Re: [PATCH] Allow prologues and epilogues to be inserted later

[Jeff Law <jeffreyalaw@gmail.com>]

On 11/11/22 09:21, Richard Sandiford via Gcc-patches wrote:
> Arm's SME adds a new processor mode called streaming mode.
> This mode enables some new (matrix-oriented) instructions and
> disables several existing groups of instructions, such as most
> Advanced SIMD vector instructions and a much smaller set of SVE
> instructions.  It can also change the current vector length.
>
> There are instructions to switch in and out of streaming mode.
> However, their effect on the ISA and vector length can't be represented
> directly in RTL, so they need to be emitted late in the pass pipeline,
> close to md_reorg.
>
> It's sometimes the responsibility of the prologue and epilogue to
> switch modes, which means we need to emit the prologue and epilogue
> sequences late as well.  (This loses shrink-wrapping and scheduling
> opportunities, but that's a price worth paying.)
>
> This patch therefore adds a target hook for forcing prologue
> and epilogue insertion to happen later in the pipeline.
>
> Tested on aarch64-linux-gnu (including with a follow-on patch)
> and x86_64-linux-gnu.  OK to install?
>   I'll ob
> Richard
>
>
> gcc/
> 	* target.def (use_late_prologue_epilogue): New hook.
> 	* doc/gccint/target-macros/miscellaneous-parameters.rst: Add
> 	TARGET_USE_LATE_PROLOGUE_EPILOGUE.
> 	* doc/gccint/target-macros/tm.rst.in: Regenerate.
> 	* passes.def (pass_late_thread_prologue_and_epilogue): New pass.
> 	* tree-pass.h (make_pass_late_thread_prologue_and_epilogue): Declare.
> 	* function.cc (pass_thread_prologue_and_epilogue::gate): New function.
> 	(pass_data_late_thread_prologue_and_epilogue): New pass variable.
> 	(pass_late_thread_prologue_and_epilogue): New pass class.
> 	(make_pass_late_thread_prologue_and_epilogue): New function.

I'm not sure how we'll enforce the no target independent code motion 
limitation that this seems to need and the exception made for reorg is 
hackish in that it appears we just rely on the fact that reorg isn't run 
for the one target where this matters.  That does make me wonder if we 
should future proof this ever so slightly -- is there a reasonably easy 
way to fail if a target were to define delay slots and the need for late 
prologue/epilogue?  If so, that seems advisable.


No objection to the meat of the patch, just wondering a bit about the 
additional sanity checking we can do...


Jeff