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Except when they appear in the condition operand of a
`GIMPLE_COND`

, logical `and' and `or' operators are simplified
as follows: `a = b && c`

becomes

T1 = (bool)b; if (T1 == true) T1 = (bool)c; a = T1;

Note that `T1`

in this example cannot be an expression temporary,
because it has two different assignments.

All gimple operands are of type `tree`

. But only certain
types of trees are allowed to be used as operand tuples. Basic
validation is controlled by the function
`get_gimple_rhs_class`

, which given a tree code, returns an
`enum`

with the following values of type ```
enum
gimple_rhs_class
```

`GIMPLE_INVALID_RHS`

The tree cannot be used as a GIMPLE operand.`GIMPLE_BINARY_RHS`

The tree is a valid GIMPLE binary operation.`GIMPLE_UNARY_RHS`

The tree is a valid GIMPLE unary operation.`GIMPLE_SINGLE_RHS`

The tree is a single object, that cannot be split into simpler operands (for instance,`SSA_NAME`

,`VAR_DECL`

,`COMPONENT_REF`

, etc).This operand class also acts as an escape hatch for tree nodes that may be flattened out into the operand vector, but would need more than two slots on the RHS. For instance, a

`COND_EXPR`

expression of the form`(a op b) ? x : y`

could be flattened out on the operand vector using 4 slots, but it would also require additional processing to distinguish`c = a op b`

from`c = a op b ? x : y`

. Something similar occurs with`ASSERT_EXPR`

. In time, these special case tree expressions should be flattened into the operand vector.

For tree nodes in the categories `GIMPLE_BINARY_RHS`

and
`GIMPLE_UNARY_RHS`

, they cannot be stored inside tuples directly.
They first need to be flattened and separated into individual
components. For instance, given the GENERIC expression

a = b + c

its tree representation is:

MODIFY_EXPR <VAR_DECL <a>, PLUS_EXPR <VAR_DECL <b>, VAR_DECL <c>>>

In this case, the GIMPLE form for this statement is logically
identical to its GENERIC form but in GIMPLE, the `PLUS_EXPR`

on the RHS of the assignment is not represented as a tree,
instead the two operands are taken out of the `PLUS_EXPR`

sub-tree
and flattened into the GIMPLE tuple as follows:

GIMPLE_ASSIGN <PLUS_EXPR, VAR_DECL <a>, VAR_DECL <b>, VAR_DECL <c>>

The operand vector is stored at the bottom of the three tuple structures that accept operands. This means, that depending on the code of a given statement, its operand vector will be at different offsets from the base of the structure. To access tuple operands use the following accessors

— GIMPLE function: unsigned **gimple_num_ops** (`gimple g`)

Returns the number of operands in statement G.

— GIMPLE function: tree ***gimple_ops** (`gimple g`)

Returns a pointer into the operand vector for statement

`G`

. This is computed using an internal table called`gimple_ops_offset_`

[]. This table is indexed by the gimple code of`G`

.When the compiler is built, this table is filled-in using the sizes of the structures used by each statement code defined in gimple.def. Since the operand vector is at the bottom of the structure, for a gimple code

`C`

the offset is computed as sizeof (struct-of`C`

) - sizeof (tree).This mechanism adds one memory indirection to every access when using

`gimple_op`

(), if this becomes a bottleneck, a pass can choose to memoize the result from`gimple_ops`

() and use that to access the operands.

When adding a new operand to a gimple statement, the operand will
be validated according to what each tuple accepts in its operand
vector. These predicates are called by the
`gimple_<name>_set_...()`

. Each tuple will use one of the
following predicates (Note, this list is not exhaustive):

— GIMPLE function: is_gimple_operand **(**`tree t`)

This is the most permissive of the predicates. It essentially checks whether t has a

`gimple_rhs_class`

of`GIMPLE_SINGLE_RHS`

.

— GIMPLE function: is_gimple_val **(**`tree t`)

Returns true if t is a "GIMPLE value", which are all the non-addressable stack variables (variables for which

`is_gimple_reg`

returns true) and constants (expressions for which`is_gimple_min_invariant`

returns true).

— GIMPLE function: is_gimple_addressable **(**`tree t`)

Returns true if t is a symbol or memory reference whose address can be taken.

— GIMPLE function: is_gimple_asm_val **(**`tree t`)

Similar to

`is_gimple_val`

but it also accepts hard registers.

— GIMPLE function: is_gimple_call_addr **(**`tree t`)

Return true if t is a valid expression to use as the function called by a

`GIMPLE_CALL`

.

— GIMPLE function: is_gimple_min_invariant **(**`tree t`)

Return true if t is a valid minimal invariant. This is different from constants, in that the specific value of t may not be known at compile time, but it is known that it doesn't change (e.g., the address of a function local variable).

— GIMPLE function: is_gimple_min_invariant_address **(**`tree t`)

Return true if t is an

`ADDR_EXPR`

that does not change once the program is running.