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9.10 Resolving Elaboration Circularities ¶

The most desirable option from the point of view of long-term maintenance is to rearrange the program so that the elaboration problems are avoided. One useful technique is to place the elaboration code into separate child packages. Another is to move some of the initialization code to explicitly invoked subprograms, where the program controls the order of initialization explicitly. Although this is the most desirable option, it may be impractical and involve too much modification, especially in the case of complex legacy code.

When faced with an elaboration circularity, the programmer should also consider the tactics given in the suggestions section of the circularity diagnostic. Depending on the units involved in the circularity, their `with' clauses, purity, preelaborability, presence of elaboration-control pragmas and invocations at elaboration time, the binder may suggest one or more of the following tactics to eliminate the circularity:

• Pragma Elaborate elimination

  remove pragma Elaborate for unit "..." in unit "..."
  

  This tactic is suggested when the binder has determined that pragma Elaborate:

  • Prevents a set of units from being elaborated.
  • The removal of the pragma will not eliminate the semantic effects of the pragma. In other words, the argument of the pragma will still be elaborated prior to the unit containing the pragma.
  • The removal of the pragma will enable the successful ordering of the units.

  The programmer should remove the pragma as advised, and rebuild the program.

• Pragma Elaborate_All elimination

  remove pragma Elaborate_All for unit "..." in unit "..."
  

  This tactic is suggested when the binder has determined that pragma Elaborate_All:

  • Prevents a set of units from being elaborated.
  • The removal of the pragma will not eliminate the semantic effects of the pragma. In other words, the argument of the pragma along with its `with' closure will still be elaborated prior to the unit containing the pragma.
  • The removal of the pragma will enable the successful ordering of the units.

  The programmer should remove the pragma as advised, and rebuild the program.

• Pragma Elaborate_All downgrade

  change pragma Elaborate_All for unit "..." to Elaborate in unit "..."
  

  This tactic is always suggested with the pragma Elaborate_All elimination tactic. It offers a different alernative of guaranteeing that the argument of the pragma will still be elaborated prior to the unit containing the pragma.

  The programmer should update the pragma as advised, and rebuild the program.

• Pragma Elaborate_Body elimination

  remove pragma Elaborate_Body in unit "..."
  

  This tactic is suggested when the binder has determined that pragma Elaborate_Body:

  • Prevents a set of units from being elaborated.
  • The removal of the pragma will enable the successful ordering of the units.

  Note that the binder cannot determine whether the pragma is required for other purposes, such as guaranteeing the initialization of a variable declared in the spec by elaboration code in the body.

  The programmer should remove the pragma as advised, and rebuild the program.

• Use of pragma Restrictions

  use pragma Restrictions (No_Entry_Calls_In_Elaboration_Code)
  

  This tactic is suggested when the binder has determined that a task activation at elaboration time:

  • Prevents a set of units from being elaborated.

  Note that the binder cannot determine with certainty whether the task will block at elaboration time.

  The programmer should create a configuration file, place the pragma within, update the general compilation arguments, and rebuild the program.

• Use of dynamic elaboration model

  use the dynamic elaboration model (compiler switch -gnatE)
  

  This tactic is suggested when the binder has determined that an invocation at elaboration time:

  • Prevents a set of units from being elaborated.
  • The use of the dynamic model will enable the successful ordering of the units.

  The programmer has two options:

  • Determine the units involved in the invocation using the detailed invocation information, and add compiler switch -gnatE to the compilation arguments of selected files only. This approach will yield safer elaboration orders compared to the other option because it will minimize the opportunities presented to the dynamic model for ignoring invocations.
  • Add compiler switch -gnatE to the general compilation arguments.
• Use of detailed invocation information

  use detailed invocation information (compiler switch -gnatd_F)
  

  This tactic is always suggested with the use of the dynamic model tactic. It causes the circularity section of the circularity diagnostic to describe the flow of elaboration code from a unit to a unit, enumerating all such paths in the process.

  The programmer should analyze this information to determine which units should be compiled with the dynamic model.

• Forced-dependency elimination

  remove the dependency of unit "..." on unit "..." from the argument of switch -f
  

  This tactic is suggested when the binder has determined that a dependency present in the forced-elaboration-order file indicated by binder switch -f:

  • Prevents a set of units from being elaborated.
  • The removal of the dependency will enable the successful ordering of the units.

  The programmer should edit the forced-elaboration-order file, remove the dependency, and rebind the program.

• All forced-dependency elimination

  remove switch -f
  

  This tactic is suggested in case editing the forced-elaboration-order file is not an option.

  The programmer should remove binder switch -f from the binder arguments, and rebind.

• Multiple-circularities diagnostic

  diagnose all circularities (binder switch -d_C)
  

  By default, the binder will diagnose only the highest-precedence circularity. If the program contains multiple circularities, the binder will suggest the use of binder switch -d_C in order to obtain the diagnostics of all circularities.

  The programmer should add binder switch -d_C to the binder arguments, and rebind.

If none of the tactics suggested by the binder eliminate the elaboration circularity, the programmer should consider using one of the legacy elaboration models, in the following order:

• Use the pre-20.x legacy elaboration-order model, with binder switch -H.
• Use both pre-18.x and pre-20.x legacy elaboration models, with compiler switch -gnatH and binder switch -H.
• Use the relaxed static-elaboration model, with compiler switches -gnatH -gnatJ and binder switch -H.
• Use the relaxed dynamic-elaboration model, with compiler switches -gnatH -gnatJ -gnatE and binder switch -H.