@findex XVECEXP
@item XVECEXP (@var{exp}, @var{idx}, @var{eltnum})
Access element number @var{eltnum} in the vector which is
in operand number @var{idx} in @var{exp}.  This value is an RTX.

It is up to you to make sure that @var{eltnum} is not negative
and is less than @code{XVECLEN (@var{exp}, @var{idx})}.
@end table

All the macros defined in this section expand into lvalues and therefore
can be used to assign the operands, lengths and vector elements as well as
to access them.

@node Flags
@section Flags in an RTL Expression
@cindex flags in RTL expression

RTL expressions contain several flags (one-bit bit-fields) and other
values that are used in certain types of expression.  Most often they
are accessed with the following macros:

@table @code
@findex MEM_VOLATILE_P
@cindex @code{mem} and @samp{/v}
@cindex @code{volatil}, in @code{mem}
@cindex @samp{/v} in RTL dump
@item MEM_VOLATILE_P (@var{x})
In @code{mem} expressions, nonzero for volatile memory references.
Stored in the @code{volatil} field and printed as @samp{/v}.

@findex MEM_IN_STRUCT_P
@cindex @code{mem} and @samp{/s}
@cindex @code{in_struct}, in @code{mem}
@cindex @samp{/s} in RTL dump
@item MEM_IN_STRUCT_P (@var{x})
In @code{mem} expressions, nonzero for reference to an entire structure,
union or array, or to a component of one.  Zero for references to a
scalar variable or through a pointer to a scalar.  Stored in the
@code{in_struct} field and printed as @samp{/s}.  If both this flag and
MEM_SCALAR_P are clear, then we don't know whether this MEM is in a
structure or not.  Both flags should never be simultaneously set.

@findex MEM_SCALAR_P
@cindex @code{mem} and @samp{/f}
@cindex @code{frame_related}, in@code{mem}
@cindex @samp{/f} in RTL dump
@item MEM_SCALAR_P (@var{x})
In @code{mem} expressions, nonzero for reference to a scalar known not
to be a member of a structure, union, or array.  Zero for such
references and for indirections through pointers, even pointers pointing
to scalar types.  If both this flag and MEM_STRUCT_P are clear, then we
don't know whether this MEM is in a structure or not.  Both flags should
never be simultaneously set.

@findex MEM_ALIAS_SET
@item MEM_ALIAS_SET (@var{x})
In @code{mem} expressions, the alias set to which @var{x} belongs.  If
zero, @var{x} is not in any alias set, and may alias anything.  If
nonzero, @var{x} may only alias objects in the same alias set.  This
value is set (in a language-specific manner) by the front-end.  This
field is not a bit-field; it is in an integer, found as the second
argument to the @code{mem}.

@findex REG_LOOP_TEST_P
@cindex @code{reg} and @samp{/s}
@cindex @code{in_struct}, in @code{reg}
@item REG_LOOP_TEST_P
In @code{reg} expressions, nonzero if this register's entire life is
contained in the exit test code for some loop.  Stored in the
@code{in_struct} field and printed as @samp{/s}.

@findex REG_USERVAR_P
@cindex @code{reg} and @samp{/v}
@cindex @code{volatil}, in @code{reg}
@item REG_USERVAR_P (@var{x})
In a @code{reg}, nonzero if it corresponds to a variable present in
the user's source code.  Zero for temporaries generated internally by
the compiler.  Stored in the @code{volatil} field and printed as
@samp{/v}.

@cindex @samp{/i} in RTL dump
@findex REG_FUNCTION_VALUE_P
@cindex @code{reg} and @samp{/i}
@cindex @code{integrated}, in @code{reg}
@item REG_FUNCTION_VALUE_P (@var{x})
Nonzero in a @code{reg} if it is the place in which this function's
value is going to be returned.  (This happens only in a hard
register.)  Stored in the @code{integrated} field and printed as
@samp{/i}.

The same hard register may be used also for collecting the values of
functions called by this one, but @code{REG_FUNCTION_VALUE_P} is zero
in this kind of use.

@findex SUBREG_PROMOTED_VAR_P
@cindex @code{subreg} and @samp{/s}
@cindex @code{in_struct}, in @code{subreg}
@item SUBREG_PROMOTED_VAR_P
Nonzero in a @code{subreg} if it was made when accessing an object that
was promoted to a wider mode in accord with the @code{PROMOTED_MODE} machine
description macro (@pxref{Storage Layout}).  In this case, the mode of
the @code{subreg} is the declared mode of the object and the mode of
@code{SUBREG_REG} is the mode of the register that holds the object.
Promoted variables are always either sign- or zero-extended to the wider
mode on every assignment.  Stored in the @code{in_struct} field and
printed as @samp{/s}.

@findex SUBREG_PROMOTED_UNSIGNED_P
@cindex @code{subreg} and @samp{/u}
@cindex @code{unchanging}, in @code{subreg}
@item SUBREG_PROMOTED_UNSIGNED_P
Nonzero in a @code{subreg} that has @code{SUBREG_PROMOTED_VAR_P} nonzero
if the object being referenced is kept zero-extended and zero if it
is kept sign-extended.  Stored in the @code{unchanging} field and
printed as @samp{/u}.

@findex RTX_UNCHANGING_P
@cindex @code{reg} and @samp{/u}
@cindex @code{mem} and @samp{/u}
@cindex @code{unchanging}, in @code{reg} and @code{mem}
@cindex @samp{/u} in RTL dump
@item RTX_UNCHANGING_P (@var{x})
Nonzero in a @code{reg} or @code{mem} if the value is not changed.
(This flag is not set for memory references via pointers to constants.
Such pointers only guarantee that the object will not be changed
explicitly by the current function.  The object might be changed by
other functions or by aliasing.)  Stored in the
@code{unchanging} field and printed as @samp{/u}.

@findex RTX_INTEGRATED_P
@cindex @code{integrated}, in @code{insn}
@item RTX_INTEGRATED_P (@var{insn})
Nonzero in an insn if it resulted from an in-line function call.
Stored in the @code{integrated} field and printed as @samp{/i}.

@findex RTX_FRAME_RELATED_P
@item RTX_FRAME_RELATED_P (@var{x})
Nonzero in an insn or expression which is part of a function prologue
and sets the stack pointer, sets the frame pointer, or saves a register.
This flag should also be set on an instruction that sets up a temporary
register to use in place of the frame pointer.

In particular, on RISC targets where there are limits on the sizes of
immediate constants, it is sometimes impossible to reach the register
save area directly from the stack pointer.  In that case, a temporary
register is used that is near enough to the register save area, and the
Canonical Frame Address, i.e., DWARF2's logical frame pointer, register
must (temporarily) be changed to be this temporary register.  So, the
instruction that sets this temporary register must be marked as
@code{RTX_FRAME_RELATED_P}.

If the marked instruction is overly complex (defined in terms of what
@code{dwarf2out_frame_debug_expr} can handle), you will also have to
create a @code{REG_FRAME_RELATED_EXPR} note and attach it to the
instruction.  This note should contain a simple expression of the
computation performed by this instruction, i.e., one that
@code{dwarf2out_frame_debug_expr} can handle.

This flag is required for exception handling support on targets with RTL
prologues.

@findex SYMBOL_REF_USED
@cindex @code{used}, in @code{symbol_ref}
@item SYMBOL_REF_USED (@var{x})
In a @code{symbol_ref}, indicates that @var{x} has been used.  This is
normally only used to ensure that @var{x} is only declared external
once.  Stored in the @code{used} field.

@findex SYMBOL_REF_FLAG
@cindex @code{symbol_ref} and @samp{/v}
@cindex @code{volatil}, in @code{symbol_ref}
@item SYMBOL_REF_FLAG (@var{x})
In a @code{symbol_ref}, this is used as a flag for machine-specific purposes.
Stored in the @code{volatil} field and printed as @samp{/v}.

@findex SYMBOL_REF_WEAK
@cindex @code{symbol_ref} and @samp{/i}
@cindex @code{integrated}, in @code{symbol_ref}
@item SYMBOL_REF_WEAK (@var{x})
In a @code{symbol_ref}, indicates that @var{x} has been declared weak.
Stored in the @code{integrated} field and printed as @samp{/i}.

@findex LABEL_OUTSIDE_LOOP_P
@cindex @code{label_ref} and @samp{/s}
@cindex @code{in_struct}, in @code{label_ref}
@item LABEL_OUTSIDE_LOOP_P
In @code{label_ref} expressions, nonzero if this is a reference to a
label that is outside the innermost loop containing the reference to the
label.  Stored in the @code{in_struct} field and printed as @samp{/s}.

@findex INSN_DELETED_P
@cindex @code{volatil}, in @code{insn}
@item INSN_DELETED_P (@var{insn})
In an insn, nonzero if the insn has been deleted.  Stored in the
@code{volatil} field and printed as @samp{/v}.

@findex INSN_ANNULLED_BRANCH_P
@cindex @code{insn} and @samp{/u}
@cindex @code{unchanging}, in @code{insn}
@item INSN_ANNULLED_BRANCH_P (@var{insn})
In an @code{insn} in the delay slot of a branch insn, indicates that an
annulling branch should be used.  See the discussion under
@code{sequence} below.  Stored in the @code{unchanging} field and printed
as @samp{/u}.

@findex INSN_FROM_TARGET_P
@cindex @code{insn} and @samp{/s}
@cindex @code{in_struct}, in @code{insn}
@cindex @samp{/s} in RTL dump
@item INSN_FROM_TARGET_P (@var{insn})
In an @code{insn} in a delay slot of a branch, indicates that the insn
is from the target of the branch.  If the branch insn has
@code{INSN_ANNULLED_BRANCH_P} set, this insn will only be executed if
the branch is taken.  For annulled branches with
@code{INSN_FROM_TARGET_P} clear, the insn will be executed only if the
branch is not taken.  When @code{INSN_ANNULLED_BRANCH_P} is not set,
this insn will always be executed.  Stored in the @code{in_struct}
field and printed as @samp{/s}.

@findex CONSTANT_POOL_ADDRESS_P
@cindex @code{symbol_ref} and @samp{/u}
@cindex @code{unchanging}, in @code{symbol_ref}
@item CONSTANT_POOL_ADDRESS_P (@var{x})
Nonzero in a @code{symbol_ref} if it refers to part of the current
function's ``constants pool''.  These are addresses close to the
beginning of the function, and GNU CC assumes they can be addressed
directly (perhaps with the help of base registers).  Stored in the
@code{unchanging} field and printed as @samp{/u}.

@findex CONST_CALL_P
@cindex @code{call_insn} and @samp{/u}
@cindex @code{unchanging}, in @code{call_insn}
@item CONST_CALL_P (@var{x})
In a @code{call_insn}, indicates that the insn represents a call to a const
function.  Stored in the @code{unchanging} field and printed as @samp{/u}.

@findex LABEL_PRESERVE_P
@cindex @code{code_label} and @samp{/i}
@cindex @code{in_struct}, in @code{code_label}
@item LABEL_PRESERVE_P (@var{x})
In a @code{code_label}, indicates that the label can never be deleted.
Labels referenced by a non-local goto will have this bit set.  Stored
in the @code{in_struct} field and printed as @samp{/s}.

@findex SCHED_GROUP_P
@cindex @code{insn} and @samp{/i}
@cindex @code{in_struct}, in @code{insn}
@item SCHED_GROUP_P (@var{insn})
During instruction scheduling, in an insn, indicates that the previous insn
must be scheduled together with this insn.  This is used to ensure that
certain groups of instructions will not be split up by the instruction
scheduling pass, for example, @code{use} insns before a @code{call_insn} may
not be separated from the @code{call_insn}.  Stored in the @code{in_struct}
field and printed as @samp{/s}.
@end table

These are the fields which the above macros refer to:

@table @code
@findex used
@item used
Normally, this flag is used only momentarily, at the end of RTL
generation for a function, to count the number of times an expression
appears in insns.  Expressions that appear more than once are copied,
according to the rules for shared structure (@pxref{Sharing}).

In a @code{symbol_ref}, it indicates that an external declaration for
the symbol has already been written.

In a @code{reg}, it is used by the leaf register renumbering code to ensure
that each register is only renumbered once.

@findex volatil
@item volatil
This flag is used in @code{mem}, @code{symbol_ref} and @code{reg}
expressions and in insns.  In RTL dump files, it is printed as
@samp{/v}.

@cindex volatile memory references
In a @code{mem} expression, it is 1 if the memory reference is volatile.
Volatile memory references may not be deleted, reordered or combined.

In a @code{symbol_ref} expression, it is used for machine-specific
purposes.

In a @code{reg} expression, it is 1 if the value is a user-level variable.
0 indicates an internal compiler temporary.

In an insn, 1 means the insn has been deleted.

@findex in_struct
@item in_struct
In @code{mem} expressions, it is 1 if the memory datum referred to is
all or part of a structure or array; 0 if it is (or might be) a scalar
variable.  A reference through a C pointer has 0 because the pointer
might point to a scalar variable.  This information allows the compiler
to determine something about possible cases of aliasing.

In an insn in the delay slot of a branch, 1 means that this insn is from
the target of the branch.

During instruction scheduling, in an insn, 1 means that this insn must be
scheduled as part of a group together with the previous insn.

In @code{reg} expressions, it is 1 if the register has its entire life
contained within the test expression of some loop.

In @code{subreg} expressions, 1 means that the @code{subreg} is accessing
an object that has had its mode promoted from a wider mode.

In @code{label_ref} expressions, 1 means that the referenced label is
outside the innermost loop containing the insn in which the @code{label_ref}
was found.

In @code{code_label} expressions, it is 1 if the label may never be deleted.
This is used for labels which are the target of non-local gotos.

In an RTL dump, this flag is represented as @samp{/s}.

@findex unchanging
@item unchanging