This patch introduces a new aspect "Subprogram_Variant" with expressions that are meant to increase/decrease with each recursive call of the annotated subprogram. It is inspired by the existing pragma Loop_Variant, whose expressions are meant to increase/decrease with each iteration of the loop. Both of those annotations are primarily used by GNATprove to verify that subprograms and loops terminate, but GNAT does all the legality checks and expansion for the dynamic semantics.
The new aspect is processed in the following steps:
1. Translate aspect to a corresponding pragma, so that we can reuse the
existing circuitry for similar aspects; in particular, we heavily
mimic processing of aspect Contract_Cases, which appears to be the
most similar one.
2. Validate placement of the pragma.
3. Analyse increase/decrease expressions, which might contain references
to both the subprogram parameters and forward references to global
objects (just like expressions of the aspects Pre/Post/Contract_Cases
and Global/Depends, so analysis is delayed).
4. Build declarations of constant objects that capture values of the
variant expressions at subprogram entry and a procedure that compares
those constants with values of the corresponding expressions at a
recursive call.
5. Add call to the above procedure at recursive calls of the subprogram.
Steps 1-3 are needed for both GNAT and GNATprove (which requires
increases/decreases expressions to be analyzed); steps 4-5 are only
needed for GNAT to implement dynamics semantics.
No existing GNAT functionality is affected by this new aspect.
Tested on x86_64-pc-linux-gnu, committed on trunk
gcc/ada/
* aspects.ads: Introduce Subprogram_Variant aspect with the
following properties: GNAT-specific, with mandatory expression,
not a representation aspect, never delayed.
* contracts.adb (Expand_Subprogram_Contract): Mention new aspect
in the comment.
(Add_Contract_Item): Support addition of pragma
Subprogram_Variant to N_Contract node.
(Analyze_Entry_Or_Subprogram_Contract): Mention new aspect in
the comment; add pragma Subprogram_Variant to N_Contract node.
(Build_Postconditions_Procedure): Adapt call to
Insert_Before_First_Source_Declaration, which is now reused in
expansion of new aspect.
(Process_Contract_Cases_For): Also process Subprogram_Variant,
which is stored in N_Contract node together with Contract_Cases.
* contracts.ads (Analyze_Entry_Or_Subprogram_Contract): Mention
new aspect in the comment.
(Analyze_Entry_Or_Subprogram_Body_Contract): Likewise.
* einfo.adb (Get_Pragma): Support retrieval of new pragma.
* einfo.ads (Get_Pragma): Likewise.
* exp_ch6.adb (Check_Subprogram_Variant): New routine for
emitting call to check Subprogram_Variant expressions at run
time.
(Expand_Call_Helper): Check Subprogram_Variant expressions at
recursive calls.
* exp_prag.adb (Make_Op): Moved from expansion of pragma
Loop_Variant to Exp_Util, so it is now reused for expansion of
pragma Subprogram_Variant.
(Process_Variant): Adapt call to Make_Op after moving it to
Exp_Util.
(Expand_Pragma_Subprogram_Variant): New routine.
* exp_prag.ads (Expand_Pragma_Subprogram_Variant): Likewise.
* exp_util.adb (Make_Variant_Comparison): Moved from Exp_Prag
(see above).
* exp_util.ads (Make_Variant_Comparison): Likewise.
* inline.adb (Remove_Aspects_And_Pragmas): Handle aspect/pragma
Subprogram_Variant just like similar contracts.
* par-prag.adb (Prag): Likewise.
* sem.adb (Insert_Before_First_Source_Declaration): Moved from
Contracts (see above).
* sem.ads (Insert_Before_First_Source_Declaration): Likewise.
* sem_ch12.adb: Mention new aspect in the comment about
"Implementation of Generic Contracts", just like similar aspects
are mentioned there.
* sem_ch13.adb (Insert_Pragma): Mention new aspect in the
comment, because this routine is now used for Subprogram_Variant
just like for other similar aspects.
(Analyze_Aspect_Specifications): Mention new aspect in comments;
it is handled just like aspect Contract_Cases.
(Check_Aspect_At_Freeze_Point): Do not expect aspect
Subprogram_Variant just like we don't expect aspect
Contract_Cases.
* sem_prag.adb (Ensure_Aggregate_Form): Now also used for pragma
Subprogram_Variant, so update comment.
(Analyze_Pragma): Add initial checks for pragma
Subprogram_Variant.
(Analyze_Subprogram_Variant_In_Decl_Part): New routine with
secondary checks on the new pragma.
(Sig_Flags): Handle references within pragma Subprogram_Variant
expression just like references in similar pragma
Contract_Cases.
(Is_Valid_Assertion_Kind): Handle Subprogram_Variant just like
other similar contracts.
* sem_prag.ads (Analyze_Subprogram_Variant_In_Decl_Part): New
routine.
* sem_res.adb (Same_Or_Aliased_Subprograms): Moved to Sem_Util,
so it can be reused for detection of recursive calls where
Subprogram_Variant needs to be verified.
* sem_util.adb (Is_Subprogram_Contract_Annotation): Handle new
Subprogram_Variant annotation just like other similar
annotations.
(Same_Or_Aliased_Subprograms): Moved from Sem_Res (see above).
* sem_util.ads (Is_Subprogram_Contract_Annotation): Mention new
aspect in the comment.
(Same_Or_Aliased_Subprograms): Moved from Sem_Res (see above).
* sinfo.ads (N_Contract): Document handling of
Subprogram_Variant.
* snames.ads-tmpl: Add name for the internally generated
procedure with checks for Subprogram_Variant expression, name
for the new aspect and new pragma corresponding to aspect
Subprogram_Variant.
patch.diff.gz
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