double_mode): Return
    temporary register when simplify_gen_subreg fails with
    the high half of the paradoxical subreg.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Pushed to master, will be backported to other release branches.

Uros.

diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
index 0fa8d45a684..9f3d41955a2 100644
--- a/gcc/config/i386/i386-expand.c
+++ b/gcc/config/i386/i386-expand.c
@@ -154,9 +154,13 @@ split_double_mode (machine_mode mode, rtx operands[],
          lo_half[num] = simplify_gen_subreg (half_mode, op,
                                              GET_MODE (op) == VOIDmode
                                              ? mode : GET_MODE (op), 0);
-         hi_half[num] = simplify_gen_subreg (half_mode, op,
-                                             GET_MODE (op) == VOIDmode
-                                             ? mode : GET_MODE (op), byte);
+
+         rtx tmp = simplify_gen_subreg (half_mode, op,
+                                        GET_MODE (op) == VOIDmode
+                                        ? mode : GET_MODE (op), byte);
+         /* simplify_gen_subreg will return NULL RTX for the
+            high half of the paradoxical subreg. */
+         hi_half[num] = tmp ? tmp : gen_reg_rtx (half_mode);
        }
     }
 }