[PATCH 1/2] Add new RTX instruction class FILLER_INSN

[Andrea Corallo]

Hi all,

I'd like to submit the following two patches implementing a new AArch64
specific back-end pass that helps optimize branch-dense code, which can
be a bottleneck for performance on some Arm cores.  This is achieved by
padding out the branch-dense sections of the instruction stream with
nops.

The original patch was already posted some time ago:

https://www.mail-archive.com/gcc-patches@gcc.gnu.org/msg200721.html

This follows up splitting as suggested in two patches, rebasing on
master and implementing the suggestions of the first code review.

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.

1/2 is requirement for 2/2.  Please see this the cover letter of this last
for more details on the pass itself.

Regards

  Andrea

gcc/ChangeLog

2020-07-17  Andrea Corallo  <andrea.corallo@arm.com>
	    Carey Williams  <carey.williams@arm.com>

	* 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.
	* emit-rtl.c (emit_filler_after): New function.
	* rtl.def (FILLER_INSN): New rtl define.
	* rtl.h (rtx_filler_insn): Define new structure.
	(FILLER_INSN_P): New macro.
	(is_a_helper <rtx_filler_insn *>::test): New test helper for
	rtx_filler_insn.
	(emit_filler_after): New extern.
	* target-insns.def: Add target insn definition.
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