What exactly does is_variable_sized test for?

[Ian Lance Taylor]

Amittai Aviram <amittai.Aviram@yale.edu> writes:

> I am trying to understand some code in gcc/omp-low.c that calls the predicate is_variable_sized:
>
> omp-low.c:is_variable_sized (const_tree expr)
>
> where const_tree is a pointer to a constant tree_node, which is a union of a bunch of structures, but, in this case, it must be (I think) a tree_var_decl structure.  It is defined as follows:
>
> static inline bool
> is_variable_sized (const_tree expr)
> {
>   return !TREE_CONSTANT (TYPE_SIZE_UNIT (TREE_TYPE (expr)));
> }
>
> Unpacking the macros, I get
>   return !expr->common.type->type.size_unit->base.constant_flag;
>
> But what exactly does this mean?  What kind of C variable counts as "variable sized"?  Does that mean an array or struct type?  Thanks!

In

void foo(int i) { int a[i]; }

the local variable a is variably sized.  Or, for that matter, in this

void foo(int i) { struct s { int a[i]; } v; }

the local variable v is variably sized.  So both structs and arrays can
be variably sized, although it is not the common case for either.

By the way, don't try to unpack the macros.  Instead, look for the
definitions in tree.h and read the comments there.  Unpacking the macros
is complicated and is far more confusing than helpful.

Ian