[ Please don't post new patch series as replies to old ]
On Wed, Jul 22, 2020 at 12:02:33PM +0200, Andrea Corallo wrote:
> This first patch implements the addition of a new RTX instruction class
> FILLER_INSN, which has been white listed to allow placement of NOPs
> outside of a basic block. This is to allow padding after unconditional
> branches. This is favorable so that any performance gained from
> diluting branches is not paid straight back via excessive eating of
> nops.
>
> It was deemed that a new RTX class was less invasive than modifying
> behavior in regards to standard UNSPEC nops.
Deemed, by whom? There are several people very against it, too. You
need to modify only one simple behaviour (maybe in a handful of places),
making a new RTX class for that is excessive.
> * cfgbuild.c (inside_basic_block_p): Handle FILLER_INSN.
> * cfgrtl.c (rtl_verify_bb_layout): Whitelist FILLER_INSN outside
> basic blocks.
> * coretypes.h: New rtx class.
coretypes.h is not a new RTX class? :-) Maybe:
* coretypes.h (rtx_filler_insn): New struct.
> @@ -3033,7 +3034,20 @@ rtl_verify_bb_layout (void)
> break;
>
> default:
> - fatal_insn ("insn outside basic block", x);
> + /* Allow nops after branches, via FILLER_INSN. */
> + bool fail = true;
> + subrtx_iterator::array_type array;
> + FOR_EACH_SUBRTX (iter, array, x, ALL)
> + {
> + const_rtx rtx = *iter;
> + if (GET_CODE (rtx) == FILLER_INSN)
> + {
> + fail = false;
> + break;
> + }
> + }
> + if (fail)
> + fatal_insn ("insn outside basic block", x);
> }
> }
It wouldn't be hard to allow some existing RTL here. Maybe something
with CODE_FOR_filler_nop or similar (after you recog () it).
It still allows anything after leading filler insns, btw; you could get
rid of the "fail" variable altogether, just call fatal_insn as soon as
you see some unexpected RTX code.
> + rtx_insn* i = make_insn_raw (pattern);
rtx_insn *i = ...
Segher
