internals.texi

基于4个mips核的noc设计

Return whether a symbol was used in an expression.

@item symbol_mark_used_in_reloc
@cindex symbol_mark_used_in_reloc
Mark a symbol as having been used by a relocation.

@item symbol_clear_used_in_reloc
@cindex symbol_clear_used_in_reloc
Clear the mark indicating that a symbol was used in a relocation.

@item symbol_used_in_reloc_p
@cindex symbol_used_in_reloc_p
Return whether a symbol was used in a relocation.

@item symbol_mark_mri_common
@cindex symbol_mark_mri_common
Mark a symbol as an MRI common symbol.

@item symbol_clear_mri_common
@cindex symbol_clear_mri_common
Clear the mark indicating that a symbol is an MRI common symbol.

@item symbol_mri_common_p
@cindex symbol_mri_common_p
Return whether a symbol is an MRI common symbol.

@item symbol_mark_written
@cindex symbol_mark_written
Mark a symbol as having been written.

@item symbol_clear_written
@cindex symbol_clear_written
Clear the mark indicating that a symbol was written.

@item symbol_written_p
@cindex symbol_written_p
Return whether a symbol was written.

@item symbol_mark_resolved
@cindex symbol_mark_resolved
Mark a symbol as having been resolved.

@item symbol_resolved_p
@cindex symbol_resolved_p
Return whether a symbol has been resolved.

@item symbol_section_p
@cindex symbol_section_p
Return whether a symbol is a section symbol.

@item symbol_equated_p
@cindex symbol_equated_p
Return whether a symbol is equated to another symbol.

@item symbol_constant_p
@cindex symbol_constant_p
Return whether a symbol has a constant value, including being an offset within
some frag.

@item symbol_get_bfdsym
@cindex symbol_get_bfdsym
Return the BFD symbol associated with a symbol.

@item symbol_set_bfdsym
@cindex symbol_set_bfdsym
Set the BFD symbol associated with a symbol.

@item symbol_get_obj
@cindex symbol_get_obj
Return a pointer to the @code{OBJ_SYMFIELD_TYPE} field of a symbol.

@item symbol_set_obj
@cindex symbol_set_obj
Set the @code{OBJ_SYMFIELD_TYPE} field of a symbol.

@item symbol_get_tc
@cindex symbol_get_tc
Return a pointer to the @code{TC_SYMFIELD_TYPE} field of a symbol.

@item symbol_set_tc
@cindex symbol_set_tc
Set the @code{TC_SYMFIELD_TYPE} field of a symbol.

@end table

When @code{BFD_ASSEMBLER} is defined, GAS attempts to store local
symbols--symbols which will not be written to the output file--using a
different structure, @code{struct local_symbol}.  This structure can only
represent symbols whose value is an offset within a frag.

Code outside of the symbol handler will always deal with @code{symbolS}
structures and use the accessor functions.  The accessor functions correctly
deal with local symbols.  @code{struct local_symbol} is much smaller than
@code{symbolS} (which also automatically creates a bfd @code{asymbol}
structure), so this saves space when assembling large files.

The first field of @code{symbolS} is @code{bsym}, the pointer to the BFD
symbol.  The first field of @code{struct local_symbol} is a pointer which is
always set to NULL.  This is how the symbol accessor functions can distinguish
local symbols from ordinary symbols.  The symbol accessor functions
automatically convert a local symbol into an ordinary symbol when necessary.

@node Expressions
@subsection Expressions
@cindex internals, expressions
@cindex expressions, internal
@cindex expressionS structure

Expressions are stored in an @code{expressionS} structure.  The structure is
defined in @file{expr.h}.

@cindex expression
The macro @code{expression} will create an @code{expressionS} structure based
on the text found at the global variable @code{input_line_pointer}.

@cindex make_expr_symbol
@cindex expr_symbol_where
A single @code{expressionS} structure can represent a single operation.
Complex expressions are formed by creating @dfn{expression symbols} and
combining them in @code{expressionS} structures.  An expression symbol is
created by calling @code{make_expr_symbol}.  An expression symbol should
naturally never appear in a symbol table, and the implementation of
@code{S_IS_LOCAL} (@pxref{Symbols}) reflects that.  The function
@code{expr_symbol_where} returns non-zero if a symbol is an expression symbol,
and also returns the file and line for the expression which caused it to be
created.

The @code{expressionS} structure has two symbol fields, a number field, an
operator field, and a field indicating whether the number is unsigned.

The operator field is of type @code{operatorT}, and describes how to interpret
the other fields; see the definition in @file{expr.h} for the possibilities.

An @code{operatorT} value of @code{O_big} indicates either a floating point
number, stored in the global variable @code{generic_floating_point_number}, or
an integer too large to store in an @code{offsetT} type, stored in the global
array @code{generic_bignum}.  This rather inflexible approach makes it
impossible to use floating point numbers or large expressions in complex
expressions.

@node Fixups
@subsection Fixups
@cindex internals, fixups
@cindex fixups
@cindex fixS structure

A @dfn{fixup} is basically anything which can not be resolved in the first
pass.  Sometimes a fixup can be resolved by the end of the assembly; if not,
the fixup becomes a relocation entry in the object file.

@cindex fix_new
@cindex fix_new_exp
A fixup is created by a call to @code{fix_new} or @code{fix_new_exp}.  Both
take a frag (@pxref{Frags}), a position within the frag, a size, an indication
of whether the fixup is PC relative, and a type.  In a @code{BFD_ASSEMBLER}
GAS, the type is nominally a @code{bfd_reloc_code_real_type}, but several
targets use other type codes to represent fixups that can not be described as
relocations.

The @code{fixS} structure has a number of fields, several of which are obsolete
or are only used by a particular target.  The important fields are:

@table @code
@item fx_frag
The frag (@pxref{Frags}) this fixup is in.

@item fx_where
The location within the frag where the fixup occurs.

@item fx_addsy
The symbol this fixup is against.  Typically, the value of this symbol is added
into the object contents.  This may be NULL.

@item fx_subsy
The value of this symbol is subtracted from the object contents.  This is
normally NULL.

@item fx_offset
A number which is added into the fixup.

@item fx_addnumber
Some CPU backends use this field to convey information between
@code{md_apply_fix} and @code{tc_gen_reloc}.  The machine independent code does
not use it.

@item fx_next
The next fixup in the section.

@item fx_r_type
The type of the fixup.  This field is only defined if @code{BFD_ASSEMBLER}, or
if the target defines @code{NEED_FX_R_TYPE}.

@item fx_size
The size of the fixup.  This is mostly used for error checking.

@item fx_pcrel
Whether the fixup is PC relative.

@item fx_done
Non-zero if the fixup has been applied, and no relocation entry needs to be
generated.

@item fx_file
@itemx fx_line
The file and line where the fixup was created.

@item tc_fix_data
This has the type @code{TC_FIX_TYPE}, and is only defined if the target defines
that macro.
@end table

@node Frags
@subsection Frags
@cindex internals, frags
@cindex frags
@cindex fragS structure.

The @code{fragS} structure is defined in @file{as.h}.  Each frag represents a
portion of the final object file.  As GAS reads the source file, it creates
frags to hold the data that it reads.  At the end of the assembly the frags and
fixups are processed to produce the final contents.

@table @code
@item fr_address
The address of the frag.  This is not set until the assembler rescans the list
of all frags after the entire input file is parsed.  The function
@code{relax_segment} fills in this field.

@item fr_next
Pointer to the next frag in this (sub)section.

@item fr_fix
Fixed number of characters we know we're going to emit to the output file.  May
be zero.

@item fr_var
Variable number of characters we may output, after the initial @code{fr_fix}
characters.  May be zero.

@item fr_offset
The interpretation of this field is controlled by @code{fr_type}.  Generally,
if @code{fr_var} is non-zero, this is a repeat count: the @code{fr_var}
characters are output @code{fr_offset} times.

@item line
Holds line number info when an assembler listing was requested.

@item fr_type
Relaxation state.  This field indicates the interpretation of @code{fr_offset},
@code{fr_symbol} and the variable-length tail of the frag, as well as the
treatment it gets in various phases of processing.  It does not affect the
initial @code{fr_fix} characters; they are always supposed to be output
verbatim (fixups aside).  See below for specific values this field can have.

@item fr_subtype
Relaxation substate.  If the macro @code{md_relax_frag} isn't defined, this is
assumed to be an index into @code{TC_GENERIC_RELAX_TABLE} for the generic
relaxation code to process (@pxref{Relaxation}).  If @code{md_relax_frag} is
defined, this field is available for any use by the CPU-specific code.

@item fr_symbol
This normally indicates the symbol to use when relaxing the frag according to
@code{fr_type}.

@item fr_opcode
Points to the lowest-addressed byte of the opcode, for use in relaxation.

@item tc_frag_data
Target specific fragment data of type TC_FRAG_TYPE.
Only present if @code{TC_FRAG_TYPE} is defined.

@item fr_file
@itemx fr_line
The file and line where this frag was last modified.

@item fr_literal
Declared as a one-character array, this last field grows arbitrarily large to
hold the actual contents of the frag.
@end table

These are the possible relaxation states, provided in the enumeration type
@code{relax_stateT}, and the interpretations they represent for the other
fields:

@table @code
@item rs_align
@itemx rs_align_code
The start of the following frag should be aligned on some boundary.  In this
frag, @code{fr_offset} is the logarithm (base 2) of the alignment in bytes.
(For example, if alignment on an 8-byte boundary were desired, @code{fr_offset}
would have a value of 3.)  The variable characters indicate the fill pattern to
be used.  The @code{fr_subtype} field holds the maximum number of bytes to skip
when doing this alignment.  If more bytes are needed, the alignment is not
done.  An @code{fr_subtype} value of 0 means no maximum, which is the normal
case.  Target backends can use @code{rs_align_code} to handle certain types of
alignment differently.

@item rs_broken_word
This indicates that ``broken word'' processing should be done (@pxref{Broken
words}).  If broken word processing is not necessary on the target machine,
this enumerator value will not be defined.

@item rs_cfa
This state is used to implement exception frame optimizations.  The
@code{fr_symbol} is an expression symbol for the subtraction which may be
relaxed.  The @code{fr_opcode} field holds the frag for the preceding command
byte.  The @code{fr_offset} field holds the offset within that frag.  The
@code{fr_subtype} field is used during relaxation to hold the current size of
the frag.

@item rs_fill
The variable characters are to be repeated @code{fr_offset} times.  If
@code{fr_offset} is 0, this frag has a length of @code{fr_fix}.  Most frags
have this type.

@item rs_leb128
This state is used to implement the DWARF ``little endian base 128''
variable length number format.  The @code{fr_symbol} is always an expression
symbol, as constant expressions are emitted directly.  The @code{fr_offset}
field is used during relaxation to hold the previous size of the number so
that we can determine if the fragment changed size.

@item rs_machine_dependent
Displacement relaxation is to be done on this frag.  The target is indicated by
@code{fr_symbol} and @code{fr_offset}, and @code{fr_subtype} indicates the
particular machine-specific addressing mode desired.  @xref{Relaxation}.

@item rs_org
The start of the following frag should be pushed back to some specific offset
within the section.  (Some assemblers use the value as an absolute address; GAS
does not handle final absolute addresses, but rather requires that the linker
set them.)  The offset is given by @code{fr_symbol} and @code{fr_offset}; one
character from the variable-length tail is used as the fill character.