xref: /freebsd-10.0-release/contrib/gcc/doc/passes.texi

@c Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
@c 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node Passes
@chapter Passes and Files of the Compiler
@cindex passes and files of the compiler
@cindex files and passes of the compiler
@cindex compiler passes and files

@cindex top level of compiler
The overall control structure of the compiler is in @file{toplev.c}.  This
file is responsible for initialization, decoding arguments, opening and
closing files, and sequencing the passes.  Routines for emitting
diagnostic messages are defined in @file{diagnostic.c}.  The files
@file{pretty-print.h} and @file{pretty-print.c} provide basic support
for language-independent pretty-printing.

@cindex parsing pass
The parsing pass is invoked only once, to parse the entire input.  A
high level tree representation is then generated from the input,
one function at a time.  This tree code is then transformed into RTL
intermediate code, and processed.  The files involved in transforming
the trees into RTL are @file{expr.c}, @file{expmed.c}, and
@file{stmt.c}.
@c Note, the above files aren't strictly the only files involved. It's
@c all over the place (function.c, final.c,etc).  However, those are
@c the files that are supposed to be directly involved, and have
@c their purpose listed as such, so i've only listed them.
The order of trees that are processed, is not
necessarily the same order they are generated from
the input, due to deferred inlining, and other considerations.

@findex rest_of_compilation
@findex rest_of_decl_compilation
Each time the parsing pass reads a complete function definition or
top-level declaration, it calls either the function
@code{rest_of_compilation}, or the function
@code{rest_of_decl_compilation} in @file{toplev.c}, which are
responsible for all further processing necessary, ending with output of
the assembler language.  All other compiler passes run, in sequence,
within @code{rest_of_compilation}.  When that function returns from
compiling a function definition, the storage used for that function
definition's compilation is entirely freed, unless it is an inline
function, or was deferred for some reason (this can occur in
templates, for example).
(@pxref{Inline,,An Inline Function is As Fast As a Macro,gcc,Using the
GNU Compiler Collection (GCC)}).

Here is a list of all the passes of the compiler and their source files.
Also included is a description of where debugging dumps can be requested
with @option{-d} options.

@itemize @bullet
@item
Parsing.  This pass reads the entire text of a function definition,
constructing a high level tree representation.  (Because of the semantic
analysis that takes place during this pass, it does more than is
formally considered to be parsing.)

The tree representation does not entirely follow C syntax, because it is
intended to support other languages as well.

Language-specific data type analysis is also done in this pass, and every
tree node that represents an expression has a data type attached.
Variables are represented as declaration nodes.

The language-independent source files for parsing are
@file{tree.c}, @file{fold-const.c}, and @file{stor-layout.c}.
There are also header files @file{tree.h} and @file{tree.def}
which define the format of the tree representation.

C preprocessing, for language front ends, that want or require it, is
performed by cpplib, which is covered in separate documentation.  In
particular, the internals are covered in @xref{Top, ,Cpplib internals,
cppinternals, Cpplib Internals}.

The source files to parse C are found in the toplevel directory, and
by convention are named @file{c-*}.  Some of these are also used by
the other C-like languages: @file{c-common.c},
@file{c-common.def},
@file{c-format.c},
@file{c-opts.c},
@file{c-pragma.c},
@file{c-semantics.c},
@file{c-lex.c},
@file{c-incpath.c},
@file{c-ppoutput.c},
@file{c-cppbuiltin.c},
@file{c-common.h},
@file{c-dump.h},
@file{c.opt},
@file{c-incpath.h}
and
@file{c-pragma.h},

Files specific to each language are in subdirectories named after the
language in question, like @file{ada}, @file{objc}, @file{cp} (for C++).

@cindex Tree optimization
@item
Tree optimization.   This is the optimization of the tree
representation, before converting into RTL code.

@cindex inline on trees, automatic
Currently, the main optimization performed here is tree-based
inlining.
This is implemented in @file{tree-inline.c} and used by both C and C++.
Note that tree based inlining turns off rtx based inlining (since it's more
powerful, it would be a waste of time to do rtx based inlining in
addition).

@cindex constant folding
@cindex arithmetic simplifications
@cindex simplifications, arithmetic
Constant folding and some arithmetic simplifications are also done
during this pass, on the tree representation.
The routines that perform these tasks are located in @file{fold-const.c}.

@cindex RTL generation
@item
RTL generation.  This is the conversion of syntax tree into RTL code.

@cindex target-parameter-dependent code
This is where the bulk of target-parameter-dependent code is found,
since often it is necessary for strategies to apply only when certain
standard kinds of instructions are available.  The purpose of named
instruction patterns is to provide this information to the RTL
generation pass.

@cindex tail recursion optimization
Optimization is done in this pass for @code{if}-conditions that are
comparisons, boolean operations or conditional expressions.  Tail
recursion is detected at this time also.  Decisions are made about how
best to arrange loops and how to output @code{switch} statements.

@c Avoiding overfull is tricky here.
The source files for RTL generation include
@file{stmt.c},
@file{calls.c},
@file{expr.c},
@file{explow.c},
@file{expmed.c},
@file{function.c},
@file{optabs.c}
and @file{emit-rtl.c}.
Also, the file
@file{insn-emit.c}, generated from the machine description by the
program @code{genemit}, is used in this pass.  The header file
@file{expr.h} is used for communication within this pass.

@findex genflags
@findex gencodes
The header files @file{insn-flags.h} and @file{insn-codes.h},
generated from the machine description by the programs @code{genflags}
and @code{gencodes}, tell this pass which standard names are available
for use and which patterns correspond to them.

Aside from debugging information output, none of the following passes
refers to the tree structure representation of the function (only
part of which is saved).

@cindex inline on rtx, automatic
The decision of whether the function can and should be expanded inline
in its subsequent callers is made at the end of rtl generation.  The
function must meet certain criteria, currently related to the size of
the function and the types and number of parameters it has.  Note that
this function may contain loops, recursive calls to itself
(tail-recursive functions can be inlined!), gotos, in short, all
constructs supported by GCC@.  The file @file{integrate.c} contains
the code to save a function's rtl for later inlining and to inline that
rtl when the function is called.  The header file @file{integrate.h}
is also used for this purpose.

@opindex dr
The option @option{-dr} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.rtl} to
the input file name.

@c Should the exception handling pass be talked about here?

@cindex sibling call optimization
@item
Sibling call optimization.   This pass performs tail recursion
elimination, and tail and sibling call optimizations.  The purpose of
these optimizations is to reduce the overhead of function calls,
whenever possible.

The source file of this pass is @file{sibcall.c}

@opindex di
The option @option{-di} causes a debugging dump of the RTL code after
this pass is run.  This dump file's name is made by appending
@samp{.sibling} to the input file name.

@cindex jump optimization
@cindex unreachable code
@cindex dead code
@item
Jump optimization.  This pass simplifies jumps to the following
instruction, jumps across jumps, and jumps to jumps.  It deletes
unreferenced labels and unreachable code, except that unreachable code
that contains a loop is not recognized as unreachable in this pass.
(Such loops are deleted later in the basic block analysis.)  It also
converts some code originally written with jumps into sequences of
instructions that directly set values from the results of comparisons,
if the machine has such instructions.

Jump optimization is performed two or three times.  The first time is
immediately following RTL generation.  The second time is after CSE,
but only if CSE says repeated jump optimization is needed.  The
last time is right before the final pass.  That time, cross-jumping
and deletion of no-op move instructions are done together with the
optimizations described above.

The source file of this pass is @file{jump.c}.

@opindex dj
The option @option{-dj} causes a debugging dump of the RTL code after
this pass is run for the first time.  This dump file's name is made by
appending @samp{.jump} to the input file name.


@cindex register use analysis
@item
Register scan.  This pass finds the first and last use of each
register, as a guide for common subexpression elimination.  Its source
is in @file{regclass.c}.

@cindex jump threading
@item
@opindex fthread-jumps
Jump threading.  This pass detects a condition jump that branches to an
identical or inverse test.  Such jumps can be @samp{threaded} through
the second conditional test.  The source code for this pass is in
@file{jump.c}.  This optimization is only performed if
@option{-fthread-jumps} is enabled.

@cindex common subexpression elimination
@cindex constant propagation
@item
Common subexpression elimination.  This pass also does constant
propagation.  Its source files are @file{cse.c}, and @file{cselib.c}.
If constant  propagation causes conditional jumps to become
unconditional or to become no-ops, jump optimization is run again when
CSE is finished.

@opindex ds
The option @option{-ds} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.cse} to
the input file name.

@cindex global common subexpression elimination
@cindex constant propagation
@cindex copy propagation
@item
Global common subexpression elimination.  This pass performs two
different types of GCSE  depending on whether you are optimizing for
size or not (LCM based GCSE tends to increase code size for a gain in
speed, while Morel-Renvoise based GCSE does not).
When optimizing for size, GCSE is done using Morel-Renvoise Partial
Redundancy Elimination, with the exception that it does not try to move
invariants out of loops---that is left to  the loop optimization pass.
If MR PRE GCSE is done, code hoisting (aka unification) is also done, as
well as load motion.
If you are optimizing for speed, LCM (lazy code motion) based GCSE is
done.  LCM is based on the work of Knoop, Ruthing, and Steffen.  LCM
based GCSE also does loop invariant code motion.  We also perform load
and store motion when optimizing for speed.
Regardless of which type of GCSE is used, the GCSE pass also performs
global constant and  copy propagation.

The source file for this pass is @file{gcse.c}, and the LCM routines
are in @file{lcm.c}.

@opindex dG
The option @option{-dG} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.gcse} to
the input file name.

@cindex loop optimization
@cindex code motion
@cindex strength-reduction
@item
Loop optimization.  This pass moves constant expressions out of loops,
and optionally does strength-reduction and loop unrolling as well.
Its source files are @file{loop.c} and @file{unroll.c}, plus the header
@file{loop.h} used for communication between them.  Loop unrolling uses
some functions in @file{integrate.c} and the header @file{integrate.h}.
Loop dependency analysis routines are contained in @file{dependence.c}.

Second loop optimization pass takes care of basic block level optimizations --
unrolling, peeling and unswitching loops. The source files are
@file{cfgloopanal.c} and @file{cfgloopmanip.c} containing generic loop
analysis and manipulation code, @file{loop-init.c} with initialization and
finalization code, @file{loop-unswitch.c} for loop unswitching and
@file{loop-unroll.c} for loop unrolling and peeling.

@opindex dL
The option @option{-dL} causes a debugging dump of the RTL code after
these passes.  The dump file names are made by appending @samp{.loop} and
@samp{.loop2} to the input file name.

@cindex jump bypassing
@item
Jump bypassing.  This pass is an aggressive form of GCSE that transforms
the control flow graph of a function by propagating constants into
conditional branch instructions.

The source file for this pass is @file{gcse.c}.

@opindex dG
The option @option{-dG} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.bypass}
to the input file name.

@cindex web construction
@item
Simple optimization pass that splits independent uses of each pseudo
increasing effect of other optimizations.  This can improve effect of the
other transformation, such as CSE or register allocation.
Its source files are @file{web.c}.

@opindex dZ
The option @option{-dZ} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.web} to
the input file name.

@item
@opindex frerun-cse-after-loop
If @option{-frerun-cse-after-loop} was enabled, a second common
subexpression elimination pass is performed after the loop optimization
pass.  Jump threading is also done again at this time if it was specified.

@opindex dt
The option @option{-dt} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.cse2} to
the input file name.

@cindex data flow analysis
@cindex analysis, data flow
@cindex basic blocks
@item
Data flow analysis (@file{flow.c}).  This pass divides the program
into basic blocks (and in the process deletes unreachable loops); then
it computes which pseudo-registers are live at each point in the
program, and makes the first instruction that uses a value point at
the instruction that computed the value.

@cindex autoincrement/decrement analysis
This pass also deletes computations whose results are never used, and
combines memory references with add or subtract instructions to make
autoincrement or autodecrement addressing.

@opindex df
The option @option{-df} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.flow} to
the input file name.  If stupid register allocation is in use, this
dump file reflects the full results of such allocation.

@cindex instruction combination
@item
Instruction combination (@file{combine.c}).  This pass attempts to
combine groups of two or three instructions that are related by data
flow into single instructions.  It combines the RTL expressions for
the instructions by substitution, simplifies the result using algebra,
and then attempts to match the result against the machine description.

@opindex dc
The option @option{-dc} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.combine}
to the input file name.

@cindex if conversion
@item
If-conversion is a transformation that transforms control dependencies
into data dependencies (IE it transforms conditional code into a
single control stream).
It is implemented in the file @file{ifcvt.c}.

@opindex dE
The option @option{-dE} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.ce} to
the input file name.

@cindex register movement
@item
Register movement (@file{regmove.c}).  This pass looks for cases where
matching constraints would force an instruction to need a reload, and
this reload would be a register-to-register move.  It then attempts
to change the registers used by the instruction to avoid the move
instruction.

@opindex dN
The option @option{-dN} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.regmove}
to the input file name.

@cindex instruction scheduling
@cindex scheduling, instruction
@item
Instruction scheduling (@file{sched.c}).  This pass looks for
instructions whose output will not be available by the time that it is
used in subsequent instructions.  (Memory loads and floating point
instructions often have this behavior on RISC machines).  It re-orders
instructions within a basic block to try to separate the definition and
use of items that otherwise would cause pipeline stalls.

Instruction scheduling is performed twice.  The first time is immediately
after instruction combination and the second is immediately after reload.

@opindex dS
The option @option{-dS} causes a debugging dump of the RTL code after this
pass is run for the first time.  The dump file's name is made by
appending @samp{.sched} to the input file name.

@cindex register allocation
@item
Register allocation.  These passes make sure that all occurrences of pseudo
registers are eliminated, either by allocating them to a hard register,
replacing them by an equivalent expression (e.g.@: a constant) or by placing
them on the stack.  This is done in several subpasses:

@itemize @bullet
@cindex register class preference pass
@item
Register class preferencing.  The RTL code is scanned to find out
which register class is best for each pseudo register.  The source
file is @file{regclass.c}.

@cindex local register allocation
@item
Local register allocation (@file{local-alloc.c}).  This pass allocates
hard registers to pseudo registers that are used only within one basic
block.  Because the basic block is linear, it can use fast and
powerful techniques to do a very good job.

@opindex dl
The option @option{-dl} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.lreg} to
the input file name.

@cindex global register allocation
@item
Global register allocation (@file{global.c}).  This pass
allocates hard registers for the remaining pseudo registers (those
whose life spans are not contained in one basic block).

@cindex graph coloring register allocation
@opindex fnew-ra
@opindex dl
@item
Graph coloring register allocator.  The files @file{ra.c}, @file{ra-build.c},
@file{ra-colorize.c}, @file{ra-debug.c}, @file{ra-rewrite.c} together with
the header @file{ra.h} contain another register allocator, which is used
when the option @option{-fnew-ra} is given.  In that case it is run instead
of the above mentioned local and global register allocation passes, and the
option @option{-dl} causes a debugging dump of its work.

@cindex reloading
@item
Reloading.  This pass renumbers pseudo registers with the hardware
registers numbers they were allocated.  Pseudo registers that did not
get hard registers are replaced with stack slots.  Then it finds
instructions that are invalid because a value has failed to end up in
a register, or has ended up in a register of the wrong kind.  It fixes
up these instructions by reloading the problematical values
temporarily into registers.  Additional instructions are generated to
do the copying.

The reload pass also optionally eliminates the frame pointer and inserts
instructions to save and restore call-clobbered registers around calls.

Source files are @file{reload.c} and @file{reload1.c}, plus the header
@file{reload.h} used for communication between them.

@opindex dg
The option @option{-dg} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.greg} to
the input file name.
@end itemize

@cindex instruction scheduling
@cindex scheduling, instruction
@item
Instruction scheduling is repeated here to try to avoid pipeline stalls
due to memory loads generated for spilled pseudo registers.

@opindex dR
The option @option{-dR} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.sched2}
to the input file name.

@cindex basic block reordering
@cindex reordering, block
@item
Basic block reordering.  This pass implements profile guided code
positioning.  If profile information is not available, various types of
static analysis are performed to make the predictions normally coming
from the profile feedback (IE execution frequency, branch probability,
etc).  It is implemented in the file @file{bb-reorder.c}, and the
various prediction routines are in @file{predict.c}.

@opindex dB
The option @option{-dB} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.bbro} to
the input file name.

@cindex delayed branch scheduling
@cindex scheduling, delayed branch
@item
Delayed branch scheduling.  This optional pass attempts to find
instructions that can go into the delay slots of other instructions,
usually jumps and calls.  The source file name is @file{reorg.c}.

@opindex dd
The option @option{-dd} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.dbr}
to the input file name.

@cindex branch shortening
@item
Branch shortening.  On many RISC machines, branch instructions have a
limited range.  Thus, longer sequences of instructions must be used for
long branches.  In this pass, the compiler figures out what how far each
instruction will be from each other instruction, and therefore whether
the usual instructions, or the longer sequences, must be used for each
branch.

@cindex register-to-stack conversion
@item
Conversion from usage of some hard registers to usage of a register
stack may be done at this point.  Currently, this is supported only
for the floating-point registers of the Intel 80387 coprocessor.   The
source file name is @file{reg-stack.c}.

@opindex dk
The options @option{-dk} causes a debugging dump of the RTL code after
this pass.  This dump file's name is made by appending @samp{.stack}
to the input file name.

@cindex final pass
@cindex peephole optimization
@item
Final.  This pass outputs the assembler code for the function.  It is
also responsible for identifying spurious test and compare
instructions.  Machine-specific peephole optimizations are performed
at the same time.  The function entry and exit sequences are generated
directly as assembler code in this pass; they never exist as RTL@.

The source files are @file{final.c} plus @file{insn-output.c}; the
latter is generated automatically from the machine description by the
tool @file{genoutput}.  The header file @file{conditions.h} is used
for communication between these files.

@cindex debugging information generation
@item
Debugging information output.  This is run after final because it must
output the stack slot offsets for pseudo registers that did not get
hard registers.  Source files are @file{dbxout.c} for DBX symbol table
format, @file{sdbout.c} for SDB symbol table format,  @file{dwarfout.c}
for DWARF symbol table format, files @file{dwarf2out.c} and
@file{dwarf2asm.c} for DWARF2 symbol table format, and @file{vmsdbgout.c}
for VMS debug symbol table format.
@end itemize

Some additional files are used by all or many passes:

@itemize @bullet
@item
Every pass uses @file{machmode.def} and @file{machmode.h} which define
the machine modes.

@item
Several passes use @file{real.h}, which defines the default
representation of floating point constants and how to operate on them.

@item
All the passes that work with RTL use the header files @file{rtl.h}
and @file{rtl.def}, and subroutines in file @file{rtl.c}.  The tools
@code{gen*} also use these files to read and work with the machine
description RTL@.

@item
All the tools that read the machine description use support routines
found in @file{gensupport.c}, @file{errors.c}, and @file{read-rtl.c}.

@findex genconfig
@item
Several passes refer to the header file @file{insn-config.h} which
contains a few parameters (C macro definitions) generated
automatically from the machine description RTL by the tool
@code{genconfig}.

@cindex instruction recognizer
@item
Several passes use the instruction recognizer, which consists of
@file{recog.c} and @file{recog.h}, plus the files @file{insn-recog.c}
and @file{insn-extract.c} that are generated automatically from the
machine description by the tools @file{genrecog} and
@file{genextract}.

@item
Several passes use the header files @file{regs.h} which defines the
information recorded about pseudo register usage, and @file{basic-block.h}
which defines the information recorded about basic blocks.

@item
@file{hard-reg-set.h} defines the type @code{HARD_REG_SET}, a bit-vector
with a bit for each hard register, and some macros to manipulate it.
This type is just @code{int} if the machine has few enough hard registers;
otherwise it is an array of @code{int} and some of the macros expand
into loops.

@item
Several passes use instruction attributes.  A definition of the
attributes defined for a particular machine is in file
@file{insn-attr.h}, which is generated from the machine description by
the program @file{genattr}.  The file @file{insn-attrtab.c} contains
subroutines to obtain the attribute values for insns and information
about processor pipeline characteristics for the instruction
scheduler.  It is generated from the machine description by the
program @file{genattrtab}.
@end itemize