[Bug rtl-optimization/84101] [7/8/9 Regression] -O3 and -ftree-vectorize trying too hard for function returning trivial pair-of-uint64_t-structure

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84101

--- Comment #22 from Richard Biener <rguenth at gcc dot gnu.org> ---
But we can side-step this issue with changing the way expand_constructor
works:

Index: gcc/expr.c
===================================================================
--- gcc/expr.c  (revision 269963)
+++ gcc/expr.c  (working copy)
@@ -7018,7 +7018,16 @@ store_field (rtx target, poly_int64 bits
            }
        }

-      temp = expand_normal (exp);
+      if (REG_P (target)
+         && known_eq (bitpos, 0)
+         && known_eq (bitsize, GET_MODE_BITSIZE (GET_MODE (target))))
+       {
+         temp = expand_expr (exp, target, VOIDmode, EXPAND_NORMAL);
+         if (temp == target)
+           return const0_rtx;
+       }
+      else
+       temp = expand_normal (exp);

       /* We don't support variable-sized BLKmode bitfields, since our
         handling of BLKmode is bound up with the ability to break
@@ -8191,7 +8202,15 @@ expand_constructor (tree exp, rtx target
       if (avoid_temp_mem)
        return NULL_RTX;

-      target = assign_temp (type, TREE_ADDRESSABLE (exp), 1);
+      if (target && REG_P (target))
+       {
+         rtx tem = gen_reg_rtx (GET_MODE (target));
+         store_constructor (exp, tem, 0, int_expr_size (exp), false);
+         emit_move_insn (target, tem);
+         return target;
+       }
+      else
+       target = assign_temp (type, TREE_ADDRESSABLE (exp), 1);
     }

   store_constructor (exp, target, 0, int_expr_size (exp), false);