[gcc r11-8468] i386: Fix split_double_mode with paradoxical subreg [PR100626]

[Uros Bizjak <uros@gcc.gnu.org>]

https://gcc.gnu.org/g:6be2c12e37b167890d68587086a2186358b64c02

commit r11-8468-g6be2c12e37b167890d68587086a2186358b64c02
Author: Uros Bizjak <ubizjak@gmail.com>
Date:   Tue May 18 15:45:54 2021 +0200

    i386: Fix split_double_mode with paradoxical subreg [PR100626]
    
    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.
    
    2021-05-18  Uroš Bizjak  <ubizjak@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 od the paradoxical subreg.
    
    (cherry picked from commit d39fbed75810fc7478842503ecb0268b85dc9c2e)

Diff:
---
 gcc/config/i386/i386-expand.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
index dda08ff67f2..c16811013f1 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);
 	}
     }
 }