Date: 05/23/2000
Author: Emmanuel STAPF
email: manus@eiffel.com


What is an execution unit?
==========================

This is an entity associated to each Eiffel routines in workbench mode only.
For a normal Eiffel features, it contains the encoded name of an Eiffel
routine (body_index + class_type), its pattern identification and its
return type if any. For an external it contains in addition the external
name, the complete signature if any and the list of header files if any.

All execution unit are put together in the execution table and they are indexed
by their `body_index'. When performing a call to a feature in workbench mode, we
find at the end a `body_index'. This info enables us to call the corresponding
Eiffel routine by simply looking at the Execution table.


Old way of computing EXECUTION_UNIT:
====================================

At degree 3, we were storing in `melted_set' the list of melted routine of
current class (stored through a TWO_WAY_SORTED_SET [MELTED_INFO]). A MELTED_INFO
contained only the feature_name of the melted feature.

Then a degree 2, we were traversing this structure and for each class, we were
loading the FEATURE_TABLE and look for the corresponding FEATURE_I with the name
stored in MELTED_INFO. With the retrieved FEATURE_I, we created the corresponding
EXECUTION_UNIT.

This was simple but there was a huge bottleneck: the use of FEATURE_TABLE.
By looking at what we needed from the FEATURE_I to create the execution unit
we found that we did not need it all the time.


New way of computing EXECUTION_UNIT:
====================================

At degree 3 instead of storing only the name, we decided to store instead:
- body_index
- pattern_id
- return_type
Also instead of using a TWO_WAY_SORTED_SET, we are using a SEARCH_TABLE.

Therefore at degree 2, we have all the required information to create the
EXECUTION_UNIT without having to go through the FEATURE_TABLE.

The speed improvement at degree 2 is about 10/20 seconds versus 3 to 8 minutes
before.


Missed details:
===============

I did not go through the details of the different kind of EXECUTION_UNIT and the
different kind of MELTED_INFO. But by looking at the code it should be enough
and simple to extend this explanation.