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During testing it was observed that quite a few info tables were not
being given locations (due to not being assigned source locations,
because they were not enclosed by a source note). We can at least give
the module name and type for such closures even if no more accurate
source information.
Especially for constructors this helps find them in the STG dumps.
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The `-fdistinct-constructor-tables` flag will generate a fresh info
table for the usage of any data constructor. This is useful for
debugging as now by inspecting the info table, you can determine which
usage of a constructor caused that allocation rather than the old
situation where the info table always mapped to the definition site of
the data constructor which is useless.
In conjunction with `-hi` and `-finfo-table-map` this gives a more fine
grained understanding of where constructor allocations arise from in a
program.
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This new flag embeds a lookup table from the address of an info table
to information about that info table.
The main interface for consulting the map is the `lookupIPE` C function
> InfoProvEnt * lookupIPE(StgInfoTable *info)
The `InfoProvEnt` has the following structure:
> typedef struct InfoProv_{
> char * table_name;
> char * closure_desc;
> char * ty_desc;
> char * label;
> char * module;
> char * srcloc;
> } InfoProv;
>
> typedef struct InfoProvEnt_ {
> StgInfoTable * info;
> InfoProv prov;
> struct InfoProvEnt_ *link;
> } InfoProvEnt;
The source positions are approximated in a similar way to the source
positions for DWARF debugging information. They are only approximate but
in our experience provide a good enough hint about where the problem
might be. It is therefore recommended to use this flag in conjunction
with `-g<n>` for more accurate locations.
The lookup table is also emitted into the eventlog when it is available
as it is intended to be used with the `-hi` profiling mode.
Using this flag will significantly increase the size of the resulting
object file but only by a factor of 2-3x in our experience.
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