Hi Andrea,
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.
So I wonder if this cannot be done with some kind of NOTE, instead?
> + /* 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);
This stops checking after finding a FILLER_INSN as first insn. Is that
on purpose?
> +/* Make an insn of code FILLER_INSN to
> + pad out the instruction stream.
> + PATTERN should be from gen_filler_insn ().
> + AFTER will typically be an unconditional
> + branch at the end of a basic block. */
Because it only allows one particular insn pattern, it should be pretty
easy to use a NOTE for this, or even a normal INSN where the ouside-of-BB
test can then just look at its pattern.
As you see, I really do not like to have another RTX class, without very
well defined semantics even. Not without first being shown no
alternatives are acceptable, anyway :-)
Segher
