GNU Compiler Collection (GCC) Internals: Comparisons |
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Comparison operators test a relation on two operands and are considered
to represent a machine-dependent nonzero value described by, but not
necessarily equal to, STORE_FLAG_VALUE
(see Misc)
if the relation holds, or zero if it does not, for comparison operators
whose results have a ‘MODE_INT’ mode,
FLOAT_STORE_FLAG_VALUE
(see Misc) if the relation holds, or
zero if it does not, for comparison operators that return floating-point
values, and a vector of either VECTOR_STORE_FLAG_VALUE
(see Misc)
if the relation holds, or of zeros if it does not, for comparison operators
that return vector results.
The mode of the comparison operation is independent of the mode
of the data being compared. If the comparison operation is being tested
(e.g., the first operand of an if_then_else
), the mode must be
VOIDmode
.
There are two ways that comparison operations may be used. The
comparison operators may be used to compare the condition codes
(cc0)
against zero, as in (eq (cc0) (const_int 0))
. Such
a construct actually refers to the result of the preceding instruction
in which the condition codes were set. The instruction setting the
condition code must be adjacent to the instruction using the condition
code; only note
insns may separate them.
Alternatively, a comparison operation may directly compare two data objects. The mode of the comparison is determined by the operands; they must both be valid for a common machine mode. A comparison with both operands constant would be invalid as the machine mode could not be deduced from it, but such a comparison should never exist in RTL due to constant folding.
In the example above, if (cc0)
were last set to
(compare x y)
, the comparison operation is
identical to (eq x y)
. Usually only one style
of comparisons is supported on a particular machine, but the combine
pass will try to merge the operations to produce the eq
shown
in case it exists in the context of the particular insn involved.
Inequality comparisons come in two flavors, signed and unsigned. Thus,
there are distinct expression codes gt
and gtu
for signed and
unsigned greater-than. These can produce different results for the same
pair of integer values: for example, 1 is signed greater-than -1 but not
unsigned greater-than, because -1 when regarded as unsigned is actually
0xffffffff
which is greater than 1.
The signed comparisons are also used for floating point values. Floating point comparisons are distinguished by the machine modes of the operands.
(eq:m x y)
STORE_FLAG_VALUE
if the values represented by x and y
are equal, otherwise 0.
(ne:m x y)
STORE_FLAG_VALUE
if the values represented by x and y
are not equal, otherwise 0.
(gt:m x y)
STORE_FLAG_VALUE
if the x is greater than y. If they
are fixed-point, the comparison is done in a signed sense.
(gtu:m x y)
Like gt
but does unsigned comparison, on fixed-point numbers only.
(lt:m x y)
(ltu:m x y)
Like gt
and gtu
but test for “less than”.
(ge:m x y)
(geu:m x y)
Like gt
and gtu
but test for “greater than or equal”.
(le:m x y)
(leu:m x y)
Like gt
and gtu
but test for “less than or equal”.
(if_then_else cond then else)
This is not a comparison operation but is listed here because it is always used in conjunction with a comparison operation. To be precise, cond is a comparison expression. This expression represents a choice, according to cond, between the value represented by then and the one represented by else.
On most machines, if_then_else
expressions are valid only
to express conditional jumps.
(cond [test1 value1 test2 value2 …] default)
Similar to if_then_else
, but more general. Each of test1,
test2, … is performed in turn. The result of this expression is
the value corresponding to the first nonzero test, or default if
none of the tests are nonzero expressions.
This is currently not valid for instruction patterns and is supported only for insn attributes. See Insn Attributes.
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