AI12-0280 relaxes the "potentially unevaluated" restrictions on the use of the Old attribute in postconditions. A complex attribute prefix is allowed in cases that were previously forbidden if the conditions governing whether the attribute value will be needed during evaluation of the postcondition are "known on entry". In this newly-allowed case, evaluation of the 'Old attribute prefix upon entry to the subprogram is performed conditionally. It turns out that this is similar to something that was already part of the implementation of the Contract_Cases attribute, and that bugs having to do with 'Old attribute references in Contract_Cases consequences are fixed by sharing code with this new implementation via a new package Sem_Util.Old_Attr_Util.Indirect_Temps. In particular, such an attribute reference would previously generate a malformed tree if the subtype of the prefix was indefinite; for example, we might previously have generated an illegal declaration like "Temp123 : String;" .
Tested on x86_64-pc-linux-gnu, committed on trunk
gcc/ada/
* sem_util.ads: Declare a new package, Old_Attr_Util, which in
turn declares two more packages, Conditional_Evaluation and
Indirect_Temps. Conditional_Evaluation provides a predicate for
deciding whether a given 'Old attribute reference is eligible
for conditional evaluation and, in the case where it is
eligible, a function that constructs the Boolean-valued
condition that is to be evaluated at run time in deciding
whether to evaluate the attribute prefix. Indirect_Temps
provides support for declaring a temporary which is only
initialized conditionally; more specifically, an access type and
a variable of that type are declared (unconditionally) and then
the variable is (conditionally) initialized with an allocator.
The existence of the access type and the pointer variable is
hidden from clients, except that a predicate,
Is_Access_Type_For_Indirect_Temp, is provided for identifying
such access types. This is needed because we want such an access
type to be treated like a "normal" access type (specifically
with respect to finalization of allocated objects). Other parts
of the compiler treat access types differently if
Comes_From_Source is False, or if the secondary stack storage
pool is used; this predicate is used to disable this special
treatment.
* sem_attr.adb (Uneval_Old_Msg): Improve message text to reflect
Ada202x changes.
(Analyze_Attribute): A previously-illegal 'Old attribute
reference is accepted in Ada2020 if it is eligible for
conditional evaluation.
* sem_res.adb (Valid_Conversion): Do not treat a rewritten 'Old
attribute like other rewrite substitutions. This makes a
difference, for example, in the case where we are generating the
expansion of a membership test of the form "Saooaaat'Old in
Named_Access_Type"; in this case Valid_Conversion needs to
return True (otherwise the expansion will be False - see the
call site in exp_ch4.adb).
* exp_attr.adb (Expand_N_Attribute_Reference): When expanding a
'Old attribute reference, test for the case where the reference
is eligible for conditional evaluation. In that case, use the
new "indirect temporary" mechanism provided by Sem_Util.
* exp_prag.adb
(Expand_Attributes_In_Consequence.Expand_Attributes): If
Sem_Util.Indirect_Temp_Needed indicates that there could be
correctness problems associated with the old expansion scheme
for dealing with 'Old attributes in contract cases consequences,
then we use the new "indirect temporary" mechanism provided by
Sem_Util instead. We do not want to do this unconditionally.
* sem_util.adb: Provide a body for the new Old_Attr_Util
package. Further work is needed in several areas for
correctness:
- The function Is_Repeatedly_Evaluated does not deal with
container aggregates yet.
- The function Is_Known_On_Entry does not deal with interactions
with the Global aspect.
Each area where more work is needed is indicated with a "???"
comment in the code; a more detailed description can be found
there. Some optimization opportunties are similarly indicated
with a "???" comment.
* exp_ch3.adb (Freeze_Type): In deciding whether to generate
expansion for the list controller of an access type, take the
predicate Is_Access_Type_For_Indirect_Temp into account. If the
predicate yields True, then generate the expansion.
* exp_util.adb (Build_Allocate_Deallocate_Proc): We don't
normally finalize allocated objects that are allocated on the
secondary stack. Add an exception to this rule if the predicate
Is_Access_Type_For_Indirect_Temp yields True. As a result of
this exception, we have to deal with the fact that gigi expects
a different parameter profile if we are using the secondary
stack pool; the Pool and Alignment parameters must be omitted in
this case.
patch.diff.gz
Description: application/gzip
