There are often cases where multiple RTL expressions could represent an operation performed by a single machine instruction. This situation is most commonly encountered with logical, branch, and multiply-accumulate instructions. In such cases, the compiler attempts to convert these multiple RTL expressions into a single canonical form to reduce the number of insn patterns required.
In addition to algebraic simplifications, following canonicalizations are performed:
neg
, not
,
mult
, plus
, or minus
expression, it will be the
first operand.
compare
operator, a constant is always the second operand
on machines where cc0
is used (see section Defining Jump Instruction Patterns). On other
machines, there are rare cases where the compiler might want to construct
a compare
with a constant as the first operand. However, these
cases are not common enough for it to be worthwhile to provide a pattern
matching a constant as the first operand unless the machine actually has
such an instruction.
An operand of neg
, not
, mult
, plus
, or
minus
is made the first operand under the same conditions as
above.
(minus x (const_int n))
is converted to
(plus x (const_int -n))
.
mem
), a left shift is
converted into the appropriate multiplication by a power of two.
not
expression, it will be the first one.
A machine that has an instruction that performs a bitwise logical-and of one
operand with the bitwise negation of the other should specify the pattern
for that instruction as
(define_insn "" [(set (match_operand:m 0 ...) (and:m (not:m (match_operand:m 1 ...)) (match_operand:m 2 ...)))] "..." "...")Similarly, a pattern for a "NAND" instruction should be written
(define_insn "" [(set (match_operand:m 0 ...) (ior:m (not:m (match_operand:m 1 ...)) (not:m (match_operand:m 2 ...))))] "..." "...")In both cases, it is not necessary to include patterns for the many logically equivalent RTL expressions.
(xor:m x y)
and (not:m (xor:m x y))
.
(plus:m (plus:m x y) constant)
cc0
,
(compare x (const_int 0))
will be converted to
x.
zero_extract
rather than the equivalent
and
or sign_extract
operations.