Using and Porting GNU CC
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. 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
If the comparison operation is producing data to be stored in some
variable, the mode must be in class
MODE_INT. All comparison
operations producing data must use the same mode, which is
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 instructing setting the
condition code must be adjacent to the instruction using the condition
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
(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
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
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)
(ne:m x y)
(gt:m x y)
(gtu:m x y)
gtbut does unsigned comparison, on fixed-point numbers only.
(lt:m x y)
(ltu:m x y)
gtubut test for ``less than''.
(ge:m x y)
(geu:m x y)
gtubut test for ``greater than or equal''.
(le:m x y)
(leu:m x y)
gtubut test for ``less than or equal''.
(if_then_else cond then else)
On most machines,
if_then_else expressions are valid only
to express conditional jumps.
(cond [test1 value1 test2 value2 ...] default)
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 non-zero test, or default if none of the tests are non-zero expressions.
This is currently not valid for instruction patterns and is supported only for insn attributes. See Insn Attributes.