Re: [PATCH 6/9] ipa-sra: Be optimistic about Fortran descriptors

[Martin Jambor <mjambor@suse.cz>]

Hi,

On Mon, Dec 12 2022, Jan Hubicka wrote:
>> Hi,
>> 
>> I'm re-posting patches which I have posted at the end of stage 1 but
>> which have not passed review yet.
>> 
>> 8<--------------------------------------------------------------------
>> 
>> Fortran descriptors are structures which are often constructed just
>> for a particular argument of a particular call where it is passed by
>> reference.  When the called function is under compiler's control, it
>> can be beneficial to split up the descriptor and pass it in individual
>> parameters.  Unfortunately, currently we allow IPA-SRA to replace a
>> pointer with a set of replacements which are at most twice as big in
>> total and for descriptors we'd need to bump that factor to seven.
>> 
>> This patch looks for parameters which are ADDR_EXPRs of local
>> variables which are written to and passed as arguments by reference
>> but are never loaded from and marks them with a flag in the call edge
>> summary.  The IPA analysis phase then identifies formal parameters
>> which are always fed such arguments and then is more lenient when it
>> comoes to size.
>> 
>> In order not to store to maximums per formal parameter, I calculate
>> the more lenient one by multiplying the existing one with a new
>> parameter.  If it is preferable to keep the maximums independent, we
>> can do that.  Documentation for the new parameter is missing as I
>> still need to re-base the patch on a version which has sphinx.  I will
>> write it before committing.
>> 
>> I have disable IPA-SRA in pr48636-2.f90 in order to be able to keep
>> using its dump-scan expressions.  The new testcase is basically a copy
>> of it with different options and IPA-SRA dump scans.
>> 
>> Bootstrapped and tested individually when I originally posted it and
>> now bootstrapped and LTO-bootstrapped and tested as part of the whole
>> series.  OK for master?
>> 
>> 
>> gcc/ChangeLog:
>> 
>> 2022-11-11  Martin Jambor  <mjambor@suse.cz>
>> 
>> 	* ipa-sra.c (isra_param_desc): New field not_specially_constructed.
>> 	(struct isra_param_flow): New field constructed_for_calls.
>> 	(isra_call_summary::dump): Dump the new flag.
>> 	(loaded_decls): New variable.
>> 	(dump_isra_param_descriptor): New parameter hints, dump
>> 	not_specially_constructed if it is true.
>> 	(dump_isra_param_descriptors): New parameter hints, pass it to
>> 	dump_isra_param_descriptor.
>> 	(ipa_sra_function_summaries::duplicate): Duplicate new flag.
>> 	(create_parameter_descriptors): Adjust comment.
>> 	(get_gensum_param_desc): Bail out when decl2desc is NULL.
>> 	(scan_expr_access): Add loaded local variables to loaded_decls.
>> 	(scan_function): Survive if final_bbs is NULL.
>> 	(isra_analyze_call): Compute constructed_for_calls flag.
>> 	(process_scan_results): Be optimistic about size limits.  Do not dump
>> 	computed param hints when dumpint IPA-SRA structures.
>> 	(isra_write_edge_summary): Stream constructed_for_calls.
>> 	(isra_read_edge_summary): Likewise.
>> 	(ipa_sra_dump_all_summaries): New parameter hints, pass it to
>> 	dump_isra_param_descriptor.
>> 	(flip_all_hints_pessimistic): New function.
>> 	(flip_all_param_hints_pessimistic): Likewise.
>> 	(propagate_param_hints): Likewise.
>> 	(disable_unavailable_parameters): Renamed to
>> 	adjust_parameter_descriptions.  Expand size limits for parameters
>> 	which are specially contstructed by all callers.  Check limits again.p
>> 	(ipa_sra_analysis): Pass required hints to ipa_sra_dump_all_summaries.
>> 	Add hint propagation.
>> 	(ipa_sra_summarize_function): Initialize and destory loaded_decls,
>> 	rearrange so that scan_function is called even when there are no
>> 	candidates.
>> 	* params.opt (ipa-sra-ptrwrap-growth-factor): New parameter.
>
> Hmm, this is quite specific heuristics, but I do not have much better
> ideas, so it is OK :)
>

Yeah, it kind of is.  I was wondering whether it should really only
target Fortran array descriptors (and have them marked some way) but
eventually decided for this - but I do not expect the code to trigger
too much for non-Fortran code.

> Can this be useful also for inlining?

IPA-SRA deallocates its summaries after the analysis phase so we'd need
to postpone that for later.  Otherwise it should be quite directly
usable - perhaps after a check that the "hint propagation" bit of
IPA-SRA has been run, the flag starts with the optimistic value.  

Martin