GNU Compiler Collection (GCC) Internals: Comparisons

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### 13.10 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` (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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