Next: Comparisons, Previous: Regs and Memory, Up: RTL
Unless otherwise specified, all the operands of arithmetic expressions
must be valid for mode m. An operand is valid for mode m
if it has mode m, or if it is a const_int
or
const_double
and m is a mode of class MODE_INT
.
For commutative binary operations, constants should be placed in the second operand.
(plus:
m x y)
(ss_plus:
m x y)
(us_plus:
m x y)
plus
wraps round modulo the width of m; ss_plus
saturates at the maximum signed value representable in m;
us_plus
saturates at the maximum unsigned value.
(lo_sum:
m x y)
high
(see Constants) to
represent the typical two-instruction sequence used in RISC machines
to reference a global memory location.
The number of low order bits is machine-dependent but is
normally the number of bits in a Pmode
item minus the number of
bits set by high
.
m should be Pmode
.
(minus:
m x y)
(ss_minus:
m x y)
(us_minus:
m x y)
plus
(see above).
(compare:
m x y)
Of course, machines can't really subtract with infinite precision.
However, they can pretend to do so when only the sign of the result will
be used, which is the case when the result is stored in the condition
code. And that is the only way this kind of expression may
validly be used: as a value to be stored in the condition codes, either
(cc0)
or a register. See Comparisons.
The mode m is not related to the modes of x and y, but
instead is the mode of the condition code value. If (cc0)
is
used, it is VOIDmode
. Otherwise it is some mode in class
MODE_CC
, often CCmode
. See Condition Code. If m
is VOIDmode
or CCmode
, the operation returns sufficient
information (in an unspecified format) so that any comparison operator
can be applied to the result of the COMPARE
operation. For other
modes in class MODE_CC
, the operation only returns a subset of
this information.
Normally, x and y must have the same mode. Otherwise,
compare
is valid only if the mode of x is in class
MODE_INT
and y is a const_int
or
const_double
with mode VOIDmode
. The mode of x
determines what mode the comparison is to be done in; thus it must not
be VOIDmode
.
If one of the operands is a constant, it should be placed in the second operand and the comparison code adjusted as appropriate.
A compare
specifying two VOIDmode
constants is not valid
since there is no way to know in what mode the comparison is to be
performed; the comparison must either be folded during the compilation
or the first operand must be loaded into a register while its mode is
still known.
(neg:
m x)
(ss_neg:
m x)
neg
, the negation of the operand may be a number not representable
in mode m, in which case it is truncated to m. ss_neg
ensures that an out-of-bounds result saturates to the maximum or minimum
representable value.
(mult:
m x y)
Some machines support a multiplication that generates a product wider than the operands. Write the pattern for this as
(mult:m (sign_extend:m x) (sign_extend:m y))
where m is wider than the modes of x and y, which need not be the same.
For unsigned widening multiplication, use the same idiom, but with
zero_extend
instead of sign_extend
.
(div:
m x y)
Some machines have division instructions in which the operands and
quotient widths are not all the same; you should represent
such instructions using truncate
and sign_extend
as in,
(truncate:m1 (div:m2 x (sign_extend:m2 y)))
(udiv:
m x y)
div
but represents unsigned division.
(mod:
m x y)
(umod:
m x y)
div
and udiv
but represent the remainder instead of
the quotient.
(smin:
m x y)
(smax:
m x y)
smin
) or larger (for smax
) of
x and y, interpreted as signed values in mode m.
When used with floating point, if both operands are zeros, or if either
operand is NaN
, then it is unspecified which of the two operands
is returned as the result.
(umin:
m x y)
(umax:
m x y)
smin
and smax
, but the values are interpreted as unsigned
integers.
(not:
m x)
(and:
m x y)
(ior:
m x y)
(xor:
m x y)
(ashift:
m x c)
(ss_ashift:
m x c)
ashift
operation is a plain shift with no special behavior
in case of a change in the sign bit; ss_ashift
saturates to the minimum
or maximum representable value if any of the bits shifted out differs from the
final sign bit.
x have mode m, a fixed-point machine mode. c
be a fixed-point mode or be a constant with mode VOIDmode
; which
mode is determined by the mode called for in the machine description
entry for the left-shift instruction. For example, on the VAX, the mode
of c is QImode
regardless of m.
(lshiftrt:
m x c)
(ashiftrt:
m x c)
ashift
but for right shift. Unlike the case for left shift,
these two operations are distinct.
(rotate:
m x c)
(rotatert:
m x c)
rotate
.
(abs:
m x)
(sqrt:
m x)
(ffs:
m x)
(clz:
m x)
CLZ_DEFINED_VALUE_AT_ZERO
. Note that this is one of
the few expressions that is not invariant under widening. The mode of
x will usually be an integer mode.
(ctz:
m x)
CTZ_DEFINED_VALUE_AT_ZERO
. Except for this case,
ctz(x)
is equivalent to ffs(
x) - 1
. The mode of
x will usually be an integer mode.
(popcount:
m x)
(parity:
m x)