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INFO-DIR-SECTION Programming
START-INFO-DIR-ENTRY
* gccint: (gccint).            Internals of the GNU Compiler Collection.
END-INFO-DIR-ENTRY
   This file documents the internals of the GNU compilers.

   Published by the Free Software Foundation
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File: gccint.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))' if 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.  In an `addr_vec',
     `JUMP_LABEL' is `NULL_RTX'.

     Return insns count as jumps, but since they do not refer to any
     labels, their `JUMP_LABEL' is `NULL_RTX'.

`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 and `MEM's used or
     clobbered by the called function.

     A `MEM' generally points to a stack slots in which arguments passed
     to the libcall by reference (*note FUNCTION_ARG_PASS_BY_REFERENCE:
     Register Arguments.) are stored.  If the argument is caller-copied
     (*note FUNCTION_ARG_CALLEE_COPIES: Register Arguments.), the stack
     slot will be mentioned in `CLOBBER' and `USE' entries; if it's
     callee-copied, only a `USE' will appear, and the `MEM' may point
     to addresses that are not stack slots.  These `MEM's are used only
     in libcalls, because, unlike regular function calls, `CONST_CALL's
     (which libcalls generally are, *note CONST_CALL_P: Flags.) aren't
     assumed to read and write all memory, so flow would consider the
     stores dead and remove them.  Note that, since a libcall must
     never return values in memory (*note RETURN_IN_MEMORY: Aggregate
     Return.), there will never be a `CLOBBER' for a memory address
     holding a return value.

     `CLOBBER'ed registers 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.

     Besides as a `code_label', a label can also be represented as a
     `note' of type `NOTE_INSN_DELETED_LABEL'.

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

     The field `LABEL_KIND' differentiates four different types of
     labels: `LABEL_NORMAL', `LABEL_STATIC_ENTRY',
     `LABEL_GLOBAL_ENTRY', and `LABEL_WEAK_ENTRY'.  The only labels
     that do not have type `LABEL_NORMAL' are "alternate entry points"
     to the current function.  These may be static (visible only in the
     containing translation unit), global (exposed to all translation
     units), or weak (global, but can be overridden by another symbol
     with the same name).

     Much of the compiler treats all four kinds of label identically.
     Some of it needs to know whether or not a label is an alternate
     entry point; for this purpose, the macro `LABEL_ALT_ENTRY_P' is
     provided.  It is equivalent to testing whether `LABEL_KIND (label)
     == LABEL_NORMAL'.  The only place that cares about the distinction
     between static, global, and weak alternate entry points, besides
     the front-end code that creates them, is the function
     `output_alternate_entry_point', in `final.c'.

     To set the kind of a label, use the `SET_LABEL_KIND' macro.

`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_DELETED_LABEL'
          This marks what used to be a `code_label', but was not used
          for other purposes than taking its address and was
          transformed to mark that no code jumps to it.

    `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' or `note'
          of type `NOTE_INSN_DELETED_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.

   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.

   The second Haifa scheduling pass, for targets that can multiple
issue, sets the mode of an insn to `TImode' when it is believed that the
instruction begins an issue group.  That is, when the instruction
cannot issue simultaneously with the previous.  This may be relied on
by later passes, in particular machine-dependent reorg.

   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', `cond_exec', 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.

     It does not follow that the register OP has no useful value after
     this insn since OP is not necessarily modified by this insn.
     Rather, no subsequent instruction uses the contents of OP.

`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_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' or a `note' of type
     `NOTE_INSN_DELETED_LABEL', but is not a `jump_insn', or it is a
     `jump_insn' that required the label to be held in a register.  The
     presence of this note allows jump optimization to be aware that OP
     is, in fact, being used, and flow optimization to build an
     accurate flow graph.

   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 their 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.

   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.

     These notes will be deleted after reload, since they are no longer
     accurate or useful.

`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.

     These notes are deleted after reload, since they are no longer
     useful or accurate.

`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_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.

`REG_BR_PRED'
     These notes are found in JUMP insns after delayed branch scheduling
     has taken place.  They indicate both the direction and the
     likelihood of the JUMP.  The format is a bitmask of ATTR_FLAG_*
     values.

`REG_FRAME_RELATED_EXPR'
     This is used on an RTX_FRAME_RELATED_P insn wherein the attached
     expression is used in place of the actual insn pattern.  This is
     done in cases where the pattern is either complex or misleading.

   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: gccint.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: gccint.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.

   * All `const_int' expressions with equal values are shared.

   * 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.

   * There is only one `const_vector' expression with value 0 for each
     vector mode, be it an integer or a double constant vector.

   * 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: gccint.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.  This routine
is defined in `read-rtl.c'.  It is not available in the compiler
itself, only the various programs that generate the compiler back end
from the machine description.

   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 GCC.
This idea is not feasible.

   GCC 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 GCC to a new language front end is with
the "tree" data structure, described in the files `tree.h' and
`tree.def'.  The documentation for this structure (*note Trees::) is
incomplete.


File: gccint.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:

* Overview::            How the machine description is used.
* 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.
* Looping Patterns::    How to define patterns for special looping insns.
* Insn Canonicalizations::Canonicalization of Instructions
* Expander Definitions::Generating a sequence of several RTL insns
                          for a standard operation.
* Insn Splitting::      Splitting Instructions into Multiple Instructions.
* Including Patterns::      Including Patterns in Machine Descriptions.
* Peephole Definitions::Defining machine-specific peephole optimizations.
* Insn Attributes::     Specifying the value of attributes for generated insns.
* Conditional Execution::Generating `define_insn' patterns for
                           predication.
* Constant Definitions::Defining symbolic constants that can be used in the
                        md file.


File: gccint.info,  Node: Overview,  Next: Patterns,  Up: Machine Desc

Overview of How the Machine Description is Used
===============================================

   There are three main conversions that happen in the compiler:

  1. The front end reads the source code and builds a parse tree.

  2. The parse tree is used to generate an RTL insn list based on named
     instruction patterns.

  3. The insn list is matched against the RTL templates to produce
     assembler code.


   For the generate pass, only the names of the insns matter, from
either a named `define_insn' or a `define_expand'.  The compiler will
choose the pattern with the right name and apply the operands according
to the documentation later in this chapter, without regard for the RTL
template or operand constraints.  Note that the names the compiler looks
for are hard-coded in the compiler--it will ignore unnamed patterns and
patterns with names it doesn't know about, but if you don't provide a
named pattern it needs, it will abort.

   If a `define_insn' is used, the template given is inserted into the
insn list.  If a `define_expand' is used, one of three things happens,
based on the condition logic.  The condition logic may manually create
new insns for the insn list, say via `emit_insn()', and invoke `DONE'.
For certain named patterns, it may invoke `FAIL' to tell the compiler
to use an alternate way of performing that task.  If it invokes neither
`DONE' nor `FAIL', the template given in the pattern is inserted, as if
the `define_expand' were a `define_insn'.

   Once the insn list is generated, various optimization passes convert,
replace, and rearrange the insns in the insn list.  This is where the
`define_split' and `define_peephole' patterns get used, for example.

   Finally, the insn list's RTL is matched up with the RTL templates in
the `define_insn' patterns, and those patterns are used to emit the
final assembly code.  For this purpose, each named `define_insn' acts
like it's unnamed, since the names are ignored.


File: gccint.info,  Node: Patterns,  Next: Example,  Prev: Overview,  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.

     For the purpose of debugging the compiler, you may also specify a
     name beginning with the `*' character.  Such a name is used only
     for identifying the instruction in RTL dumps; it is entirely
     equivalent to having a nameless pattern for all other purposes.

  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'.  For an insn where the condition has
     once matched, it can't be used to control register allocation, for
     example by excluding certain hard registers or hard register
     combinations.

  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: gccint.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 can also be written using braced strings:

     (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.

