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Martin Jambor [Fri, 24 Nov 2023 16:32:35 +0000 (17:32 +0100)]

sra: SRA of non-escaped aggregates passed by reference to calls

PR109849 shows that a loop that heavily pushes and pops from a stack
implemented by a C++ std::vec results in slow code, mainly because the
vector structure is not split by SRA and so we end up in many loads
and stores into it.  This is because it is passed by reference
to (re)allocation methods and so needs to live in memory, even though
it does not escape from them and so we could SRA it if we
re-constructed it before the call and then separated it to distinct
replacements afterwards.

This patch does exactly that, first relaxing the selection of
candidates to also include those which are addressable but do not
escape and then adding code to deal with the calls.  The
micro-benchmark that is also the (scan-dump) testcase in this patch
runs twice as fast with it than with current trunk.  Honza measured
its effect on the libjxl benchmark and it almost closes the
performance gap between Clang and GCC while not requiring excessive
inlining and thus code growth.

The patch disallows creation of replacements for such aggregates which
are also accessed with a precision smaller than their size because I
have observed that this led to excessive zero-extending of data
leading to slow-downs of perlbench (on some CPUs).  Apart from this
case I have not noticed any regressions, at least not so far.

Gimple call argument flags can tell if an argument is unused (and then
we do not need to generate any statements for it) or if it is not
written to and then we do not need to generate statements loading
replacements from the original aggregate after the call statement.
Unfortunately, we cannot symmetrically use flags that an aggregate is
not read because to avoid re-constructing the aggregate before the
call because flags don't tell which what parts of aggregates were not
written to, so we load all replacements, and so all need to have the
correct value before the call.

This version of the patch also takes care to avoid attempts to modify
abnormal edges, something which was missing in the previosu version.

gcc/ChangeLog:

2023-11-23  Martin Jambor  <mjambor@suse.cz>

PR middle-end/109849
* tree-sra.cc (passed_by_ref_in_call): New.
(sra_initialize): Allocate passed_by_ref_in_call.
(sra_deinitialize): Free passed_by_ref_in_call.
(create_access): Add decl pool candidates only if they are not
already candidates.
(build_access_from_expr_1): Bail out on ADDR_EXPRs.
(build_access_from_call_arg): New function.
(asm_visit_addr): Rename to scan_visit_addr, change the
disqualification dump message.
(scan_function): Check taken addresses for all non-call statements,
including phi nodes.  Process all call arguments, including the static
chain, build_access_from_call_arg.
(maybe_add_sra_candidate): Relax need_to_live_in_memory check to allow
non-escaped local variables.
(sort_and_splice_var_accesses): Disallow smaller-than-precision
replacements for aggregates passed by reference to functions.
(sra_modify_expr): Use a separate stmt iterator for adding satements
before the processed statement and after it.
(enum out_edge_check): New type.
(abnormal_edge_after_stmt_p): New function.
(sra_modify_call_arg): New function.
(sra_modify_assign): Adjust calls to sra_modify_expr.
(sra_modify_function_body): Likewise, use sra_modify_call_arg to
process call arguments, including the static chain.

gcc/testsuite/ChangeLog:

2023-11-23  Martin Jambor  <mjambor@suse.cz>

PR middle-end/109849
* g++.dg/tree-ssa/pr109849.C: New test.
* g++.dg/tree-ssa/sra-eh-1.C: Likewise.
* gcc.dg/tree-ssa/pr109849.c: Likewise.
* gcc.dg/tree-ssa/sra-longjmp-1.c: Likewise.
* gfortran.dg/pr43984.f90: Added -fno-tree-sra to dg-options.

(cherry picked from commit aae723d360ca26cd9fd0b039fb0a616bd0eae363)