as: MIPS Macros
9.26.2 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 (1).
Some MIPS macro instructions extend an underlying architectural
instruction while others are entirely new. An example of the former
type is 'and', which allows the third operand to be either a register or
an arbitrary immediate value. Examples of the latter type include
'bgt', which branches to the third operand when the first operand is
greater than the second operand, and '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 'my_data + 4' to be used in place of integer
constants. For example, the architectural instruction 'lbu' allows only
a signed 16-bit offset, whereas the macro 'lbu' allows code such as 'lbu
$4,array+32769($5)'. The implementation of these symbolic offsets
depends on several factors, such as whether the assembler is generating
DONTPRINTYET SVR4-style PIC (selected by '-KPIC', 
Assembler options MIPS
Options.), the size of symbols (*noteDirectives to override the size of
DONTPRINTYET DONTPRINTYET SVR4-style PIC (selected by '-KPIC', Assembler options MIPS
Options.), the size of symbols (Directives to override the size of
symbols MIPS Symbol Sizes.), and the small data limit (*noteDONTPRINTYET DONTPRINTYET SVR4-style PIC (selected by '-KPIC', Assembler options MIPS
Options.), the size of symbols (Directives to override the size of
symbols MIPS Symbol Sizes.), and the small data limit (
Controlling the use of small data accesses MIPS Small Data.).
Sometimes it is undesirable to have one assembly instruction expand
to several machine instructions. The directive '.set nomacro' tells the
assembler to warn when this happens. '.set macro' restores the default
behavior.
Some macro instructions need a temporary register to store
intermediate results. This register is usually '$1', also known as
'$at', but it can be changed to any core register REG using '.set
at=REG'. Note that '$at' always refers to '$1' regardless of which
register is being used as the temporary register.
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 '.set noat' silences this warning while '.set at' restores the
default behavior. It is safe to use '.set noat' while '.set nomacro' is
in effect since single-instruction macros never need a temporary
register.
Note that while the GNU assembler provides these macros for
compatibility, it does not make any attempt to optimize them with the
surrounding code.
---------- Footnotes ----------
(1) The term "macro" is somewhat overloaded here, since these macros
have no relation to those defined by '.macro', '.macro' Macro.
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