[Bug target/99648] [11 regression] gcc.dg/torture/pr71522.c fails starting with r11-165 for 32 bits

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99648

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
--- gcc/simplify-rtx.c.jj       2021-04-09 16:18:24.275668496 +0200
+++ gcc/simplify-rtx.c  2021-04-09 19:26:24.963134240 +0200
@@ -7059,12 +7059,19 @@ simplify_immed_subreg (fixed_size_mode o
       while (buffer.length () < buffer_bytes)
        buffer.quick_push (filler);
     }
-  else
+  else if (!native_encode_rtx (innermode, x, buffer, first_byte, inner_bytes))
+    return NULL_RTX;
+  rtx ret = native_decode_rtx (outermode, buffer, 0);
+  if (ret && MODE_COMPOSITE_P (outermode))
     {
-      if (!native_encode_rtx (innermode, x, buffer, first_byte, inner_bytes))
+      auto_vec<target_unit, 128> buffer2 (buffer_bytes);
+      if (!native_encode_rtx (outermode, ret, buffer2, 0, buffer_bytes))
        return NULL_RTX;
-      }
-  return native_decode_rtx (outermode, buffer, 0);
+      for (unsigned int i = 0; i < buffer_bytes; ++i)
+       if (buffer[i] != buffer2[i])
+         return NULL_RTX;
+    }
+  return ret;
 }

 /* Simplify SUBREG:OUTERMODE(OP:INNERMODE, BYTE)

Makes simplify_subreg fail when trying to subreg a constant into a composite
floating mode that doesn't decode back.
Unfortunately, this causes ICE:
pr71522.c: In function ‘main’:
pr71522.c:27:1: error: unrecognizable insn:
   27 | }
      | ^
(insn 5 2 6 2 (set (reg/v:TF 118 [ d ])
        (subreg:TF (const_vector:V16QI [
                    (const_int 65 [0x41]) repeated x15
                    (const_int 0 [0])
                ]) 0)) "pr71522.c":20:3 -1
     (nil))
during RTL pass: vregs

store_bit_field_1 calls simplify_gen_subreg and that when simplify_immed_subreg
fails will happily create the above subreg.
So I'm quite afraid the above change could break quite a lot and so might be
better to defer it for GCC12.