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gfc-internals.texi (section gfc_code): Extended documentation about gfc_code in the internal datastructures chapter...

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2008-06-28  Daniel Kraft  <d@domob.eu>

	* gfc-internals.texi (section gfc_code):  Extended documentation about
	gfc_code in the internal datastructures chapter including details about
	how IF, DO and SELECT blocks look like and an example for how the
	block-chaining works.

From-SVN: r137226
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Daniel Kraft 2008-06-28 17:11:25 +02:00 • committed by Daniel Kraft
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commit b66c86cbee
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7
gcc/fortran/ChangeLog
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@ -1,3 +1,10 @@
2008-06-28 Daniel Kraft <d@domob.eu>
* gfc-internals.texi (section gfc_code): Extended documentation about
gfc_code in the internal datastructures chapter including details about
how IF, DO and SELECT blocks look like and an example for how the
block-chaining works.
2008-06-25 Paul Thomas <pault@gcc.gnu.org>
PR fortran/36526

99
gcc/fortran/gfc-internals.texi
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@ -284,9 +284,12 @@ should exhaust all possible valid combinations of content for these
structures.
@menu
* gfc_code:: Representation of Executable Statements
* gfc_code:: Representation of Executable Statements.
@end menu
@c gfc_code
@c --------
@node gfc_code
@section @code{gfc_code}
@cindex statement chaining
@ -309,13 +312,95 @@ current statement.
If the current statement is one of @code{IF}, @code{DO}, @code{SELECT}
it starts a block, i.e.@: a nested level in the program. In order to
represent this, the @code{block} member is set to point to a
@code{gfc_code} structure whose @code{block} member points to the
block in question. The @code{SELECT} and @code{IF} statements may
contain various blocks (the chain of @code{ELSE IF} and @code{ELSE}
blocks or the various @code{CASE}s, respectively).
@code{gfc_code} structure whose @code{next} member starts the chain of
statements inside the block; this structure's @code{op} member should be set to
the same value as the parent structure's @code{op} member. The @code{SELECT}
and @code{IF} statements may contain various blocks (the chain of @code{ELSE IF}
and @code{ELSE} blocks or the various @code{CASE}s, respectively). These chains
are linked-lists formed by the @code{block} members.
@c What would be nice here would be an example program together with
@c an image that says more than the mythical thousand words.
Consider the following example code:
@example
IF (foo < 20) THEN
PRINT *, "Too small"
foo = 20
ELSEIF (foo > 50) THEN
PRINT *, "Too large"
foo = 50
ELSE
PRINT *, "Good"
END IF
@end example
This statement-block will be represented in the internal gfortran tree as
follows, were the horizontal link-chains are those induced by the @code{next}
members and vertical links down are those of @code{block}. @samp{==|} and
@samp{--|} mean @code{NULL} pointers to mark the end of a chain:
@example
... ==> IF ==> ...
|
+--> IF foo < 20 ==> PRINT *, "Too small" ==> foo = 20 ==|
|
+--> IF foo > 50 ==> PRINT *, "Too large" ==> foo = 50 ==|
|
+--> ELSE ==> PRINT *, "Good" ==|
|
+--|
@end example
@subsection IF Blocks
Conditionals are represented by @code{gfc_code} structures with their
@code{op} member set to @code{EXEC_IF}. This structure's @code{block}
member must point to another @code{gfc_code} node that is the header of the
if-block. This header's @code{op} member must be set to @code{EXEC_IF}, too,
its @code{expr} member holds the condition to check for, and its @code{next}
should point to the code-chain of the statements to execute if the condition is
true.
If in addition an @code{ELSEIF} or @code{ELSE} block is present, the
@code{block} member of the if-block-header node points to yet another
@code{gfc_code} structure that is the header of the elseif- or else-block. Its
structure is identical to that of the if-block-header, except that in case of an
@code{ELSE} block without a new condition the @code{expr} member should be
@code{NULL}. This block can itself have its @code{block} member point to the
next @code{ELSEIF} or @code{ELSE} block if there's a chain of them.
@subsection Loops
@code{DO} loops are stored in the tree as @code{gfc_code} nodes with their
@code{op} set to @code{EXEC_DO} for a @code{DO} loop with iterator variable and
to @code{EXEC_DO_WHILE} for infinite @code{DO}s and @code{DO WHILE} blocks.
Their @code{block} member should point to a @code{gfc_code} structure heading
the code-chain of the loop body; its @code{op} member should be set to
@code{EXEC_DO} or @code{EXEC_DO_WHILE}, too, respectively.
For @code{DO WHILE} loops, the loop condition is stored on the top
@code{gfc_code} structure's @code{expr} member; @code{DO} forever loops are
simply @code{DO WHILE} loops with a constant @code{.TRUE.} loop condition in
the internal representation.
Similarly, @code{DO} loops with an iterator have instead of the condition their
@code{ext.iterator} member set to the correct values for the loop iterator
variable and its range.
@subsection @code{SELECT} Statements
A @code{SELECT} block is introduced by a @code{gfc_code} structure with an
@code{op} member of @code{EXEC_SELECT} and @code{expr} containing the expression
to evaluate and test. Its @code{block} member starts a list of @code{gfc_code}
structures linked together by their @code{block} members that stores the various
@code{CASE} parts.
Each @code{CASE} node has its @code{op} member set to @code{EXEC_SELECT}, too,
its @code{next} member points to the code-chain to be executed in the current
case-block, and @code{extx.case_list} contains the case-values this block
corresponds to. The @code{block} member links to the next case in the list.
@c ---------------------------------------------------------------------