This is Info file gcc.info, produced by Makeinfo version 1.68 from the
input file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

   Published by the Free Software Foundation 59 Temple Place - Suite 330
Boston, MA 02111-1307 USA

   Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998
Free Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.


File: gcc.info,  Node: Insns,  Next: Calls,  Prev: Assembler,  Up: RTL

Insns
=====

   The RTL representation of the code for a function is a doubly-linked
chain of objects called "insns".  Insns are expressions with special
codes that are used for no other purpose.  Some insns are actual
instructions; others represent dispatch tables for `switch' statements;
others represent labels to jump to or various sorts of declarative
information.

   In addition to its own specific data, each insn must have a unique
id-number that distinguishes it from all other insns in the current
function (after delayed branch scheduling, copies of an insn with the
same id-number may be present in multiple places in a function, but
these copies will always be identical and will only appear inside a
`sequence'), and chain pointers to the preceding and following insns.
These three fields occupy the same position in every insn, independent
of the expression code of the insn.  They could be accessed with `XEXP'
and `XINT', but instead three special macros are always used:

`INSN_UID (I)'
     Accesses the unique id of insn I.

`PREV_INSN (I)'
     Accesses the chain pointer to the insn preceding I.  If I is the
     first insn, this is a null pointer.

`NEXT_INSN (I)'
     Accesses the chain pointer to the insn following I.  If I is the
     last insn, this is a null pointer.

   The first insn in the chain is obtained by calling `get_insns'; the
last insn is the result of calling `get_last_insn'.  Within the chain
delimited by these insns, the `NEXT_INSN' and `PREV_INSN' pointers must
always correspond: if INSN is not the first insn,

     NEXT_INSN (PREV_INSN (INSN)) == INSN

is always true and if INSN is not the last insn,

     PREV_INSN (NEXT_INSN (INSN)) == INSN

is always true.

   After delay slot scheduling, some of the insns in the chain might be
`sequence' expressions, which contain a vector of insns.  The value of
`NEXT_INSN' in all but the last of these insns is the next insn in the
vector; the value of `NEXT_INSN' of the last insn in the vector is the
same as the value of `NEXT_INSN' for the `sequence' in which it is
contained.  Similar rules apply for `PREV_INSN'.

   This means that the above invariants are not necessarily true for
insns inside `sequence' expressions.  Specifically, if INSN is the
first insn in a `sequence', `NEXT_INSN (PREV_INSN (INSN))' is the insn
containing the `sequence' expression, as is the value of `PREV_INSN
(NEXT_INSN (INSN))' is INSN is the last insn in the `sequence'
expression.  You can use these expressions to find the containing
`sequence' expression.

   Every insn has one of the following six expression codes:

`insn'
     The expression code `insn' is used for instructions that do not
     jump and do not do function calls.  `sequence' expressions are
     always contained in insns with code `insn' even if one of those
     insns should jump or do function calls.

     Insns with code `insn' have four additional fields beyond the three
     mandatory ones listed above.  These four are described in a table
     below.

`jump_insn'
     The expression code `jump_insn' is used for instructions that may
     jump (or, more generally, may contain `label_ref' expressions).  If
     there is an instruction to return from the current function, it is
     recorded as a `jump_insn'.

     `jump_insn' insns have the same extra fields as `insn' insns,
     accessed in the same way and in addition contain a field
     `JUMP_LABEL' which is defined once jump optimization has completed.

     For simple conditional and unconditional jumps, this field
     contains the `code_label' to which this insn will (possibly
     conditionally) branch.  In a more complex jump, `JUMP_LABEL'
     records one of the labels that the insn refers to; the only way to
     find the others is to scan the entire body of the insn.

     Return insns count as jumps, but since they do not refer to any
     labels, they have zero in the `JUMP_LABEL' field.

`call_insn'
     The expression code `call_insn' is used for instructions that may
     do function calls.  It is important to distinguish these
     instructions because they imply that certain registers and memory
     locations may be altered unpredictably.

     `call_insn' insns have the same extra fields as `insn' insns,
     accessed in the same way and in addition contain a field
     `CALL_INSN_FUNCTION_USAGE', which contains a list (chain of
     `expr_list' expressions) containing `use' and `clobber'
     expressions that denote hard registers used or clobbered by the
     called function.  A register specified in a `clobber' in this list
     is modified *after* the execution of the `call_insn', while a
     register in a `clobber' in the body of the `call_insn' is
     clobbered before the insn completes execution.  `clobber'
     expressions in this list augment registers specified in
     `CALL_USED_REGISTERS' (*note Register Basics::.).

`code_label'
     A `code_label' insn represents a label that a jump insn can jump
     to.  It contains two special fields of data in addition to the
     three standard ones.  `CODE_LABEL_NUMBER' is used to hold the
     "label number", a number that identifies this label uniquely among
     all the labels in the compilation (not just in the current
     function).  Ultimately, the label is represented in the assembler
     output as an assembler label, usually of the form `LN' where N is
     the label number.

     When a `code_label' appears in an RTL expression, it normally
     appears within a `label_ref' which represents the address of the
     label, as a number.

     The field `LABEL_NUSES' is only defined once the jump optimization
     phase is completed and contains the number of times this label is
     referenced in the current function.

`barrier'
     Barriers are placed in the instruction stream when control cannot
     flow past them.  They are placed after unconditional jump
     instructions to indicate that the jumps are unconditional and
     after calls to `volatile' functions, which do not return (e.g.,
     `exit').  They contain no information beyond the three standard
     fields.

`note'
     `note' insns are used to represent additional debugging and
     declarative information.  They contain two nonstandard fields, an
     integer which is accessed with the macro `NOTE_LINE_NUMBER' and a
     string accessed with `NOTE_SOURCE_FILE'.

     If `NOTE_LINE_NUMBER' is positive, the note represents the
     position of a source line and `NOTE_SOURCE_FILE' is the source
     file name that the line came from.  These notes control generation
     of line number data in the assembler output.

     Otherwise, `NOTE_LINE_NUMBER' is not really a line number but a
     code with one of the following values (and `NOTE_SOURCE_FILE' must
     contain a null pointer):

    `NOTE_INSN_DELETED'
          Such a note is completely ignorable.  Some passes of the
          compiler delete insns by altering them into notes of this
          kind.

    `NOTE_INSN_BLOCK_BEG'
    `NOTE_INSN_BLOCK_END'
          These types of notes indicate the position of the beginning
          and end of a level of scoping of variable names.  They
          control the output of debugging information.

    `NOTE_INSN_EH_REGION_BEG'
    `NOTE_INSN_EH_REGION_END'
          These types of notes indicate the position of the beginning
          and end of a level of scoping for exception handling.
          `NOTE_BLOCK_NUMBER' identifies which `CODE_LABEL' is
          associated with the given region.

    `NOTE_INSN_LOOP_BEG'
    `NOTE_INSN_LOOP_END'
          These types of notes indicate the position of the beginning
          and end of a `while' or `for' loop.  They enable the loop
          optimizer to find loops quickly.

    `NOTE_INSN_LOOP_CONT'
          Appears at the place in a loop that `continue' statements
          jump to.

    `NOTE_INSN_LOOP_VTOP'
          This note indicates the place in a loop where the exit test
          begins for those loops in which the exit test has been
          duplicated.  This position becomes another virtual start of
          the loop when considering loop invariants.

    `NOTE_INSN_FUNCTION_END'
          Appears near the end of the function body, just before the
          label that `return' statements jump to (on machine where a
          single instruction does not suffice for returning).  This
          note may be deleted by jump optimization.

    `NOTE_INSN_SETJMP'
          Appears following each call to `setjmp' or a related function.

     These codes are printed symbolically when they appear in debugging
     dumps.

   The machine mode of an insn is normally `VOIDmode', but some phases
use the mode for various purposes; for example, the reload pass sets it
to `HImode' if the insn needs reloading but not register elimination
and `QImode' if both are required.  The common subexpression
elimination pass sets the mode of an insn to `QImode' when it is the
first insn in a block that has already been processed.

   Here is a table of the extra fields of `insn', `jump_insn' and
`call_insn' insns:

`PATTERN (I)'
     An expression for the side effect performed by this insn.  This
     must be one of the following codes: `set', `call', `use',
     `clobber', `return', `asm_input', `asm_output', `addr_vec',
     `addr_diff_vec', `trap_if', `unspec', `unspec_volatile',
     `parallel', or `sequence'.  If it is a `parallel', each element of
     the `parallel' must be one these codes, except that `parallel'
     expressions cannot be nested and `addr_vec' and `addr_diff_vec'
     are not permitted inside a `parallel' expression.

`INSN_CODE (I)'
     An integer that says which pattern in the machine description
     matches this insn, or -1 if the matching has not yet been
     attempted.

     Such matching is never attempted and this field remains -1 on an
     insn whose pattern consists of a single `use', `clobber',
     `asm_input', `addr_vec' or `addr_diff_vec' expression.

     Matching is also never attempted on insns that result from an `asm'
     statement.  These contain at least one `asm_operands' expression.
     The function `asm_noperands' returns a non-negative value for such
     insns.

     In the debugging output, this field is printed as a number
     followed by a symbolic representation that locates the pattern in
     the `md' file as some small positive or negative offset from a
     named pattern.

`LOG_LINKS (I)'
     A list (chain of `insn_list' expressions) giving information about
     dependencies between instructions within a basic block.  Neither a
     jump nor a label may come between the related insns.

`REG_NOTES (I)'
     A list (chain of `expr_list' and `insn_list' expressions) giving
     miscellaneous information about the insn.  It is often information
     pertaining to the registers used in this insn.

   The `LOG_LINKS' field of an insn is a chain of `insn_list'
expressions.  Each of these has two operands: the first is an insn, and
the second is another `insn_list' expression (the next one in the
chain).  The last `insn_list' in the chain has a null pointer as second
operand.  The significant thing about the chain is which insns appear
in it (as first operands of `insn_list' expressions).  Their order is
not significant.

   This list is originally set up by the flow analysis pass; it is a
null pointer until then.  Flow only adds links for those data
dependencies which can be used for instruction combination.  For each
insn, the flow analysis pass adds a link to insns which store into
registers values that are used for the first time in this insn.  The
instruction scheduling pass adds extra links so that every dependence
will be represented.  Links represent data dependencies,
antidependencies and output dependencies; the machine mode of the link
distinguishes these three types: antidependencies have mode
`REG_DEP_ANTI', output dependencies have mode `REG_DEP_OUTPUT', and
data dependencies have mode `VOIDmode'.

   The `REG_NOTES' field of an insn is a chain similar to the
`LOG_LINKS' field but it includes `expr_list' expressions in addition
to `insn_list' expressions.  There are several kinds of register notes,
which are distinguished by the machine mode, which in a register note
is really understood as being an `enum reg_note'.  The first operand OP
of the note is data whose meaning depends on the kind of note.

   The macro `REG_NOTE_KIND (X)' returns the kind of register note.
Its counterpart, the macro `PUT_REG_NOTE_KIND (X, NEWKIND)' sets the
register note type of X to be NEWKIND.

   Register notes are of three classes: They may say something about an
input to an insn, they may say something about an output of an insn, or
they may create a linkage between two insns.  There are also a set of
values that are only used in `LOG_LINKS'.

   These register notes annotate inputs to an insn:

`REG_DEAD'
     The value in OP dies in this insn; that is to say, altering the
     value immediately after this insn would not affect the future
     behavior of the program.

     This does not necessarily mean that the register OP has no useful
     value after this insn since it may also be an output of the insn.
     In such a case, however, a `REG_DEAD' note would be redundant and
     is usually not present until after the reload pass, but no code
     relies on this fact.

`REG_INC'
     The register OP is incremented (or decremented; at this level
     there is no distinction) by an embedded side effect inside this
     insn.  This means it appears in a `post_inc', `pre_inc',
     `post_dec' or `pre_dec' expression.

`REG_NONNEG'
     The register OP is known to have a nonnegative value when this
     insn is reached.  This is used so that decrement and branch until
     zero instructions, such as the m68k dbra, can be matched.

     The `REG_NONNEG' note is added to insns only if the machine
     description has a `decrement_and_branch_until_zero' pattern.

`REG_NO_CONFLICT'
     This insn does not cause a conflict between OP and the item being
     set by this insn even though it might appear that it does.  In
     other words, if the destination register and OP could otherwise be
     assigned the same register, this insn does not prevent that
     assignment.

     Insns with this note are usually part of a block that begins with a
     `clobber' insn specifying a multi-word pseudo register (which will
     be the output of the block), a group of insns that each set one
     word of the value and have the `REG_NO_CONFLICT' note attached,
     and a final insn that copies the output to itself with an attached
     `REG_EQUAL' note giving the expression being computed.  This block
     is encapsulated with `REG_LIBCALL' and `REG_RETVAL' notes on the
     first and last insns, respectively.

`REG_LABEL'
     This insn uses OP, a `code_label', but is not a `jump_insn'.  The
     presence of this note allows jump optimization to be aware that OP
     is, in fact, being used.

   The following notes describe attributes of outputs of an insn:

`REG_EQUIV'
`REG_EQUAL'
     This note is only valid on an insn that sets only one register and
     indicates that that register will be equal to OP at run time; the
     scope of this equivalence differs between the two types of notes.
     The value which the insn explicitly copies into the register may
     look different from OP, but they will be equal at run time.  If the
     output of the single `set' is a `strict_low_part' expression, the
     note refers to the register that is contained in `SUBREG_REG' of
     the `subreg' expression.

     For `REG_EQUIV', the register is equivalent to OP throughout the
     entire function, and could validly be replaced in all its
     occurrences by OP.  ("Validly" here refers to the data flow of the
     program; simple replacement may make some insns invalid.)  For
     example, when a constant is loaded into a register that is never
     assigned any other value, this kind of note is used.

     When a parameter is copied into a pseudo-register at entry to a
     function, a note of this kind records that the register is
     equivalent to the stack slot where the parameter was passed.
     Although in this case the register may be set by other insns, it
     is still valid to replace the register by the stack slot
     throughout the function.

     A `REG_EQUIV' note is also used on an instruction which copies a
     register parameter into a pseudo-register at entry to a function,
     if there is a stack slot where that parameter could be stored.
     Although other insns may set the pseudo-register, it is valid for
     the compiler to replace the pseudo-register by stack slot
     throughout the function, provided the compiler ensures that the
     stack slot is properly initialized by making the replacement in
     the initial copy instruction as well.  This is used on machines
     for which the calling convention allocates stack space for
     register parameters.  See `REG_PARM_STACK_SPACE' in *Note Stack
     Arguments::.

     In the case of `REG_EQUAL', the register that is set by this insn
     will be equal to OP at run time at the end of this insn but not
     necessarily elsewhere in the function.  In this case, OP is
     typically an arithmetic expression.  For example, when a sequence
     of insns such as a library call is used to perform an arithmetic
     operation, this kind of note is attached to the insn that produces
     or copies the final value.

     These two notes are used in different ways by the compiler passes.
     `REG_EQUAL' is used by passes prior to register allocation (such as
     common subexpression elimination and loop optimization) to tell
     them how to think of that value.  `REG_EQUIV' notes are used by
     register allocation to indicate that there is an available
     substitute expression (either a constant or a `mem' expression for
     the location of a parameter on the stack) that may be used in
     place of a register if insufficient registers are available.

     Except for stack homes for parameters, which are indicated by a
     `REG_EQUIV' note and are not useful to the early optimization
     passes and pseudo registers that are equivalent to a memory
     location throughout there entire life, which is not detected until
     later in the compilation, all equivalences are initially indicated
     by an attached `REG_EQUAL' note.  In the early stages of register
     allocation, a `REG_EQUAL' note is changed into a `REG_EQUIV' note
     if OP is a constant and the insn represents the only set of its
     destination register.

     Thus, compiler passes prior to register allocation need only check
     for `REG_EQUAL' notes and passes subsequent to register allocation
     need only check for `REG_EQUIV' notes.

`REG_UNUSED'
     The register OP being set by this insn will not be used in a
     subsequent insn.  This differs from a `REG_DEAD' note, which
     indicates that the value in an input will not be used subsequently.
     These two notes are independent; both may be present for the same
     register.

`REG_WAS_0'
     The single output of this insn contained zero before this insn.
     OP is the insn that set it to zero.  You can rely on this note if
     it is present and OP has not been deleted or turned into a `note';
     its absence implies nothing.

   These notes describe linkages between insns.  They occur in pairs:
one insn has one of a pair of notes that points to a second insn, which
has the inverse note pointing back to the first insn.

`REG_RETVAL'
     This insn copies the value of a multi-insn sequence (for example, a
     library call), and OP is the first insn of the sequence (for a
     library call, the first insn that was generated to set up the
     arguments for the library call).

     Loop optimization uses this note to treat such a sequence as a
     single operation for code motion purposes and flow analysis uses
     this note to delete such sequences whose results are dead.

     A `REG_EQUAL' note will also usually be attached to this insn to
     provide the expression being computed by the sequence.

`REG_LIBCALL'
     This is the inverse of `REG_RETVAL': it is placed on the first
     insn of a multi-insn sequence, and it points to the last one.

`REG_CC_SETTER'
`REG_CC_USER'
     On machines that use `cc0', the insns which set and use `cc0' set
     and use `cc0' are adjacent.  However, when branch delay slot
     filling is done, this may no longer be true.  In this case a
     `REG_CC_USER' note will be placed on the insn setting `cc0' to
     point to the insn using `cc0' and a `REG_CC_SETTER' note will be
     placed on the insn using `cc0' to point to the insn setting `cc0'.

   These values are only used in the `LOG_LINKS' field, and indicate
the type of dependency that each link represents.  Links which indicate
a data dependence (a read after write dependence) do not use any code,
they simply have mode `VOIDmode', and are printed without any
descriptive text.

`REG_DEP_ANTI'
     This indicates an anti dependence (a write after read dependence).

`REG_DEP_OUTPUT'
     This indicates an output dependence (a write after write
     dependence).

   These notes describe information gathered from gcov profile data.
They are stored in the `REG_NOTES' field of an insn as an `expr_list'.

`REG_EXEC_COUNT'
     This is used to indicate the number of times a basic block was
     executed according to the profile data.  The note is attached to
     the first insn in the basic block.

`REG_BR_PROB'
     This is used to specify the ratio of branches to non-branches of a
     branch insn according to the profile data.  The value is stored as
     a value between 0 and REG_BR_PROB_BASE; larger values indicate a
     higher probability that the branch will be taken.

   For convenience, the machine mode in an `insn_list' or `expr_list'
is printed using these symbolic codes in debugging dumps.

   The only difference between the expression codes `insn_list' and
`expr_list' is that the first operand of an `insn_list' is assumed to
be an insn and is printed in debugging dumps as the insn's unique id;
the first operand of an `expr_list' is printed in the ordinary way as
an expression.


File: gcc.info,  Node: Calls,  Next: Sharing,  Prev: Insns,  Up: RTL

RTL Representation of Function-Call Insns
=========================================

   Insns that call subroutines have the RTL expression code `call_insn'.
These insns must satisfy special rules, and their bodies must use a
special RTL expression code, `call'.

   A `call' expression has two operands, as follows:

     (call (mem:FM ADDR) NBYTES)

Here NBYTES is an operand that represents the number of bytes of
argument data being passed to the subroutine, FM is a machine mode
(which must equal as the definition of the `FUNCTION_MODE' macro in the
machine description) and ADDR represents the address of the subroutine.

   For a subroutine that returns no value, the `call' expression as
shown above is the entire body of the insn, except that the insn might
also contain `use' or `clobber' expressions.

   For a subroutine that returns a value whose mode is not `BLKmode',
the value is returned in a hard register.  If this register's number is
R, then the body of the call insn looks like this:

     (set (reg:M R)
          (call (mem:FM ADDR) NBYTES))

This RTL expression makes it clear (to the optimizer passes) that the
appropriate register receives a useful value in this insn.

   When a subroutine returns a `BLKmode' value, it is handled by
passing to the subroutine the address of a place to store the value.
So the call insn itself does not "return" any value, and it has the
same RTL form as a call that returns nothing.

   On some machines, the call instruction itself clobbers some register,
for example to contain the return address.  `call_insn' insns on these
machines should have a body which is a `parallel' that contains both
the `call' expression and `clobber' expressions that indicate which
registers are destroyed.  Similarly, if the call instruction requires
some register other than the stack pointer that is not explicitly
mentioned it its RTL, a `use' subexpression should mention that
register.

   Functions that are called are assumed to modify all registers listed
in the configuration macro `CALL_USED_REGISTERS' (*note Register
Basics::.) and, with the exception of `const' functions and library
calls, to modify all of memory.

   Insns containing just `use' expressions directly precede the
`call_insn' insn to indicate which registers contain inputs to the
function.  Similarly, if registers other than those in
`CALL_USED_REGISTERS' are clobbered by the called function, insns
containing a single `clobber' follow immediately after the call to
indicate which registers.


File: gcc.info,  Node: Sharing,  Next: Reading RTL,  Prev: Calls,  Up: RTL

Structure Sharing Assumptions
=============================

   The compiler assumes that certain kinds of RTL expressions are
unique; there do not exist two distinct objects representing the same
value.  In other cases, it makes an opposite assumption: that no RTL
expression object of a certain kind appears in more than one place in
the containing structure.

   These assumptions refer to a single function; except for the RTL
objects that describe global variables and external functions, and a
few standard objects such as small integer constants, no RTL objects
are common to two functions.

   * Each pseudo-register has only a single `reg' object to represent
     it, and therefore only a single machine mode.

   * For any symbolic label, there is only one `symbol_ref' object
     referring to it.

   * There is only one `const_int' expression with value 0, only one
     with value 1, and only one with value -1.  Some other integer
     values are also stored uniquely.

   * There is only one `pc' expression.

   * There is only one `cc0' expression.

   * There is only one `const_double' expression with value 0 for each
     floating point mode.  Likewise for values 1 and 2.

   * No `label_ref' or `scratch' appears in more than one place in the
     RTL structure; in other words, it is safe to do a tree-walk of all
     the insns in the function and assume that each time a `label_ref'
     or `scratch' is seen it is distinct from all others that are seen.

   * Only one `mem' object is normally created for each static variable
     or stack slot, so these objects are frequently shared in all the
     places they appear.  However, separate but equal objects for these
     variables are occasionally made.

   * When a single `asm' statement has multiple output operands, a
     distinct `asm_operands' expression is made for each output operand.
     However, these all share the vector which contains the sequence of
     input operands.  This sharing is used later on to test whether two
     `asm_operands' expressions come from the same statement, so all
     optimizations must carefully preserve the sharing if they copy the
     vector at all.

   * No RTL object appears in more than one place in the RTL structure
     except as described above.  Many passes of the compiler rely on
     this by assuming that they can modify RTL objects in place without
     unwanted side-effects on other insns.

   * During initial RTL generation, shared structure is freely
     introduced.  After all the RTL for a function has been generated,
     all shared structure is copied by `unshare_all_rtl' in
     `emit-rtl.c', after which the above rules are guaranteed to be
     followed.

   * During the combiner pass, shared structure within an insn can exist
     temporarily.  However, the shared structure is copied before the
     combiner is finished with the insn.  This is done by calling
     `copy_rtx_if_shared', which is a subroutine of `unshare_all_rtl'.


File: gcc.info,  Node: Reading RTL,  Prev: Sharing,  Up: RTL

Reading RTL
===========

   To read an RTL object from a file, call `read_rtx'.  It takes one
argument, a stdio stream, and returns a single RTL object.

   Reading RTL from a file is very slow.  This is not currently a
problem since reading RTL occurs only as part of building the compiler.

   People frequently have the idea of using RTL stored as text in a
file as an interface between a language front end and the bulk of GNU
CC.  This idea is not feasible.

   GNU CC was designed to use RTL internally only.  Correct RTL for a
given program is very dependent on the particular target machine.  And
the RTL does not contain all the information about the program.

   The proper way to interface GNU CC to a new language front end is
with the "tree" data structure.  There is no manual for this data
structure, but it is described in the files `tree.h' and `tree.def'.


File: gcc.info,  Node: Machine Desc,  Next: Target Macros,  Prev: RTL,  Up: Top

Machine Descriptions
********************

   A machine description has two parts: a file of instruction patterns
(`.md' file) and a C header file of macro definitions.

   The `.md' file for a target machine contains a pattern for each
instruction that the target machine supports (or at least each
instruction that is worth telling the compiler about).  It may also
contain comments.  A semicolon causes the rest of the line to be a
comment, unless the semicolon is inside a quoted string.

   See the next chapter for information on the C header file.

* Menu:

* Patterns::            How to write instruction patterns.
* Example::             An explained example of a `define_insn' pattern.
* RTL Template::        The RTL template defines what insns match a pattern.
* Output Template::     The output template says how to make assembler code
                          from such an insn.
* Output Statement::    For more generality, write C code to output
                          the assembler code.
* Constraints::         When not all operands are general operands.
* Standard Names::      Names mark patterns to use for code generation.
* Pattern Ordering::    When the order of patterns makes a difference.
* Dependent Patterns::  Having one pattern may make you need another.
* Jump Patterns::       Special considerations for patterns for jump insns.
* Insn Canonicalizations::Canonicalization of Instructions
* Peephole Definitions::Defining machine-specific peephole optimizations.
* Expander Definitions::Generating a sequence of several RTL insns
                         for a standard operation.
* Insn Splitting::    Splitting Instructions into Multiple Instructions
* Insn Attributes::     Specifying the value of attributes for generated insns.


File: gcc.info,  Node: Patterns,  Next: Example,  Up: Machine Desc

Everything about Instruction Patterns
=====================================

   Each instruction pattern contains an incomplete RTL expression, with
pieces to be filled in later, operand constraints that restrict how the
pieces can be filled in, and an output pattern or C code to generate
the assembler output, all wrapped up in a `define_insn' expression.

   A `define_insn' is an RTL expression containing four or five
operands:

  1. An optional name.  The presence of a name indicate that this
     instruction pattern can perform a certain standard job for the
     RTL-generation pass of the compiler.  This pass knows certain
     names and will use the instruction patterns with those names, if
     the names are defined in the machine description.

     The absence of a name is indicated by writing an empty string
     where the name should go.  Nameless instruction patterns are never
     used for generating RTL code, but they may permit several simpler
     insns to be combined later on.

     Names that are not thus known and used in RTL-generation have no
     effect; they are equivalent to no name at all.

  2. The "RTL template" (*note RTL Template::.) is a vector of
     incomplete RTL expressions which show what the instruction should
     look like.  It is incomplete because it may contain
     `match_operand', `match_operator', and `match_dup' expressions
     that stand for operands of the instruction.

     If the vector has only one element, that element is the template
     for the instruction pattern.  If the vector has multiple elements,
     then the instruction pattern is a `parallel' expression containing
     the elements described.

  3. A condition.  This is a string which contains a C expression that
     is the final test to decide whether an insn body matches this
     pattern.

     For a named pattern, the condition (if present) may not depend on
     the data in the insn being matched, but only the
     target-machine-type flags.  The compiler needs to test these
     conditions during initialization in order to learn exactly which
     named instructions are available in a particular run.

     For nameless patterns, the condition is applied only when matching
     an individual insn, and only after the insn has matched the
     pattern's recognition template.  The insn's operands may be found
     in the vector `operands'.

  4. The "output template": a string that says how to output matching
     insns as assembler code.  `%' in this string specifies where to
     substitute the value of an operand.  *Note Output Template::.

     When simple substitution isn't general enough, you can specify a
     piece of C code to compute the output.  *Note Output Statement::.

  5. Optionally, a vector containing the values of attributes for insns
     matching this pattern.  *Note Insn Attributes::.


File: gcc.info,  Node: Example,  Next: RTL Template,  Prev: Patterns,  Up: Machine Desc

Example of `define_insn'
========================

   Here is an actual example of an instruction pattern, for the
68000/68020.

     (define_insn "tstsi"
       [(set (cc0)
             (match_operand:SI 0 "general_operand" "rm"))]
       ""
       "*
     { if (TARGET_68020 || ! ADDRESS_REG_P (operands[0]))
         return \"tstl %0\";
       return \"cmpl #0,%0\"; }")

   This is an instruction that sets the condition codes based on the
value of a general operand.  It has no condition, so any insn whose RTL
description has the form shown may be handled according to this
pattern.  The name `tstsi' means "test a `SImode' value" and tells the
RTL generation pass that, when it is necessary to test such a value, an
insn to do so can be constructed using this pattern.

   The output control string is a piece of C code which chooses which
output template to return based on the kind of operand and the specific
type of CPU for which code is being generated.

   `"rm"' is an operand constraint.  Its meaning is explained below.


File: gcc.info,  Node: RTL Template,  Next: Output Template,  Prev: Example,  Up: Machine Desc

RTL Template
============

   The RTL template is used to define which insns match the particular
pattern and how to find their operands.  For named patterns, the RTL
template also says how to construct an insn from specified operands.

   Construction involves substituting specified operands into a copy of
the template.  Matching involves determining the values that serve as
the operands in the insn being matched.  Both of these activities are
controlled by special expression types that direct matching and
substitution of the operands.

`(match_operand:M N PREDICATE CONSTRAINT)'
     This expression is a placeholder for operand number N of the insn.
     When constructing an insn, operand number N will be substituted
     at this point.  When matching an insn, whatever appears at this
     position in the insn will be taken as operand number N; but it
     must satisfy PREDICATE or this instruction pattern will not match
     at all.

     Operand numbers must be chosen consecutively counting from zero in
     each instruction pattern.  There may be only one `match_operand'
     expression in the pattern for each operand number.  Usually
     operands are numbered in the order of appearance in `match_operand'
     expressions.  In the case of a `define_expand', any operand numbers
     used only in `match_dup' expressions have higher values than all
     other operand numbers.

     PREDICATE is a string that is the name of a C function that
     accepts two arguments, an expression and a machine mode.  During
     matching, the function will be called with the putative operand as
     the expression and M as the mode argument (if M is not specified,
     `VOIDmode' will be used, which normally causes PREDICATE to accept
     any mode).  If it returns zero, this instruction pattern fails to
     match.  PREDICATE may be an empty string; then it means no test is
     to be done on the operand, so anything which occurs in this
     position is valid.

     Most of the time, PREDICATE will reject modes other than M--but
     not always.  For example, the predicate `address_operand' uses M
     as the mode of memory ref that the address should be valid for.
     Many predicates accept `const_int' nodes even though their mode is
     `VOIDmode'.

     CONSTRAINT controls reloading and the choice of the best register
     class to use for a value, as explained later (*note
     Constraints::.).

     People are often unclear on the difference between the constraint
     and the predicate.  The predicate helps decide whether a given
     insn matches the pattern.  The constraint plays no role in this
     decision; instead, it controls various decisions in the case of an
     insn which does match.

     On CISC machines, the most common PREDICATE is
     `"general_operand"'.  This function checks that the putative
     operand is either a constant, a register or a memory reference,
     and that it is valid for mode M.

     For an operand that must be a register, PREDICATE should be
     `"register_operand"'.  Using `"general_operand"' would be valid,
     since the reload pass would copy any non-register operands through
     registers, but this would make GNU CC do extra work, it would
     prevent invariant operands (such as constant) from being removed
     from loops, and it would prevent the register allocator from doing
     the best possible job.  On RISC machines, it is usually most
     efficient to allow PREDICATE to accept only objects that the
     constraints allow.

     For an operand that must be a constant, you must be sure to either
     use `"immediate_operand"' for PREDICATE, or make the instruction
     pattern's extra condition require a constant, or both.  You cannot
     expect the constraints to do this work!  If the constraints allow
     only constants, but the predicate allows something else, the
     compiler will crash when that case arises.

`(match_scratch:M N CONSTRAINT)'
     This expression is also a placeholder for operand number N and
     indicates that operand must be a `scratch' or `reg' expression.

     When matching patterns, this is equivalent to

          (match_operand:M N "scratch_operand" PRED)

     but, when generating RTL, it produces a (`scratch':M) expression.

     If the last few expressions in a `parallel' are `clobber'
     expressions whose operands are either a hard register or
     `match_scratch', the combiner can add or delete them when
     necessary.  *Note Side Effects::.

`(match_dup N)'
     This expression is also a placeholder for operand number N.  It is
     used when the operand needs to appear more than once in the insn.

     In construction, `match_dup' acts just like `match_operand': the
     operand is substituted into the insn being constructed.  But in
     matching, `match_dup' behaves differently.  It assumes that operand
     number N has already been determined by a `match_operand'
     appearing earlier in the recognition template, and it matches only
     an identical-looking expression.

`(match_operator:M N PREDICATE [OPERANDS...])'
     This pattern is a kind of placeholder for a variable RTL expression
     code.

     When constructing an insn, it stands for an RTL expression whose
     expression code is taken from that of operand N, and whose
     operands are constructed from the patterns OPERANDS.

     When matching an expression, it matches an expression if the
     function PREDICATE returns nonzero on that expression *and* the
     patterns OPERANDS match the operands of the expression.

     Suppose that the function `commutative_operator' is defined as
     follows, to match any expression whose operator is one of the
     commutative arithmetic operators of RTL and whose mode is MODE:

          int
          commutative_operator (x, mode)
               rtx x;
               enum machine_mode mode;
          {
            enum rtx_code code = GET_CODE (x);
            if (GET_MODE (x) != mode)
              return 0;
            return (GET_RTX_CLASS (code) == 'c'
                    || code == EQ || code == NE);
          }

     Then the following pattern will match any RTL expression consisting
     of a commutative operator applied to two general operands:

          (match_operator:SI 3 "commutative_operator"
            [(match_operand:SI 1 "general_operand" "g")
             (match_operand:SI 2 "general_operand" "g")])

     Here the vector `[OPERANDS...]' contains two patterns because the
     expressions to be matched all contain two operands.

     When this pattern does match, the two operands of the commutative
     operator are recorded as operands 1 and 2 of the insn.  (This is
     done by the two instances of `match_operand'.)  Operand 3 of the
     insn will be the entire commutative expression: use `GET_CODE
     (operands[3])' to see which commutative operator was used.

     The machine mode M of `match_operator' works like that of
     `match_operand': it is passed as the second argument to the
     predicate function, and that function is solely responsible for
     deciding whether the expression to be matched "has" that mode.

     When constructing an insn, argument 3 of the gen-function will
     specify the operation (i.e. the expression code) for the
     expression to be made.  It should be an RTL expression, whose
     expression code is copied into a new expression whose operands are
     arguments 1 and 2 of the gen-function.  The subexpressions of
     argument 3 are not used; only its expression code matters.

     When `match_operator' is used in a pattern for matching an insn,
     it usually best if the operand number of the `match_operator' is
     higher than that of the actual operands of the insn.  This improves
     register allocation because the register allocator often looks at
     operands 1 and 2 of insns to see if it can do register tying.

     There is no way to specify constraints in `match_operator'.  The
     operand of the insn which corresponds to the `match_operator'
     never has any constraints because it is never reloaded as a whole.
     However, if parts of its OPERANDS are matched by `match_operand'
     patterns, those parts may have constraints of their own.

`(match_op_dup:M N[OPERANDS...])'
     Like `match_dup', except that it applies to operators instead of
     operands.  When constructing an insn, operand number N will be
     substituted at this point.  But in matching, `match_op_dup' behaves
     differently.  It assumes that operand number N has already been
     determined by a `match_operator' appearing earlier in the
     recognition template, and it matches only an identical-looking
     expression.

`(match_parallel N PREDICATE [SUBPAT...])'
     This pattern is a placeholder for an insn that consists of a
     `parallel' expression with a variable number of elements.  This
     expression should only appear at the top level of an insn pattern.

     When constructing an insn, operand number N will be substituted at
     this point.  When matching an insn, it matches if the body of the
     insn is a `parallel' expression with at least as many elements as
     the vector of SUBPAT expressions in the `match_parallel', if each
     SUBPAT matches the corresponding element of the `parallel', *and*
     the function PREDICATE returns nonzero on the `parallel' that is
     the body of the insn.  It is the responsibility of the predicate
     to validate elements of the `parallel' beyond those listed in the
     `match_parallel'.

     A typical use of `match_parallel' is to match load and store
     multiple expressions, which can contain a variable number of
     elements in a `parallel'.  For example,

          (define_insn ""
            [(match_parallel 0 "load_multiple_operation"
               [(set (match_operand:SI 1 "gpc_reg_operand" "=r")
                     (match_operand:SI 2 "memory_operand" "m"))
                (use (reg:SI 179))
                (clobber (reg:SI 179))])]
            ""
            "loadm 0,0,%1,%2")

     This example comes from `a29k.md'.  The function
     `load_multiple_operations' is defined in `a29k.c' and checks that
     subsequent elements in the `parallel' are the same as the `set' in
     the pattern, except that they are referencing subsequent registers
     and memory locations.

     An insn that matches this pattern might look like:

          (parallel
           [(set (reg:SI 20) (mem:SI (reg:SI 100)))
            (use (reg:SI 179))
            (clobber (reg:SI 179))
            (set (reg:SI 21)
                 (mem:SI (plus:SI (reg:SI 100)
                                  (const_int 4))))
            (set (reg:SI 22)
                 (mem:SI (plus:SI (reg:SI 100)
                                  (const_int 8))))])

`(match_par_dup N [SUBPAT...])'
     Like `match_op_dup', but for `match_parallel' instead of
     `match_operator'.

`(address (match_operand:M N "address_operand" ""))'
     This complex of expressions is a placeholder for an operand number
     N in a "load address" instruction: an operand which specifies a
     memory location in the usual way, but for which the actual operand
     value used is the address of the location, not the contents of the
     location.

     `address' expressions never appear in RTL code, only in machine
     descriptions.  And they are used only in machine descriptions that
     do not use the operand constraint feature.  When operand
     constraints are in use, the letter `p' in the constraint serves
     this purpose.

     M is the machine mode of the *memory location being addressed*,
     not the machine mode of the address itself.  That mode is always
     the same on a given target machine (it is `Pmode', which normally
     is `SImode'), so there is no point in mentioning it; thus, no
     machine mode is written in the `address' expression.  If some day
     support is added for machines in which addresses of different
     kinds of objects appear differently or are used differently (such
     as the PDP-10), different formats would perhaps need different
     machine modes and these modes might be written in the `address'
     expression.