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* doc/c-mips.texi (MIPS Macros): New section.
	(MIPS Object): Replace with...
	(MIPS Small Data): ...this new section.
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Richard Sandiford 2013-06-22 16:09:55 +00:00
parent 5a7560b5ec
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@ -1,3 +1,9 @@
2013-06-22 Richard Sandiford <rdsandiford@googlemail.com>
* doc/c-mips.texi (MIPS Macros): New section.
(MIPS Object): Replace with...
(MIPS Small Data): ...this new section.
2013-06-22 Richard Sandiford <rdsandiford@googlemail.com>
* doc/c-mips.texi (MIPS symbol sizes): Move section further up file.

149
gas/doc/c-mips.texi
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@ -23,8 +23,9 @@ Assembly Language Programming'' in the same work.
@menu
* MIPS Opts:: Assembler options
* MIPS Macros:: High-level assembly macros
* MIPS Symbol Sizes:: Directives to override the size of symbols
* MIPS Object:: ECOFF object code
* MIPS Small Data:: Controlling the use of small data accesses
* MIPS ISA:: Directives to override the ISA level
* MIPS autoextend:: Directives for extending MIPS 16 bit instructions
* MIPS insn:: Directive to mark data as an instruction
@ -44,9 +45,8 @@ special options:
@table @code
@cindex @code{-G} option (MIPS)
@item -G @var{num}
This option sets the largest size of an object that can be referenced
implicitly with the @code{gp} register. It is only accepted for targets
that use @sc{ecoff} format. The default value is 8.
Set the ``small data'' limit to @var{n} bytes. The default limit is 8 bytes.
@xref{MIPS Small Data,, Controlling the use of small data accesses}.
@cindex @code{-EB} option (MIPS)
@cindex @code{-EL} option (MIPS)
@ -439,6 +439,66 @@ efficient. This option only affects the handling of the
@samp{.cpload} and @samp{.cpsetup} pseudo-ops.
@end table
@node MIPS Macros
@section High-level assembly macros
MIPS assemblers have traditionally provided a wider range of
instructions than the MIPS architecture itself. These extra
instructions are usually referred to as ``macro'' instructions
@footnote{The term ``macro'' is somewhat overloaded here, since
these macros have no relation to those defined by @code{.macro},
@pxref{Macro,, @code{.macro}}.}.
Some MIPS macro instructions extend an underlying architectural instruction
while others are entirely new. An example of the former type is @code{and},
which allows the third operand to be either a register or an arbitrary
immediate value. Examples of the latter type include @code{bgt}, which
branches to the third operand when the first operand is greater than
the second operand, and @code{ulh}, which implements an unaligned
2-byte load.
One of the most common extensions provided by macros is to expand
memory offsets to the full address range (32 or 64 bits) and to allow
symbolic offsets such as @samp{my_data + 4} to be used in place of
integer constants. For example, the architectural instruction
@code{lbu} allows only a signed 16-bit offset, whereas the macro
@code{lbu} allows code such as @samp{lbu $4,array+32769($5)}.
The implementation of these symbolic offsets depends on several factors,
such as whether the assembler is generating SVR4-style PIC (selected
by @option{-KPIC}, @pxref{MIPS Opts,, Assembler options}), the size of symbols
(@pxref{MIPS Symbol Sizes,, Directives to override the size of symbols}),
and the small data limit (@pxref{MIPS Small Data,, Controlling the use
of small data accesses}).
@kindex @code{.set macro}
@kindex @code{.set nomacro}
Sometimes it is undesirable to have one assembly instruction expand
to several machine instructions. The directive @code{.set nomacro}
tells the assembler to warn when this happens. @code{.set macro}
restores the default behavior.
@cindex @code{at} register, MIPS
@kindex @code{.set at=@var{reg}}
Some macro instructions need a temporary register to store intermediate
results. This register is usually @code{$1}, also known as @code{$at},
but it can be changed to any core register @var{reg} using
@code{.set at=@var{reg}}. Note that @code{$at} always refers
to @code{$1} regardless of which register is being used as the
temporary register.
@kindex @code{.set at}
@kindex @code{.set noat}
Implicit uses of the temporary register in macros could interfere with
explicit uses in the assembly code. The assembler therefore warns
whenever it sees an explicit use of the temporary register. The directive
@code{.set noat} silences this warning while @code{.set at} restores
the default behavior. It is safe to use @code{.set noat} while
@code{.set nomacro} is in effect since single-instruction macros
never need a temporary register.
Note that while the @sc{gnu} assembler provides these macros for compatibility,
it does not make any attempt to optimize them with the surrounding code.
@node MIPS Symbol Sizes
@section Directives to override the size of symbols
@ -494,38 +554,61 @@ symbol size using the command-line options @option{-msym32} and
These options and directives are always accepted, but at present,
they have no effect for anything other than n64.
@node MIPS Object
@section MIPS ECOFF object code
@node MIPS Small Data
@section Controlling the use of small data accesses
@cindex ECOFF sections
@cindex MIPS ECOFF sections
Assembling for a @sc{mips} @sc{ecoff} target supports some additional sections
besides the usual @code{.text}, @code{.data} and @code{.bss}. The
additional sections are @code{.rdata}, used for read-only data,
@code{.sdata}, used for small data, and @code{.sbss}, used for small
common objects.
@cindex small objects, MIPS ECOFF
@c This section deliberately glosses over the possibility of using -G
@c in SVR4-style PIC, as could be done on IRIX. We don't support that.
@cindex small data, MIPS
@cindex @code{gp} register, MIPS
When assembling for @sc{ecoff}, the assembler uses the @code{$gp} (@code{$28})
register to form the address of a ``small object''. Any object in the
@code{.sdata} or @code{.sbss} sections is considered ``small'' in this sense.
For external objects, or for objects in the @code{.bss} section, you can use
the @code{@value{GCC}} @samp{-G} option to control the size of objects addressed via
@code{$gp}; the default value is 8, meaning that a reference to any object
eight bytes or smaller uses @code{$gp}. Passing @samp{-G 0} to
@code{@value{AS}} prevents it from using the @code{$gp} register on the basis
of object size (but the assembler uses @code{$gp} for objects in @code{.sdata}
or @code{sbss} in any case). The size of an object in the @code{.bss} section
is set by the @code{.comm} or @code{.lcomm} directive that defines it. The
size of an external object may be set with the @code{.extern} directive. For
example, @samp{.extern sym,4} declares that the object at @code{sym} is 4 bytes
in length, whie leaving @code{sym} otherwise undefined.
It often takes several instructions to load the address of a symbol.
For example, when @samp{addr} is a 32-bit symbol, the non-PIC expansion
of @samp{dla $4,addr} is usually:
Using small @sc{ecoff} objects requires linker support, and assumes that the
@code{$gp} register is correctly initialized (normally done automatically by
the startup code). @sc{mips} @sc{ecoff} assembly code must not modify the
@code{$gp} register.
@smallexample
lui $4,%hi(addr)
daddiu $4,$4,%lo(addr)
@end smallexample
The sequence is much longer when @samp{addr} is a 64-bit symbol.
@xref{MIPS Symbol Sizes,, Directives to override the size of symbols}.
In order to cut down on this overhead, most embedded MIPS systems
set aside a 64-kilobyte ``small data'' area and guarantee that all
data of size @var{n} and smaller will be placed in that area.
The limit @var{n} is passed to both the assembler and the linker
using the command-line option @option{-G @var{n}}, @pxref{MIPS Opts,,
Assembler options}. Note that the same value of @var{n} must be used
when linking and when assembling all input files to the link; any
inconsistency could cause a relocation overflow error.
The size of an object in the @code{.bss} section is set by the
@code{.comm} or @code{.lcomm} directive that defines it. The size of
an external object may be set with the @code{.extern} directive. For
example, @samp{.extern sym,4} declares that the object at @code{sym}
is 4 bytes in length, while leaving @code{sym} otherwise undefined.
When no @option{-G} option is given, the default limit is 8 bytes.
The option @option{-G 0} prevents any data from being automatically
classified as small.
It is also possible to mark specific objects as small by putting them
in the special sections @code{.sdata} and @code{.sbss}, which are
``small'' counterparts of @code{.data} and @code{.bss} respectively.
The toolchain will treat such data as small regardless of the
@option{-G} setting.
On startup, systems that support a small data area are expected to
initialize register @code{$28}, also known as @code{$gp}, in such a
way that small data can be accessed using a 16-bit offset from that
register. For example, when @samp{addr} is small data,
the @samp{dla $4,addr} instruction above is equivalent to:
@smallexample
daddiu $4,$28,%gp_rel(addr)
@end smallexample
Small data is not supported for SVR4-style PIC.
@node MIPS ISA
@section Directives to override the ISA level