AI12-0075 adds a new aspect Static that can be applied to expression functions to specify that the functions are static. When a static expression function is called with actual parameters that are static expressions, the function call is itself treated as a static expression and can be used in contexts that are required to be static (such as named numbers and case choices). The expression of a static expression function is required to be a "potentially static expression", as defined in the Ada 202x RM in 6.8(3.2-3.4)), basically meaning that the expression is like a static expression but can also contain references to the formal parameters of its containing expression function. There are other requirements on static expression functions as given in RM 6.8(5.1-5.8), such as that all of its formal parameter and results must have scalar or string subtypes that are static. The two main elements of the implementation are a special preanalysis of the expression to ensure that it satisfies the requirements for potentially static expressions and a static evaluation of calls to these functions based on the parameter substitution approach used in Expand_Inlined_Call.
Note: The one piece of semantic checking that is not implemented by this
set of changes is the requirement in RM202x 6.8(5.8) that if the
function is a boundary entity for result type R, then no type invariant
applies to type R.
Tested on x86_64-pc-linux-gnu, committed on trunk
2020-06-17 Gary Dismukes <dismu...@adacore.com>
gcc/ada/
* aspects.ads (type Aspect_Id): Add Aspect_Static as a Boolean
aspect, and update the Is_Representation_Aspect, Aspect_Names,
and Aspect_Delay arrays.
* exp_ch6.adb (Expand_Simple_Function_Return): In the case of a
return for a static expression function, capture a copy of the
expression of the return statement before it's expanded and
reset its Analyzed flags. Then, just before leaving this
procedure, if the expression was rewritten, set the
Original_Node of the rewritten expression to the new copy and
also set the Expression of the associated static expression
function to designate that copy. This ensures that later copies
of the expression made via New_Copy_Tree will fully copy all
nodes of the expression tree.
* inline.ads (Inline_Static_Expression_Function_Call): New
procedure to evaluate and produce the result of a static call to
a static expression function.
* inline.adb: Add with and use for Sem_Res.
(Establish_Actual_Mapping_For_Inlined_Call): New procedure
extracted from code in Expand_Inlined_Call that traverses the
actuals and formals of an inlined call and in some cases creates
temporaries for holding the actuals, plus establishes an
association between formals and actuals (via the Renamed_Object
fields of the formals).
(Formal_Is_Used_Once): Function removed from Expand_Inlined_Call
and now nested in the above procedure.
(Expand_Inlined_Call): Code for doing the formal/actual
traversal is moved to Create_Actual_Temporaries and replaced
with a call to that new procedure.
(Inline_Static_Expression_Function_Call): New procedure to
evaluate a static call to a static expression function,
substituting actuals for their corresponding formals and
producing a fully folded and static result expression. The
function has subsidiary functions Replace_Formal and Reset_Sloc
that take care of doing the mapping of formals to actuals and
resetting the Slocs of subnodes of the mapped expression to that
of the call so errors will be flagged on the call rather than
function.
* sem_ch6.adb (Analyze_Expression_Function): In the case of a
static expression function, perform an additional preanalysis of
the function's expression to ensure that it's a potentially
static expression (according to the requirements of
6.8(3.2/5-3.4/5)), and issue an error if it's not. The global
flag Checking_Potentially_Static_Expression is set and unset
around this checking.
* sem_ch13.adb (Analyze_Aspect_Static): New procedure to enforce
selected requirements of the new aspect Static on expression
functions, including checking that the language version is
Ada_2020 and that the entity to which it applies is an
expression function whose formal parameters are of a static
subtype and have mode 'in', its result subtype is a static
subtype, and it has no pre- or postcondition aspects. A ???
comment is added to indicate the need for adding checking that
type invariants don't apply to the result type if the function
is a boundary entity.
(Analyze_One_Aspect): Call Analyze_Aspect_Static for aspect
Static.
* sem_elab.adb (Build_Call_Marker): Return without creating a
call marker when the subprogram is a static expression function,
since no ABE checking is needed for such functions.
* sem_eval.ads (Checking_Potentially_Static_Expression): New
function to return whether the checking for potentially static
expressions is enabled.
(Set_Checking_Potentially_Static_Expression): New procedure to
enable or disable checking of potentially static expressions.
* sem_eval.adb (Checking_For_Potentially_Static_Expression): New
global flag for determining whether preanalysis of potentially
static expression is being done, which affects the behavior of
certain static evaluation routines.
(Checking_Potentially_Static_Expression): New function to return
whether the checking for potentially static expressions is
enabled.
(Eval_Call): When evaluating a call within a static expression
function with checking of potentially static expression
functions enabled, substitutes a static value in place of the
call to allow folding of the expression.
(Eval_Entity_Name): When evaluating a formal parameter of a
static expression function with checking of potentially static
expression functions enabled, substitutes a static value in
place of the reference to the formal to allow folding of the
expression.
(Set_Checking_Potentially_Static_Expression): New procedure to
enable or disable checking of potentially static expressions.
* sem_res.adb (Resolve_Call): Test for a recursive call
occurring within a static expression function and issue an error
for such a call. Prevent the establishment of a transient scope
in the case this is a call to a (string-returning) static
expression function. When calling a static expression function,
if no error has been posted on the function, call
Inline_Static_Expression_Function_Call to convert the call into
its equivalent static value.
* sem_util.ads (Is_Static_Expression_Function): New function
returning whether the subprogram entity passed to it is a static
expression function.
(Is_Static_Expression_Function_Call): New function to determine
whether the call node passed to it is a static call to a static
expression function.
* sem_util.adb (Compile_Time_Constraint_Error): Suppress
compile-time Constraint_Error reporting when checking for a
potentially static expression.
(Is_Static_Expression_Function): New function returning whether
the subprogram entity passed to it is a static expression
function by testing for the presence of aspect Static.
(Has_All_Static_Actuals): New function in
Is_Static_Expression_Function_Call that traverses the actual
parameters of a function call and returns True only when all of
the actuals are given by static expressions. In the case of a
string-returning function, we call Resolve on each actual to
ensure that their Is_Static_Expression flag properly reflects
whether they're static, to allow suppressing creation of a
transient scope within Resolve_Call. A prominent ??? comment is
added to explain this rather unconventional call to Resolve.
(Is_Static_Expression_Function_Call): New function that
determines whether a node passed to it is a call to a static
expression function all of whose actual parameters are given by
static expressions.
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