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Re: [PR lto/41528] Add internal documentation in doc/lto.texi

Hi Diego, I have some random comments inlined below.

On Sun, Nov 14, 2010 at 10:23 PM, Diego Novillo <> wrote:
> This patch adds internal documentation for LTO. ?Much of it comes
> from Honza's GCC Summit paper, wiki pages and source comments. ?I
> also moved the internal flags from invoke.texi and added several
> pointers to the source code.
> It can still use more information, but this is a start.
> Tested with make doc, make pdf and visual inspection.
> OK for mainline?
> Diego.
> 2010-11-14 ?Jan Hubicka ?<>
> ? ? ? ? ? ?Diego Novillo ?<>
> ? ? ? ?PR lto/41528
> ? ? ? ?* doc/lto.texi: Add.
> ? ? ? ?* doc/gccint.texi: Add reference to lto.texi.
> ? ? ? ?* doc/invoke.texi: Update user documentation for LTO.
> ? ? ? ?Move internal flags to lto.texi
> Index: doc/lto.texi
> ===================================================================
> --- doc/lto.texi ? ? ? ?(revision 0)
> +++ doc/lto.texi ? ? ? ?(revision 0)
> @@ -0,0 +1,568 @@
> +@c Copyright (c) 2010 Free Software Foundation, Inc.
> +@c Free Software Foundation, Inc.
> +@c This is part of the GCC manual.
> +@c For copying conditions, see the file gcc.texi.
> +@c Contributed by Jan Hubicka <> and
> +@c Diego Novillo <>
> +
> +@node LTO
> +@chapter Link Time Optimization
> +@cindex lto
> +@cindex whopr
> +@cindex wpa
> +@cindex ltrans
> +
> +@section Design Overview
> +
> +Link time optimization is implemented as a GCC front end for a
> +bytecode representation of GIMPLE that is emitted in special sections
> +of @code{.o} files. ?Currently, LTO support is enabled in most
> +ELF-based systems, as well as darwin, cygwin and mingw systems.
> +
> +Since GIMPLE bytecode is saved alongside final object code, object
> +files generated with LTO support are larger than regular object files.
> +This ``fat'' object format makes it easy to integrate LTO into
> +existing build systems, as one can, for instance, produce archives of
> +the files. ?Additionally, one might be able to ship one set of fat
> +objects which could be used both for development and the production of
> +optimized builds. ?A, perhaps surprising, side effect of this feature
> +is that any mistake in the toolchain that leads to LTO information not
> +being used (e.g. an older @code{libtool} calling @code{ld} directly).
> +This is both an advantage, as the system is more robust, and a
> +disadvantage, as the user is not informed that the optimization has
> +been disabled.
> +
> +The current implementation only produces ``fat'' objects, effectively
> +doubling compilation time and increasing file sizes up to 5x the
> +original size. ?This hides the problem that some tools, such as
> +@code{ar} and @code{nm}, need to understand symbol tables of LTO
> +sections. ?These tools were extended to use the plugin infrastructure,
> +and with these problems solved, GCC will also support ``slim'' objects
> +consisting of the intermediate code alone.
> +
> +At the highest level, LTO splits the compiler in two. ?The first half
> +(the ``writer'') produces a streaming representation of all the
> +internal data structures needed to optimize and generate code. ?This
> +includes declarations, types, the callgraph and the GIMPLE representation
> +of function bodies.
> +
> +When @option{-flto} is given during compilation of a source file, the
> +pass manager executes all the passes in @code{all_lto_gen_passes}.
> +Currently, this phase is composed of two IPA passes:
> +
> +@itemize @bullet
> +@item @code{pass_ipa_lto_gimple_out}
> +This pass executes the function @code{lto_output} in
> +@file{lto-streamer-out.c}, which traverses the call graph encoding
> +every reachable declaration, type and function. This generates a
> +memory representation of all the file sections described below.
> +
> +@item @code{pass_ipa_lto_finish_out}
> +This pass executes the function @code{produce_asm_for_decls} in
> +@file{lto-streamer-out.c}, which takes the memory image built in the
> +previous pass and encodes it in the corresponding ELF file sections.
> +@end itemize
> +
> +The second half of LTO support is the ``reader''. ?This is implemented
> +as the GCC front end @file{lto1} in @file{lto/lto.c}. ?When
> +@file{collect2} detects a link set of @code{.o}/@code{.a} files with
> +LTO information and the @option{-flto} is enabled, it invokes
> +@file{lto1} which reads the set of files and aggregates them into a
> +single translation unit for optimization. ?The main entry point for
> +the reader is @file{lto/lto.c}:@code{lto_main}.

cc1, cc1plus, f951, and lto1 -- where does '1' here mean? It always
got me thinking where cc2/lto2/... is.

> +
> +@subsection LTO modes of operation
> +
> +One of the main goals of the GCC link-time infrastructure was to allow
> +effective compilation of large programs. ?For this reason GCC implements two
> +link-time compilation modes.
> +
> +@enumerate
> +@item ?@emph{LTO mode}, in which the whole program is read into the
> +compiler at link-time and optimized in a similar way as if it
> +were a single source-level compilation unit.
> +
> +@item ?@emph{WHOPR or partitioned mode}, designed to utilize multiple
> +CPUs and/or a distributed compilation environment to quickly link
> +large applications. ?WHOPR stands for WHOle Program optimizeR (not to
> +be confused with the semantics of @option{-fwhole-program}).

WHOPR --- the Whole part is not accurate -- it can be any part of the
whole program. The 'Whole Program' should be 'the set of IL units' to
be more exact.

> ?It
> +partitions the aggregated callgraph from many different @code{.o}
> +files and distributes the compilation of the sub-graphs to different
> +CPUs.
> +
> +Note that distributed compilation is not implemented yet, but since
> +the parallelism is facilitated via generating a @code{Makefile}, it
> +would be easy to implement.
> +@end enumerate
> +
> +WHOPR splits LTO into three main stages:
> +@enumerate
> +@item Local generation (LGEN)
> +This stage executes in parallel. Every file in the program is compiled
> +into the intermediate language and packaged together with the local
> +call-graph and summary information. ?This stage is the same for both
> +the LTO and WHOPR compilation mode.
> +
> +@item Whole Program Analysis (WPA)
> +WPA is performed sequentially. The global call-graph is generated, and
> +a global analysis procedure makes transformation decisions. The global
> +call-graph is partitioned to facilitate parallel optimization during
> +phase 3. The results of the WPA stage are stored into new object files
> +which contain the partitions of program expressed in the intermediate
> +language and the optimization decisions.

Would it better to mention 'do not confuse with -fwhole-program' here?

> +
> +@item Local transformations (LTRANS)
> +This stage executes in parallel. All the decisions made during phase 2
> +are implemented locally in each partitioned object file, and the final
> +object code is generated. Optimizations which cannot be decided
> +efficiently during the phase 2 may be performed on the local
> +call-graph partitions.
> +@end enumerate
> +
> +WHOPR can be seen as an extension of the usual LTO mode of
> +compilation. ?In LTO, WPA and LTRANS and are executed within a single
> +execution of the compiler, after the whole program has been read into
> +memory.

The use of 'whole program' here and other places do not seem to be
accurate -- it is essentially the set of object files with IL/IR in

> +
> +When compiling in WHOPR mode the callgraph is partitioned during
> +the WPA stage. ?The whole program is split into a given number of
> +partitions of roughly the same size. ?The compiler tries to
> +minimize the number of references which cross partition boundaries.
> +The main advantage of WHOPR is to allow the parallel execution of
> +LTRANS stages, which are the most time-consuming part of the
> +compilation process. ?Additionally, it avoids the need to load the
> +whole program into memory.
> +
> +
> +@section LTO file sections
> +
> +LTO information is stored in several ELF sections inside object files.
> +Data structures and enum codes for sections are defined in
> +@file{lto-streamer.h}.
> +
> +These sections are emitted from @file{lto-streamer-out.c} and mapped
> +in all at once from @file{lto/lto.c}:@code{lto_file_read}. ?The
> +individual functions dealing with the reading/writing of each section
> +are described below.
> +
> +@itemize @bullet
> +@item Command line options (@code{.gnu.lto_.opts})
> +
> +This section contains the command line options used to generate the
> +object files. ?This is used at link-time to determine the optimization
> +level and other settings when they are not explicitly specified at the
> +linker command line.
> +
> +Currently, GCC does not support combining LTO object files compiled
> +with different set of the command line options into a single binary.
> +At link-time, the options given on the command line and the options
> +saved on all the files in a link-time set are applied globally. ?No
> +attempt is made at validating the combination of flags (other than the
> +usual validation done by option processing). ?This is implemented in
> +@file{lto/lto.c}:@code{lto_read_all_file_options}.

This can be a big limiting factor for the wide adoption of LTO. In
LIPO, incompatible options are detected and modules not safe to
include are banned.



> +
> +
> +@item Symbol table (@code{.gnu.lto_.symtab})
> +
> +This table replaces the ELF symbol table for functions and variables
> +represented in the LTO IL. Symbols used and exported by the optimized
> +assembly code of ``fat'' objects might not match the ones used and
> +exported by the intermediate code. ?This table is necessary because
> +the intermediate code is less optimized and thus requires a separate
> +symbol table.
> +
> +Additionally, the binary code in the ``fat'' object will lack a call
> +to a function, since the call was optimized out at compilation time
> +after the intermediate language was streamed out. ?In some special
> +cases, the same optimization may not happen ?during link-time
> +optimization. ?This would lead to an undefined symbol if only one
> +symbol table was used.
> +
> +The symbol table is emitted in
> +@file{lto-streamer-out.c}:@code{produce_symtab}.
> +
> +
> +@item Global declarations and types (@code{.gnu.lto_.decls})
> +
> +This section contains an intermediate language dump of all
> +declarations and types required to represent the callgraph, static
> +variables and top-level debug info.
> +
> +The contents of this section are emitted in
> +@file{lto-streamer-out.c}:@code{produce_asm_for_decls}. ?Types and
> +symbols are emitted in a topological order that preserves the sharing
> +of pointers when the file is read back in
> +(@file{lto.c}:@code{read_cgraph_and_symbols}).
> +
> +
> +@item The callgraph (@code{.gnu.lto_.cgraph})
> +
> +This section contains the basic data structure used by the GCC
> +inter-procedural optimization infrastructure. This section stores an
> +annotated multi-graph which represents the functions and call sites as
> +well as the variables, aliases and top-level @code{asm} statements.
> +
> +This section is emitted in
> +@file{lto-streamer-out.c}:@code{output_cgraph} and read in
> +@file{lto-cgraph.c}:@code{input_cgraph}.
> +
> +
> +@item IPA references (@code{.gnu.lto_.refs})
> +
> +This section contains references between function and static
> +variables. ?It is emitted by @file{lto-cgraph.c}:@code{output_refs}
> +and read by @file{lto-cgraph.c}:@code{input_refs}.
> +
> +
> +@item Function bodies (@code{.gnu.lto_.function_body.<name>})
> +
> +This section contains function bodies in the intermediate language
> +representation. Every function body is in a separate section to allow
> +copying of the section independently to different object files or
> +reading the function on demand.
> +
> +Functions are emitted in
> +@file{lto-streamer-out.c}:@code{output_function} and read in
> +@file{lto-streamer-in.c}:@code{input_function}.
> +
> +
> +@item Static variable initializers (@code{.gnu.lto_.vars})
> +
> +This section contains all the symbols in the global variable pool. ?It
> +is emitted by @file{lto-cgraph.c}:@code{output_varpool} and read in
> +@file{lto-cgraph.c}:@code{input_cgraph}.
> +
> +@item Summaries and optimization summaries used by IPA passes
> +(@code{.gnu.lto_.<xxx>}, where @code{<xxx>} is one of @code{jmpfuncs},
> +@code{pureconst} or @code{reference})
> +
> +These sections are used by IPA passes that need to emit summary
> +information during LTO generation to be read and aggregated at
> +link time. ?Each pass is responsible for implementing two pass manager
> +hooks: one for writing the summary and another for reading it in. ?The
> +format of these sections is entirely up to each individual pass. ?The
> +only requirement is that the writer and reader hooks agree on the
> +format.
> +@end itemize
> +
> +
> +@section Using summary information in IPA passes
> +
> +Programs are represented internally as a @emph{callgraph} (a
> +multi-graph where nodes are functions and edges are call sites)
> +and a @emph{varpool} (a list of static and external variables in
> +the program).
> +
> +The inter-procedural optimization is organized as a sequence of
> +individual passes, which operate on the callgraph and the
> +varpool. ?To make the implementation of WHOPR possible, every
> +inter-procedural optimization pass is split into several stages
> +that are executed at different times during WHOPR compilation:
> +
> +@itemize @bullet
> +@item LGEN time
> +@enumerate
> +@item @emph{Generate summary} (@code{generate_summary} in
> +@code{struct ipa_opt_pass_d}). This stage analyzes every function
> +body and variable initializer is examined and stores relevant
> +information into a pass-specific data structure.
> +
> +@item @emph{Write summary} (@code{write_summary} in
> +@code{struct ipa_opt_pass_d}. This stage writes all the
> +pass-specific information generated by @code{generate_summary}.
> +Summaries go into their own @code{LTO_section_*} sections that
> +have to be declared in @file{lto-streamer.h}:@code{enum
> +lto_section_type}. ?A new section is created by calling
> +@code{create_output_block} and data can be written using the
> +@code{lto_output_*} routines.
> +@end enumerate
> +
> +@item WPA time
> +@enumerate
> +@item @emph{Read summary} (@code{read_summary} in
> +@code{struct ipa_opt_pass_d}). This stage reads all the
> +pass-specific information in exactly the same order that it was
> +written by @code{write_summary}.
> +
> +@item @emph{Execute} (@code{execute} in @code{struct
> +opt_pass}). ?This performs inter-procedural propagation. ?This
> +must be done without actual access to the individual function
> +bodies or variable initializers. ?Typically, this results in a
> +transitive closure operation over the summary information of all
> +the nodes in the callgraph.
> +
> +@item @emph{Write optimization summary}
> +(@code{write_optimization_summary} in @code{struct
> +ipa_opt_pass_d}). ?This writes the result of the inter-procedural
> +propagation into the object file. ?This can use the same data
> +structures and helper routines used in @code{write_summary}.
> +@end enumerate
> +
> +@item LTRANS time
> +@enumerate
> +@item @emph{Read optimization summary}
> +(@code{read_optimization_summary} in @code{struct
> +ipa_opt_pass_d}). ?The counterpart to
> +@code{write_optimization_summary}. ?This reads the interprocedural
> +optimization decisions in exactly the same format emitted by
> +@code{write_optimization_summary}.
> +
> +@item @emph{Transform} (@code{function_transform} and
> +@code{variable_transform} in @code{struct ipa_opt_pass_d}).
> +The actual function bodies and variable initializers are updated
> +based on the information passed down from the @emph{Execute} stage.
> +@end enumerate
> +@end itemize
> +
> +The implementation of the inter-procedural passes are shared
> +between LTO, WHOPR and classic non-LTO compilation.
> +
> +@itemize
> +@item During the traditional file-by-file mode every pass executes its
> +own @emph{Generate summary}, @emph{Execute}, and @emph{Transform}
> +stages within the single execution context of the compiler.
> +
> +@item In LTO compilation mode, every pass uses @emph{Generate
> +summary} and @emph{Write summary} stages at compilation time,
> +while the @emph{Read summary}, @emph{Execute}, and
> +@emph{Transform} stages are executed at link time.
> +
> +@item In WHOPR mode all stages are used.
> +@end itemize
> +
> +To simplify development, the GCC pass manager differentiates
> +between normal inter-procedural passes and small inter-procedural
> +passes. ?A @emph{small inter-procedural pass}
> +(@code{SIMPLE_IPA_PASS}) is a pass that does
> +everything at once and thus it can not be executed during WPA in
> +WHOPR mode. It defines only the @emph{Execute} stage and during
> +this stage it accesses and modifies the function bodies. ?Such
> +passes are useful for optimization at LGEN or LTRANS time and are
> +used, for example, to implement early optimization before writing
> +object files. ?The simple inter-procedural passes can also be used
> +for easier prototyping and development of a new inter-procedural
> +pass.
> +
> +
> +@subsection Virtual clones
> +
> +One of the main challenges of introducing the WHOPR compilation
> +mode was addressing the interactions between optimization passes.
> +In LTO compilation mode, the passes are executed in a sequence,
> +each of which consists of analysis (or @emph{Generate summary}),
> +propagation (or @emph{Execute}) and @emph{Transform} stages.
> +Once the work of one pass is finished, the next pass sees the
> +updated program representation and can execute. ?This makes the
> +individual passes dependent on each other.
> +
> +In WHOPR mode all passes first execute their @emph{Generate
> +summary} stage. ?Then summary writing marks the end of the LGEN
> +stage. ?At WPA time,
> +the summaries are read back into memory and all passes run the
> +@emph{Execute} stage. ?Optimization summaries are streamed and
> +sent to LTRANS, where all the passes execute the @emph{Transform}
> +stage.
> +
> +Most optimization passes split naturally into analysis,
> +propagation and transformation stages. ?But some do not. ?The
> +main problem arises when one pass performs changes and the
> +following pass gets confused by seeing different callgraphs
> +betwee the @emph{Transform} stage and the @emph{Generate summary}
> +or @emph{Execute} stage. ?This means that the passes are required
> +to communicate their decisions with each other.
> +
> +To facilitate this communication, the GCC callgraph
> +infrastructure implements @emph{virtual clones}, a method of
> +representing the changes performed by the optimization passes in
> +the callgraph without needing to update function bodies.
> +
> +A @emph{virtual clone} in the callgraph is a function that has no
> +associated body, just a description of how to create its body based
> +on a different function (which itself may be a virtual clone).
> +
> +The description of function modifications includes adjustments to
> +the function's signature (which allows, for example, removing or
> +adding function arguments), substitutions to perform on the
> +function body, and, for inlined functions, a pointer to the
> +function that it will be inlined into.
> +
> +It is also possible to redirect any edge of the callgraph from a
> +function to its virtual clone. ?This implies updating of the call
> +site to adjust for the new function signature.
> +
> +Most of the transformations performed by inter-procedural
> +optimizations can be represented via virtual clones. ?For
> +instance, a constant propagation pass can produce a virtual clone
> +of the function which replaces one of its arguments by a
> +constant. ?The inliner can represent its decisions by producing a
> +clone of a function whose body will be later integrated into
> +a given function.
> +
> +Using @emph{virtual clones}, the program can be easily updated
> +during the @emph{Execute} stage, solving most of pass interactions
> +problems that would otherwise occur during @emph{Transform}.
> +
> +Virtual clones are later materialized in the LTRANS stage and
> +turned into real functions. ?Passes executed after the virtual
> +clone were introduced also perform their @emph{Transform} stage
> +on new functions, so for a pass there is no significant
> +difference between operating on a real function or a virtual
> +clone introduced before its @emph{Execute} stage.
> +
> +Optimization passes then work on virtual clones introduced before
> +their @emph{Execute} stage as if they were real functions. ?The
> +only difference is that clones are not visible during the
> +@emph{Generate Summary} stage.
> +
> +To keep function summaries updated, the callgraph interface
> +allows an optimizer to register a callback that is called every
> +time a new clone is introduced as well as when the actual
> +function or variable is generated or when a function or variable
> +is removed. ?These hooks are registered in the @emph{Generate
> +summary} stage and allow the pass to keep its information intact
> +until the @emph{Execute} stage. ?The same hooks can also be
> +registered during the @emph{Execute} stage to keep the
> +optimization summaries updated for the @emph{Transform} stage.
> +
> +@subsection IPA references
> +
> +GCC represents IPA references in the callgraph. ?For a function
> +or variable @code{A}, the @emph{IPA reference} is a list of all
> +locations where the address of @code{A} is taken and, when
> +@code{A} is a variable, a list of all direct stores and reads
> +to/from @code{A}. References represent an oriented multi-graph on
> +the union of nodes of the callgraph and the varpool. ?See
> +@file{ipa-reference.c}:@code{ipa_reference_write_optimization_summary}
> +and
> +@file{ipa-reference.c}:@code{ipa_reference_read_optimization_summary}
> +for details.
> +
> +@subsection Jump functions
> +Suppose that an optimization pass sees a function @code{A} and it
> +knows the values of (some of) its arguments. ?The @emph{jump
> +function} describes the value of a parameter of a given function
> +call in function @code{A} based on this knowledge.
> +
> +Jump functions are used by several optimizations, such as the
> +inter-procedural constant propagation pass and the
> +devirtualization pass. ?The inliner also uses jump functions to
> +perform inlining of callbacks.
> +
> +@section Whole program assumptions, linker plugin and symbol visibilities
> +
> +Link-time optimization gives relatively minor benefits when used
> +alone. ?The problem is that propagation of inter-procedural
> +information does not work well across functions and variables
> +that are called or referenced by other compilation units (such as
> +from a dynamically linked library). We say that such functions
> +are variables are @emph{externally visible}.
> +
> +To make the situation even more difficult, many applications
> +organize themselves as a set of shared libraries, and the default
> +ELF visibility rules allow one to overwrite any externally
> +visible symbol with a different symbol at runtime. ?This
> +basically disables any optimizations across such functions and
> +variables, because the compiler cannot be sure that the function
> +body it is seeing is the same function body that will be used at
> +runtime. ?Any function or variable not declared @code{static} in
> +the sources degrades the quality of inter-procedural
> +optimization.
> +
> +To avoid this problem the compiler must assume that it sees the
> +whole program when doing link-time optimization. ?Strictly
> +speaking, the whole program is rarely visible even at link-time.
> +Standard system libraries are usually linked dynamically or not
> +provided with the link-time information. ?In GCC, the whole
> +program option (@option{-fwhole-program}) asserts that every
> +function and variable defined in the current compilation
> +unit is static, except for function @code{main} (note: at
> +link-time, the current unit is the union of all objects compiled
> +with LTO). ?Since some functions and variables need to
> +be referenced externally, for example by another DSO or from an
> +assembler file, GCC also provides the function and variable
> +attribute @code{externally_visible} which can be used to disable
> +the effect of @option{-fwhole-program} on a specific symbol.
> +
> +The whole program mode assumptions are slightly more complex in
> +C++, where inline functions in headers are put into @emph{COMDAT}
> +sections. COMDAT function and variables can be defined by
> +multiple object files and their bodies are unified at link-time
> +and dynamic link-time. ?COMDAT functions are changed to local only
> +when their address is not taken and thus un-sharing them with a
> +library is not harmful. ?COMDAT variables always remain externally
> +visible, however for readonly variables it is assumed that their
> +initializers cannot be overwritten by a different value.
> +
> +GCC provides the function and variable attribute
> +@code{visibility} that can be used to specify the visibility of
> +externally visible symbols (or alternatively an
> +@option{-fdefault-visibility} command line option). ?ELF defines
> +the @code{default}, @code{protected}, @code{hidden} and
> +@code{internal} visibilities.
> +
> +The most commonly used is visibility is @code{hidden}. It
> +specifies that the symbol cannot be referenced from outside of
> +the current shared library. Unfortunately, this information
> +cannot be used directly by the link-time optimization in the
> +compiler since the whole shared library also might contain
> +non-LTO objects and those are not visible to the compiler.
> +
> +GCC solves this problem using linker plugins. ?A @emph{linker
> +plugin} is an interface to the linker that allows an external
> +program to claim the ownership of a given object file. ?The linker
> +then performs the linking procedure by querying the plugin about
> +the symbol table of the claimed objects and once the linking
> +decisions are complete, the plugin is allowed to provide the
> +final object file before the actual linking is made. ?The linker
> +plugin obtains the symbol resolution information which specifies
> +which symbols provided by the claimed objects are bound from the
> +rest of a binary being linked.
> +
> +Currently, the linker plugin ?works only in combination
> +with the Gold linker, ?but a GNU ld implementation is under
> +development.
> +
> +GCC is designed to be independent of the rest of the toolchain
> +and aims to support linkers without plugin support. ?For this
> +reason it does not use the linker plugin by default. ?Instead,
> +the object files are examined by @command{collect2} before being
> +passed to the linker and objects found to have LTO sections are
> +passed to @command{lto1} first. ?This mode does not work for
> +library archives. The decision on what object files from the
> +archive are needed depends on the actual linking and thus GCC
> +would have to implement the linker itself. ?The resolution
> +information is missing too and thus GCC needs to make an educated
> +guess based on @option{-fwhole-program}. ?Without the linker
> +plugin GCC also assumes that symbols are declared @code{hidden}
> +and not referred by non-LTO code by default.
> +
> +@section Internal flags controlling @code{lto1}
> +
> +The following flags are passed into @command{lto1} and are not
> +meant to be used directly from the command line.
> +
> +@itemize
> +@item -fwpa
> +@opindex fwpa
> +This option runs the serial part of the link-time optimizer
> +performing the inter-procedural propagation (WPA mode). ?The
> +compiler reads in summary information from all inputs and
> +performs an analysis based on summary information only. ?It
> +generates object files for subsequent runs of the link-time
> +optimizer where individual object files are optimized using both
> +summary information from the WPA mode and the actual function
> +bodies. ?It then drives the LTRANS phase.
> +
> +@item -fltrans
> +@opindex fltrans
> +This option runs the link-time optimizer in the
> +local-transformation (LTRANS) mode, which reads in output from a
> +previous run of the LTO in WPA mode. In the LTRANS mode, LTO
> +optimizes an object and produces the final assembly.
> +
> +@item -fltrans-output-list=@var{file}
> +@opindex fltrans-output-list
> +This option specifies a file to which the names of LTRANS output
> +files are written. ?This option is only meaningful in conjunction
> +with @option{-fwpa}.
> +@end itemize
> Index: doc/gccint.texi
> ===================================================================
> --- doc/gccint.texi ? ? (revision 166733)
> +++ doc/gccint.texi ? ? (working copy)
> @@ -123,6 +123,7 @@ Additional tutorial information is linke
> ?* Header Dirs:: ? ? Understanding the standard header file directories.
> ?* Type Information:: GCC's memory management; generating type information.
> ?* Plugins:: ? ? ? ? Extending the compiler with plugins.
> +* LTO:: ? ? ? ? ? ? Using Link-Time Optimization.
> ?* Funding:: ? ? ? ? How to help assure funding for free software.
> ?* GNU Project:: ? ? The GNU Project and GNU/Linux.
> @@ -158,6 +159,7 @@ Additional tutorial information is linke
> ?@include headerdirs.texi
> ?@include gty.texi
> ?@include plugins.texi
> +@include lto.texi
> ?@include funding.texi
> ?@include gnu.texi
> Index: doc/invoke.texi
> ===================================================================
> --- doc/invoke.texi ? ? (revision 166733)
> +++ doc/invoke.texi ? ? (working copy)
> @@ -356,8 +356,8 @@ Objective-C and Objective-C++ Dialects}.
> ?-fno-ira-share-spill-slots -fira-verbose=@var{n} @gol
> ?-fivopts -fkeep-inline-functions -fkeep-static-consts @gol
> ?-floop-block -floop-flatten -floop-interchange -floop-strip-mine @gol
> --floop-parallelize-all -flto -flto-compression-level -flto-partition=@var{alg} @gol
> --flto-report -fltrans -fltrans-output-list -fmerge-all-constants @gol
> +-floop-parallelize-all -flto -flto-compression-level
> +-flto-partition=@var{alg} -flto-report -fmerge-all-constants @gol
> ?-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves @gol
> ?-fmove-loop-invariants fmudflap -fmudflapir -fmudflapth -fno-branch-count-reg @gol
> ?-fno-default-inline @gol
> @@ -399,7 +399,7 @@ Objective-C and Objective-C++ Dialects}.
> ?-funit-at-a-time -funroll-all-loops -funroll-loops @gol
> ?-funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops @gol
> ?-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb @gol
> --fwhole-program -fwhopr[=@var{n}] -fwpa -fuse-linker-plugin @gol
> +-fwhole-program -fwpa -fuse-linker-plugin @gol
> ?--param @var{name}=@var{value}
> ?-O ?-O0 ?-O1 ?-O2 ?-O3 ?-Os -Ofast}
> @@ -7489,6 +7489,16 @@ The only important thing to keep in mind
> ?optimizations the @option{-flto} flag needs to be passed to both the
> ?compile and the link commands.
> +To make whole program optimization effective, it is necesary to make
> +certain whole program assumptions. ?The compiler needs to know
> +what functions and variables can be accessed by libraries and runtime
> +outside of the link time optimized unit. ?When supported by the linker,
> +the linker plugin (see @option{-fuse-linker-plugin}) passes to the
> +compiler information about used and externally visible symbols. ?When
> +the linker plugin is not available, @option{-fwhole-program} should be
> +used to allow the compiler to make these assumptions, which will lead
> +to more aggressive optimization decisions.
> +
> ?Note that when a file is compiled with @option{-flto}, the generated
> ?object file will be larger than a regular object file because it will
> ?contain GIMPLE bytecodes and the usual final code. ?This means that
> @@ -7601,16 +7611,18 @@ GCC will not work with an older/newer ve
> ?Link time optimization does not play well with generating debugging
> ?information. ?Combining @option{-flto} with
> -@option{-g} is experimental.
> +@option{-g} is currently experimental and expected to produce wrong
> +results.
> -If you specify the optional @var{n} the link stage is executed in
> -parallel using @var{n} parallel jobs by utilizing an installed
> -@command{make} program. ?The environment variable @env{MAKE} may be
> -used to override the program used.
> +If you specify the optional @var{n}, the optimization and code
> +generation done at link time is executed in parallel using @var{n}
> +parallel jobs by utilizing an installed @command{make} program. ?The
> +environment variable @env{MAKE} may be used to override the program
> +used. ?The default value for @var{n} is 1.
> -You can also specify @option{-fwhopr=jobserver} to use GNU make's
> +You can also specify @option{-flto=jobserver} to use GNU make's
> ?job server mode to determine the number of parallel jobs. This
> -is useful when the Makefile calling GCC is already parallel.
> +is useful when the Makefile calling GCC is already executing in parallel.
> ?The parent Makefile will need a @samp{+} prepended to the command recipe
> ?for this to work. This will likely only work if @env{MAKE} is
> ?GNU make.
> @@ -7619,53 +7631,17 @@ This option is disabled by default.
> ?@item -flto-partition=@var{alg}
> ?@opindex flto-partition
> -Specify partitioning algorithm used by @option{-fwhopr} mode. ?The value is
> -either @code{1to1} to specify partitioning corresponding to source files
> -or @code{balanced} to specify partitioning into, if possible, equally sized
> -chunks. ?Specifying @code{none} as an algorithm disables partitioning
> -and streaming completely.
> -The default value is @code{balanced}.
> -
> -@item -fwpa
> -@opindex fwpa
> -This is an internal option used by GCC when compiling with
> -@option{-fwhopr}. ?You should never need to use it.
> -
> -This option runs the link-time optimizer in the whole-program-analysis
> -(WPA) mode, which reads in summary information from all inputs and
> -performs a whole-program analysis based on summary information only.
> -It generates object files for subsequent runs of the link-time
> -optimizer where individual object files are optimized using both
> -summary information from the WPA mode and the actual function bodies.
> -It then drives the LTRANS phase.
> -
> -Disabled by default.
> -
> -@item -fltrans
> -@opindex fltrans
> -This is an internal option used by GCC when compiling with
> -@option{-fwhopr}. ?You should never need to use it.
> -
> -This option runs the link-time optimizer in the local-transformation (LTRANS)
> -mode, which reads in output from a previous run of the LTO in WPA mode.
> -In the LTRANS mode, LTO optimizes an object and produces the final assembly.
> -
> -Disabled by default.
> -
> -@item -fltrans-output-list=@var{file}
> -@opindex fltrans-output-list
> -This is an internal option used by GCC when compiling with
> -@option{-fwhopr}. ?You should never need to use it.
> -
> -This option specifies a file to which the names of LTRANS output files are
> -written. ?This option is only meaningful in conjunction with @option{-fwpa}.
> -
> -Disabled by default.
> +Specify the partitioning algorithm used by the link time optimizer.
> +The value is either @code{1to1} to specify a partitioning mirroring
> +the original source files or @code{balanced} to specify partitioning
> +into equally sized chunks (whenever possible). ?Specifying @code{none}
> +as an algorithm disables partitioning and streaming completely. The
> +default value is @code{balanced}.
> ?@item -flto-compression-level=@var{n}
> ?This option specifies the level of compression used for intermediate
> ?language written to LTO object files, and is only meaningful in
> -conjunction with LTO mode (@option{-fwhopr}, @option{-flto}). ?Valid
> +conjunction with LTO mode (@option{-flto}). ?Valid
> ?values are 0 (no compression) to 9 (maximum compression). ?Values
> ?outside this range are clamped to either 0 or 9. ?If the option is not
> ?given, a default balanced compression setting is used.
> @@ -7674,7 +7650,7 @@ given, a default balanced compression se
> ?Prints a report with internal details on the workings of the link-time
> ?optimizer. ?The contents of this report vary from version to version,
> ?it is meant to be useful to GCC developers when processing object
> -files in LTO mode (via @option{-fwhopr} or @option{-flto}).
> +files in LTO mode (via @option{-flto}).
> ?Disabled by default.

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