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(gcc-295.info)Comparisons

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Comparison Operations
=====================

   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' (Note: 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 `VOIDmode'.  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 machine-specific.

   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 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)'
     1 if the values represented by X and Y are equal, otherwise 0.

`(ne:M X Y)'
     1 if the values represented by X and Y are not equal, otherwise 0.

`(gt:M X Y)'
     1 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 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.  Note: Insn Attributes.