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10.2.4.1 Blocks
Block scopes and the variables they declare in GENERIC and GIMPLE are
expressed using the BIND_EXPR code, which in previous versions of
GCC was primarily used for the C statement-expression extension.
Variables in a block are collected into BIND_EXPR_VARS in
declaration order. Any runtime initialization is moved out of
DECL_INITIAL and into a statement in the controlled block. When
gimplifying from C or C++, this initialization replaces the
DECL_STMT.
Variable-length arrays (VLAs) complicate this process, as their size often
refers to variables initialized earlier in the block. To handle this, we
currently split the block at that point, and move the VLA into a new, inner
BIND_EXPR. This strategy may change in the future.
DECL_SAVED_TREE for a GIMPLE function will always be a
BIND_EXPR which contains declarations for the temporary variables
used in the function.
A C++ program will usually contain more BIND_EXPRs than there are
syntactic blocks in the source code, since several C++ constructs have
implicit scopes associated with them. On the other hand, although the C++
front end uses pseudo-scopes to handle cleanups for objects with
destructors, these don't translate into the GIMPLE form; multiple
declarations at the same level use the same BIND_EXPR.