https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99648
--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #7)
> --- 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)
I think this makes sense from a consistency point of view.
> 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.
Hmm, yeah. I suppose we can force the constant to memory and do the subreg
via a load?
