forgot to add ffe.texi, the actual g77 front-end internals docs

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+@node Front End
+@chapter Front End
+@cindex GNU Fortran Front End (FFE)
+@cindex FFE
+@cindex @code{g77}, front end
+@cindex front end, @code{g77}
+
+This chapter describes some aspects of the design and implementation
+of the @code{g77} front end.
+
+@menu
+* Philosophy of Code Generation::
+* Two-pass Design::
+* Challenges Posed::
+* Transforming Statements::
+* Transforming Expressions::
+@end menu
+
+@node Philosophy of Code Generation
+@section Philosophy of Code Generation
+
+Don't poke the bear.
+
+The @code{g77} front end generates code
+via the @code{gcc} back end.
+
+@cindex GNU Back End (GBE)
+@cindex GBE
+@cindex @code{gcc}, back end
+@cindex back end, gcc
+@cindex code generator
+The @code{gcc} back end (GBE) is a large, complex
+labyrinth of intricate code
+written in a combination of the C language
+and specialized languages internal to @code{gcc}.
+
+While the @emph{code} that implements the GBE
+is written in a combination of languages,
+the GBE itself is,
+to the front end for a language like Fortran,
+best viewed as a @emph{compiler}
+that compiles its own, unique, language.
+
+The GBE's ``source'', then, is written in this language,
+which consists primarily of
+a combination of calls to GBE functions
+and @dfn{tree} nodes
+(which are, themselves, created
+by calling GBE functions).
+
+So, the @code{g77} generates code by, in effect,
+translating the Fortran code it reads
+into a form ``written'' in the ``language''
+of the @code{gcc} back end.
+
+@cindex GBEL
+@cindex GNU Back End Language (GBEL)
+This language will heretofore be referred to as @dfn{GBEL},
+for GNU Back End Language.
+
+GBEL is an evolving language,
+not fully specified in any published form
+as of this writing.
+It offers many facilities,
+but its ``core'' facilities
+are those that corresponding most directly
+to those needed to support @code{gcc}
+(compiling code written in GNU C).
+
+The @code{g77} Fortran Front End (FFE)
+is designed and implemented
+to navigate the currents and eddies
+of ongoing GBEL and @code{gcc} development
+while also delivering on the potential
+of an integrated FFE
+(as compared to using a converter like @code{f2c}
+and feeding the output into @code{gcc}).
+
+Goals of the FFE's code-generation strategy include:
+
+@itemize @bullet
+@item
+High likelihood of generation of correct code,
+or, failing that, producing a fatal diagnostic or crashing.
+
+@item
+Generation of highly optimized code,
+as directed by the user
+via GBE-specific (versus @code{g77}-specific) constructs,
+such as command-line options.
+
+@item
+Fast overall (FFE plus GBE) compilation.
+
+@item
+Preservation of source-level debugging information.
+@end itemize
+
+The strategies historically, and currently, used by the FFE
+to achieve these goals include:
+
+@itemize @bullet
+@item
+Use of GBEL constructs that most faithfully encapsulate
+the semantics of Fortran.
+
+@item
+Avoidance of GBEL constructs that are so rarely used,
+or limited to use in specialized situations not related to Fortran,
+that their reliability and performance has not yet been established
+as sufficient for use by the FFE.
+
+@item
+Flexible design, to readily accommodate changes to specific
+code-generation strategies, perhaps governed by command-line options.
+@end itemize
+
+@cindex Bear-poking
+@cindex Poking the bear
+``Don't poke the bear'' somewhat summarizes the above strategies.
+The GBE is the bear.
+The FFE is designed and implemented to avoid poking it
+in ways that are likely to just annoy it.
+The FFE usually either tackles it head-on,
+or avoids treating it in ways dissimilar to how
+the @code{gcc} front end treats it.
+
+For example, the FFE uses the native array facility in the back end
+instead of the lower-level pointer-arithmetic facility
+used by @code{gcc} when compiling @code{f2c} output).
+Theoretically, this presents more opportunities for optimization,
+faster compile times,
+and the production of more faithful debugging information.
+These benefits were not, however, immediately realized,
+mainly because @code{gcc} itself makes little or no use
+of the native array facility.
+
+Complex arithmetic is a case study of the evolution of this strategy.
+When originally implemented,
+the GBEL had just evolved its own native complex-arithmetic facility,
+so the FFE took advantage of that.
+
+When porting @code{g77} to 64-bit systems,
+it was discovered that the GBE didn't really
+implement its native complex-arithmetic facility properly.
+
+The short-term solution was to rewrite the FFE
+to instead use the lower-level facilities
+that'd be used by @code{gcc}-compiled code
+(assuming that code, itself, didn't use the native complex type
+provided, as an extension, by @code{gcc}),
+since these were known to work,
+and, in any case, if shown to not work,
+would likely be rapidly fixed
+(since they'd likely not work for vanilla C code in similar circumstances).
+
+However, the rewrite accommodated the original, native approach as well
+by offering a command-line option to select it over the emulated approach.
+This allowed users, and especially GBE maintainers, to try out
+fixes to complex-arithmetic support in the GBE
+while @code{g77} continued to default to compiling more code correctly,
+albeit producing (typically) slower executables.
+
+As of April 1999, it appeared that the last few bugs
+in the GBE's support of its native complex-arithmetic facility
+were worked out.
+The FFE was changed back to default to using that native facility,
+leaving emulation as an option.
+
+Other Fortran constructs---arrays, character strings,
+complex division, @code{COMMON} and @code{EQUIVALENCE} aggregates,
+and so on---involve issues similar to those pertaining to complex arithmetic.
+
+So, it is possible that the history
+of how the FFE handled complex arithmetic
+will be repeated, probably in modified form
+(and hopefully over shorter timeframes),
+for some of these other facilities.
+
+@node Two-pass Design
+@section Two-pass Design
+
+The FFE does not tell the GBE anything about a program unit
+until after the last statement in that unit has been parsed.
+(A program unit is a Fortran concept that corresponds, in the C world,
+mostly closely to functions definitions in ISO C.
+That is, a program unit in Fortran is like a top-level function in C.
+Nested functions, found among the extensions offered by GNU C,
+correspond roughly to Fortran's statement functions.)
+
+So, while parsing the code in a program unit,
+the FFE saves up all the information
+on statements, expressions, names, and so on,
+until it has seen the last statement.
+
+At that point, the FFE revisits the saved information
+(in what amounts to a second @dfn{pass} over the program unit)
+to perform the actual translation of the program unit into GBEL,
+ultimating in the generation of assembly code for it.
+
+Some lookahead is performed during this second pass,
+so the FFE could be viewed as a ``two-plus-pass'' design.
+
+@menu
+* Two-pass Code::
+* Why Two Passes::
+@end menu
+
+@node Two-pass Code
+@subsection Two-pass Code
+
+Most of the code that turns the first pass (parsing)
+into a second pass for code generation
+is in @file{@value{path-g77}/std.c}.
+
+It has external functions,
+called mainly by siblings in @file{@value{path-g77}/stc.c},
+that record the information on statements and expressions
+in the order they are seen in the source code.
+These functions save that information.
+
+It also has an external function that revisits that information,
+calling the siblings in @file{@value{path-g77}/ste.c},
+which handles the actual code generation
+(by generating GBEL code,
+that is, by calling GBE routines
+to represent and specify expressions, statements, and so on).
+
+@node Why Two Passes
+@subsection Why Two Passes
+
+The need for two passes was not immediately evident
+during the design and implementation of the code in the FFE
+that was to produce GBEL.
+Only after a few kludges,
+to handle things like incorrectly-guessed @code{ASSIGN} label nature,
+had been implemented,
+did enough evidence pile up to make it clear
+that @file{std.c} had to be introduced to intercept,
+save, then revisit as part of a second pass,
+the digested contents of a program unit.
+
+Other such missteps have occurred during the evolution of the FFE,
+because of the different goals of the FFE and the GBE.
+
+Because the GBE's original, and still primary, goal
+was to directly support the GNU C language,
+the GBEL, and the GBE itself,
+requires more complexity
+on the part of most front ends
+than it requires of @code{gcc}'s.
+
+For example,
+the GBEL offers an interface that permits the @code{gcc} front end
+to implement most, or all, of the language features it supports,
+without the front end having to
+make use of non-user-defined variables.
+(It's almost certainly the case that all of K&R C,
+and probably ANSI C as well,
+is handled by the @code{gcc} front end
+without declaring such variables.)
+
+The FFE, on the other hand, must resort to a variety of ``tricks''
+to achieve its goals.
+
+Consider the following C code:
+
+@smallexample
+int
+foo (int a, int b)
+@{
+  int c = 0;
+
+  if ((c = bar (c)) == 0)
+    goto done;
+
+  quux (c << 1);
+
+done:
+  return c;
+@}
+@end smallexample
+
+Note what kinds of objects are declared, or defined, before their use,
+and before any actual code generation involving them
+would normally take place:
+
+@itemize @bullet
+@item
+Return type of function
+
+@item
+Entry point(s) of function
+
+@item
+Dummy arguments
+
+@item
+Variables
+
+@item
+Initial values for variables
+@end itemize
+
+Whereas, the following items can, and do,
+suddenly appear ``out of the blue'' in C:
+
+@itemize @bullet
+@item
+Label references
+
+@item
+Function references
+@end itemize
+
+Not surprisingly, the GBE faithfully permits the latter set of items
+to be ``discovered'' partway through GBEL ``programs'',
+just as they are permitted to in C.
+
+Yet, the GBE has tended, at least in the past,
+to be reticent to fully support similar ``late'' discovery
+of items in the former set.
+
+This makes Fortran a poor fit for the ``safe'' subset of GBEL.
+Consider:
+
+@smallexample
+      FUNCTION X (A, ARRAY, ID1)
+      CHARACTER*(*) A
+      DOUBLE PRECISION X, Y, Z, TMP, EE, PI
+      REAL ARRAY(ID1*ID2)
+      COMMON ID2
+      EXTERNAL FRED
+
+      ASSIGN 100 TO J
+      CALL FOO (I)
+      IF (I .EQ. 0) PRINT *, A(0)
+      GOTO 200
+
+      ENTRY Y (Z)
+      ASSIGN 101 TO J
+200   PRINT *, A(1)
+      READ *, TMP
+      GOTO J
+100   X = TMP * EE
+      RETURN
+101   Y = TMP * PI
+      CALL FRED
+      DATA EE, PI /2.71D0, 3.14D0/
+      END
+@end smallexample
+
+Here are some observations about the above code,
+which, while somewhat contrived,
+conforms to the FORTRAN 77 and Fortran 90 standards:
+
+@itemize @bullet
+@item
+The return type of function @samp{X} is not known
+until the @samp{DOUBLE PRECISION} line has been parsed.
+
+@item
+Whether @samp{A} is a function or a variable
+is not known until the @samp{PRINT *, A(0)} statement
+has been parsed.
+
+@item
+The bounds of the array of argument @samp{ARRAY}
+depend on a computation involving
+the subsequent argument @samp{ID1}
+and the blank-common member @samp{ID2}.
+
+@item
+Whether @samp{Y} and @samp{Z} are local variables,
+additional function entry points,
+or dummy arguments to additional entry points
+is not known
+until the @samp{ENTRY} statement is parsed.
+
+@item
+Similarly, whether @samp{TMP} is a local variable is not known
+until the @samp{READ *, TMP} statement is parsed.
+
+@item
+The initial values for @samp{EE} and @samp{PI}
+are not known until after the @samp{DATA} statement is parsed.
+
+@item
+Whether @samp{FRED} is a function returning type @code{REAL}
+or a subroutine
+(which can be thought of as returning type @code{void}
+@emph{or}, to support alternate returns in a simple way,
+type @code{int})
+is not known
+until the @samp{CALL FRED} statement is parsed.
+
+@item
+Whether @samp{100} is a @code{FORMAT} label
+or the label of an executable statement
+is not known
+until the @samp{X =} statement is parsed.
+(These two types of labels get @emph{very} different treatment,
+especially when @code{ASSIGN}'ed.)
+
+@item
+That @samp{J} is a local variable is not known
+until the first @samp{ASSIGN} statement is parsed.
+(This happens @emph{after} executable code has been seen.)
+@end itemize
+
+Very few of these ``discoveries''
+can be accommodated by the GBE as it has evolved over the years.
+The GBEL doesn't support several of them,
+and those it might appear to support
+don't always work properly,
+especially in combination with other GBEL and GBE features,
+as implemented in the GBE.
+
+(Had the GBE and its GBEL originally evolved to support @code{g77},
+the shoe would be on the other foot, so to speak---most, if not all,
+of the above would be directly supported by the GBEL,
+and a few C constructs would probably not, as they are in reality,
+be supported.
+Both this mythical, and today's real, GBE caters to its GBEL
+by, sometimes, scrambling around, cleaning up after itself---after
+discovering that assumptions it made earlier during code generation
+are incorrect.)
+
+So, the FFE handles these discrepancies---between the order in which
+it discovers facts about the code it is compiling,
+and the order in which the GBEL and GBE support such discoveries---by
+performing what amounts to two
+passes over each program unit.
+
+(A few ambiguities can remain at that point,
+such as whether, given @samp{EXTERNAL BAZ}
+and no other reference to @samp{BAZ} in the program unit,
+it is a subroutine, a function, or a block-data---which, in C-speak,
+governs its declared return type.
+Fortunately, these distinctions are easily finessed
+for the procedure, library, and object-file interfaces
+supported by @code{g77}.)
+
+@node Challenges Posed
+@section Challenges Posed
+
+Consider the following Fortran code, which uses various extensions
+(including some to Fortran 90):
+
+@smallexample
+SUBROUTINE X(A)
+CHARACTER*(*) A
+COMPLEX CFUNC
+INTEGER*2 CLOCKS(200)
+INTEGER IFUNC
+
+CALL SYSTEM_CLOCK (CLOCKS (IFUNC (CFUNC ('('//A//')'))))
+@end smallexample
+
+The above poses the following challenges to any Fortran compiler
+that uses run-time interfaces, and a run-time library, roughly similar
+to those used by @code{g77}:
+
+@itemize @bullet
+@item
+Assuming the library routine that supports @code{SYSTEM_CLOCK}
+expects to set an @code{INTEGER*4} variable via its @code{COUNT} argument,
+the compiler must make available to it a temporary variable of that type.
+
+@item
+Further, after the @code{SYSTEM_CLOCK} library routine returns,
+the compiler must ensure that the temporary variable it wrote
+is copied into the appropriate element of the @samp{CLOCKS} array.
+(This assumes the compiler doesn't just reject the code,
+which it should if it is compiling under some kind of a "strict" option.)
+
+@item
+To determine the correct index into the @samp{CLOCKS} array,
+(putting aside the fact that the index, in this particular case,
+need not be computed until after
+the @code{SYSTEM_CLOCK} library routine returns),
+the compiler must ensure that the @code{IFUNC} function is called.
+
+That requires evaluating its argument,
+which requires, for @code{g77}
+(assuming @code{-ff2c} is in force),
+reserving a temporary variable of type @code{COMPLEX}
+for use as a repository for the return value
+being computed by @samp{CFUNC}.
+
+@item
+Before invoking @samp{CFUNC},
+is argument must be evaluated,
+which requires allocating, at run time,
+a temporary large enough to hold the result of the concatenation,
+as well as actually performing the concatenation.
+
+@item
+The large temporary needed during invocation of @code{CFUNC}
+should, ideally, be deallocated
+(or, at least, left to the GBE to dispose of, as it sees fit)
+as soon as @code{CFUNC} returns,
+which means before @code{IFUNC} is called
+(as it might need a lot of dynamically allocated memory).
+@end itemize
+
+@code{g77} currently doesn't support all of the above,
+but, so that it might someday, it has evolved to handle
+at least some of the above requirements.
+
+Meeting the above requirements is made more challenging
+by conforming to the requirements of the GBEL/GBE combination.
+
+@node Transforming Statements
+@section Transforming Statements
+
+Most Fortran statements are given their own block,
+and, for temporary variables they might need, their own scope.
+(A block is what distinguishes @samp{@{ foo (); @}}
+from just @samp{foo ();} in C.
+A scope is included with every such block,
+providing a distinct name space for local variables.)
+
+Label definitions for the statement precede this block,
+so @samp{10 PRINT *, I} is handled more like
+@samp{fl10: @{ @dots{} @}} than @samp{@{ fl10: @dots{} @}}
+(where @samp{fl10} is just a notation meaning ``Fortran Label 10''
+for the purposes of this document).
+
+@menu
+* Statements Needing Temporaries::
+* Transforming DO WHILE::
+* Transforming Iterative DO::
+* Transforming Block IF::
+* Transforming SELECT CASE::
+@end menu
+
+@node Statements Needing Temporaries
+@subsection Statements Needing Temporaries
+
+Any temporaries needed during, but not beyond,
+execution of a Fortran statement,
+are made local to the scope of that statement's block.
+
+This allows the GBE to share storage for these temporaries
+among the various statements without the FFE
+having to manage that itself.
+
+(The GBE could, of course, decide to optimize 
+management of these temporaries.
+For example, it could, theoretically,
+schedule some of the computations involving these temporaries
+to occur in parallel.
+More practically, it might leave the storage for some temporaries
+``live'' beyond their scopes, to reduce the number of
+manipulations of the stack pointer at run time.)
+
+Temporaries needed across distinct statement boundaries usually
+are associated with Fortran blocks (such as @code{DO}/@code{END DO}).
+(Also, there might be temporaries not associated with blocks at all---these
+would be in the scope of the entire program unit.)
+
+Each Fortran block @emph{should} get its own block/scope in the GBE.
+This is best, because it allows temporaries to be more naturally handled.
+However, it might pose problems when handling labels
+(in particular, when they're the targets of @code{GOTO}s outside the Fortran
+block), and generally just hassling with replicating
+parts of the @code{gcc} front end
+(because the FFE needs to support
+an arbitrary number of nested back-end blocks
+if each Fortran block gets one).
+
+So, there might still be a need for top-level temporaries, whose
+``owning'' scope is that of the containing procedure.
+
+Also, there seems to be problems declaring new variables after
+generating code (within a block) in the back end, leading to, e.g.,
+@samp{label not defined before binding contour} or similar messages,
+when compiling with @samp{-fstack-check} or
+when compiling for certain targets.
+
+Because of that, and because sometimes these temporaries are not
+discovered until in the middle of of generating code for an expression
+statement (as in the case of the optimization for @samp{X**I}),
+it seems best to always
+pre-scan all the expressions that'll be expanded for a block
+before generating any of the code for that block.
+
+This pre-scan then handles discovering and declaring, to the back end,
+the temporaries needed for that block.
+
+It's also important to treat distinct items in an I/O list as distinct
+statements deserving their own blocks.
+That's because there's a requirement
+that each I/O item be fully processed before the next one,
+which matters in cases like @samp{READ (*,*), I, A(I)}---the
+element of @samp{A} read in the second item
+@emph{must} be determined from the value
+of @samp{I} read in the first item.
+
+@node Transforming DO WHILE
+@subsection Transforming DO WHILE
+
+@samp{DO WHILE(expr)} @emph{must} be implemented
+so that temporaries needed to evaluate @samp{expr}
+are generated just for the test, each time.
+
+Consider how @samp{DO WHILE (A//B .NE. 'END'); @dots{}; END DO} is transformed:
+
+@smallexample
+for (;;)
+  @{
+    int temp0;
+
+    @{
+      char temp1[large];
+
+      libg77_catenate (temp1, a, b);
+      temp0 = libg77_ne (temp1, 'END');
+    @}
+
+    if (! temp0)
+      break;
+
+    @dots{}
+  @}
+@end smallexample
+
+In this case, it seems like a time/space tradeoff
+between allocating and deallocating @samp{temp1} for each iteration
+and allocating it just once for the entire loop.
+
+However, if @samp{temp1} is allocated just once for the entire loop,
+it could be the wrong size for subsequent iterations of that loop
+in cases like @samp{DO WHILE (A(I:J)//B .NE. 'END')},
+because the body of the loop might modify @samp{I} or @samp{J}.
+
+So, the above implementation is used,
+though a more optimal one can be used
+in specific circumstances.
+
+@node Transforming Iterative DO
+@subsection Transforming Iterative DO
+
+An iterative @code{DO} loop
+(one that specifies an iteration variable)
+is required by the Fortran standards
+to be implemented as though an iteration count
+is computed before entering the loop body,
+and that iteration count used to determine
+the number of times the loop body is to be performed
+(assuming the loop isn't cut short via @code{GOTO} or @code{EXIT}).
+
+The FFE handles this by allocating a temporary variable
+to contain the computed number of iterations.
+Since this variable must be in a scope that includes the entire loop,
+a GBEL block is created for that loop,
+and the variable declared as belonging to the scope of that block.
+
+@node Transforming Block IF
+@subsection Transforming Block IF
+
+Consider:
+
+@smallexample
+SUBROUTINE X(A,B,C)
+CHARACTER*(*) A, B, C
+LOGICAL LFUNC
+
+IF (LFUNC (A//B)) THEN
+  CALL SUBR1
+ELSE IF (LFUNC (A//C)) THEN
+  CALL SUBR2
+ELSE
+  CALL SUBR3
+END
+@end smallexample
+
+The arguments to the two calls to @samp{LFUNC}
+require dynamic allocation (at run time),
+but are not required during execution of the @samp{CALL} statements.
+
+So, the scopes of those temporaries must be within blocks inside
+the block corresponding to the Fortran @code{IF} block.
+
+This cannot be represented ``naturally''
+in vanilla C, nor in GBEL.
+The @samp{if}, @samp{elseif}, @samp{else},
+and @samp{endif} constructs
+provided by both languages must,
+for a given @samp{if} block,
+share the same C/GBE block.
+
+Therefore, any temporaries needed during evaluation of @samp{expr}
+while executing @samp{ELSE IF(expr)}
+must either have been predeclared
+at the top of the corresponding @code{IF} block,
+or declared within a new block for that @code{ELSE IF}---a block that,
+since it cannot contain the @code{else} or @code{else if} itself
+(due to the above requirement),
+actually implements the rest of the @code{IF} block's
+@code{ELSE IF} and @code{ELSE} statements
+within an inner block.
+
+The FFE takes the latter approach.
+
+@node Transforming SELECT CASE
+@subsection Transforming SELECT CASE
+
+@code{SELECT CASE} poses a few interesting problems for code generation,
+if efficiency and frugal stack management are important.
+
+Consider @samp{SELECT CASE (I('PREFIX'//A))},
+where @samp{A} is @code{CHARACTER*(*)}.
+In a case like this---basically,
+in any case where largish temporaries are needed
+to evaluate the expression---those temporaries should
+not be ``live'' during execution of any of the @code{CASE} blocks.
+
+So, evaluation of the expression is best done within its own block,
+which in turn is within the @code{SELECT CASE} block itself
+(which contains the code for the CASE blocks as well,
+though each within their own block).
+
+Otherwise, we'd have the rough equivalent of this pseudo-code:
+
+@smallexample
+@{
+  char temp[large];
+
+  libg77_catenate (temp, 'prefix', a);
+
+  switch (i (temp))
+    @{
+    case 0:
+      @dots{}
+    @}
+@}
+@end smallexample
+
+And that would leave temp[large] in scope during the CASE blocks
+(although a clever back end *could* see that it isn't referenced
+in them, and thus free that temp before executing the blocks).
+
+So this approach is used instead:
+
+@smallexample
+@{
+  int temp0;
+
+  @{
+    char temp1[large];
+
+    libg77_catenate (temp1, 'prefix', a);
+    temp0 = i (temp1);
+  @}
+
+  switch (temp0)
+    @{
+    case 0:
+      @dots{}
+    @}
+@}
+@end smallexample
+
+Note how @samp{temp1} goes out of scope before starting the switch,
+thus making it easy for a back end to free it.
+
+The problem @emph{that} solution has, however,
+is with @samp{SELECT CASE('prefix'//A)}
+(which is currently not supported).
+
+Unless the GBEL is extended to support arbitrarily long character strings
+in its @code{case} facility,
+the FFE has to implement @code{SELECT CASE} on @code{CHARACTER}
+(probably excepting @code{CHARACTER*1})
+using a cascade of
+@code{if}, @code{elseif}, @code{else}, and @code{endif} constructs
+in GBEL.
+
+To prevent the (potentially large) temporary,
+needed to hold the selected expression itself (@samp{'prefix'//A}),
+from being in scope during execution of the @code{CASE} blocks,
+two approaches are available:
+
+@itemize @bullet
+@item
+Pre-evaluate all the @code{CASE} tests,
+producing an integer ordinal that is used,
+a la @samp{temp0} in the earlier example,
+as if @samp{SELECT CASE(temp0)} had been written.
+
+Each corresponding @code{CASE} is replaced with @samp{CASE(@var{i})},
+where @var{i} is the ordinal for that case,
+determined while, or before,
+generating the cascade of @samp{if}-related constructs
+to cope with @code{CHARACTER} selection.
+
+@item
+Make @samp{temp0} above just
+large enough to hold the longest @code{CASE} string
+that'll actually be compared against the expression
+(in this case, @samp{'prefix'//A}).
+
+Since that length must be constant
+(because @code{CASE} expressions are all constant),
+it won't be so large,
+and, further, @samp{temp1} need not be dynamically allocated,
+since normal @code{CHARACTER} assignment can be used
+into the fixed-length @samp{temp0}.
+@end itemize
+
+Both of these solutions require @code{SELECT CASE} implementation
+to be changed so all the corresponding @code{CASE} statements
+are seen during the actual code generation for @code{SELECT CASE}.
+
+@node Transforming Expressions
+@section Transforming Expressions
+
+The interactions between statements, expressions, and subexpressions
+at program run time can be viewed as:
+
+@smallexample
+@var{action}(@var{expr})
+@end smallexample
+
+Here, @var{action} is the series of steps
+performed to effect the statement,
+and @var{expr} is the expression
+whose value is used by @var{action}.
+
+Expanding the above shows a typical order of events at run time:
+
+@smallexample
+Evaluate @var{expr}
+Perform @var{action}, using result of evaluation of @var{expr}
+Clean up after evaluating @var{expr}
+@end smallexample
+
+So, if evaluating @var{expr} requires allocating memory,
+that memory can be freed before performing @var{action}
+only if it is not needed to hold the result of evaluating @var{expr}.
+Otherwise, it must be freed no sooner than
+after @var{action} has been performed.
+
+The above are recursive definitions,
+in the sense that they apply to subexpressions of @var{expr}.
+
+That is, evaluating @var{expr} involves
+evaluating all of its subexpressions,
+performing the @var{action} that computes the
+result value of @var{expr},
+then cleaning up after evaluating those subexpressions.
+
+The recursive nature of this evaluation is implemented
+via recursive-descent transformation of the top-level statements,
+their expressions, @emph{their} subexpressions, and so on.
+
+However, that recursive-descent transformation is,
+due to the nature of the GBEL,
+focused primarily on generating a @emph{single} stream of code
+to be executed at run time.
+
+Yet, from the above, it's clear that multiple streams of code
+must effectively be simultaneously generated
+during the recursive-descent analysis of statements.
+
+The primary stream implements the primary @var{action} items,
+while at least two other streams implement
+the evaluation and clean-up items.
+
+Requirements imposed by expressions include:
+
+@itemize @bullet
+@item
+Whether the caller needs to have a temporary ready
+to hold the value of the expression.
+
+@item
+Other stuff???
+
+
+@end itemize