Interprocedural scalar replacement of aggregates
Copyright (C) 2019-2024 Free Software Foundation, Inc.
Contributed by Martin Jambor <mjambor@suse.cz>
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IPA-SRA is an interprocedural pass that removes unused function return
values (turning functions returning a value which is never used into void
functions) and removes unused function parameters. It can also replace an
aggregate parameter by a set of other parameters representing part of the
original, turning those passed by reference into new ones which pass the
value directly.
The pass is a true IPA one, which means that it works in three stages in
order to be able to take advantage of LTO. First, summaries about functions
and each calls are generated. Function summaries (often called call graph
node summaries) contain mainly information about which parameters are
potential transformation candidates and which bits of candidates are
accessed. We differentiate between accesses done as a part of a call
statement (which might be not necessary if the callee is also transformed)
and others (which are mandatory). Call summaries (often called call graph
edge summaries) contain information about which function formal parameters
feed into which actual call arguments so that if two parameters are only
used in a sum which is then passed to another function which then however
does not use this parameter, all three parameters of the two functions can
be eliminated. Edge summaries also have flags whether the return value is
used or if it is only returned in the caller too. In LTO mode these
summaries are then streamed to the object file in the compilation phase and
streamed back in in the WPA analysis stage.
The interprocedural analysis phase traverses the graph in topological order
in two sweeps, one in each direction. First, from callees to callers for
parameter removal and splitting. Each strongly-connected component is
processed iteratively until the situation in it stabilizes. The pass from
callers to callees is then carried out to remove unused return values in a
very similar fashion.
Because parameter manipulation has big implications for call redirection
which is done only after all call graph nodes materialize, the
transformation phase is not part of this patch but is carried out by the
clone materialization and edge redirection itself, see comments in
ipa-param-manipulation.h for more details.
Intraprocedural part of IPA-SRA analysis. Scan function body of NODE and
create a summary structure describing IPA-SRA opportunities and constraints
in it.
References BITMAP_ALLOC, BITMAP_FREE, count, symtab_node::decl, DECL_ARGUMENTS, DECL_CHAIN, DECL_STRUCT_FUNCTION, dump_file, free(), func_sums, gcc_obstack_init, last_basic_block_for_fn, isra_func_summary::m_candidate, isra_func_summary::m_returns_value, symtab_node::name(), NULL, opt_for_fn, symtab_node::order, pop_cfun(), push_cfun(), scan_function(), TREE_CODE, and TREE_TYPE.
