Re: [PATCH 1/3] Add new commands for controlling type signature fallback.

[Dominik Mascherbauer <dominik.mascherbauer@gmail.com>]

Thanks for the review.


>> From: dominikmascherbauer <dominik.mascherbauer@gmail.com>
>> Cc: dominikmascherbauer <dominik.mascherbauer@oracle.com>
>> Date: Tue, 11 Mar 2025 15:57:18 +0100
>>
>> 'set dwarf-type-signature-fallback (off|main|full)':
>> Enable type signature fallback.
>> If a type signature is not found in the current objfile, either looks up the 'main' objfile or all objfiles in a progspace to find the type signature.
>> Usually a type signature is unique, ther should only be one type unit for each type signature.
>> Therefore, it should be okay to check other objfile for a type signature.
>> However, whether this is really the case depends on the debug info producer.
>> This is why its disabled by default.
>>
>> 'set dwarf-type-signature-fallback-jit':
>> Whether the type signature fallback is enabled for only JIT objfiles.
>> JIT objfiles are self-contained, but usually share most debug info with some already loaded objfile.
>> This is why JIT objfiles are a major use case for the type signature fallback.
>> This option is added to restrict the type signature fallback to match this exact use-case and provides a more granular use of the type signature fallback.
>> ---
>>  gdb/NEWS            | 14 +++++++++
>>  gdb/doc/gdb.texinfo | 26 +++++++++++++++++
>>  gdb/dwarf2/read.c   | 70 +++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 110 insertions(+)
> 
> Thanks.
> 
>> diff --git a/gdb/NEWS b/gdb/NEWS
>> index 2c504a9b603..53368d7463c 100644
>> --- a/gdb/NEWS
>> +++ b/gdb/NEWS
>> @@ -261,6 +261,20 @@ disable missing-objfile-handler LOCUS HANDLER
>>    or can be a regular expression which is matched against the filename
>>    of the primary executable in each program space.
>>  
>> +set dwarf-type-signature-fallback (off|main|full)
>> +  New command which enables a type signature lookup fallback, if a
>> +  dwarf objfile from a JIT compilation does not provide its own type
>> +  units, but relies on type units from other objfiles.  The 'main'
>> +  setting allows to limit the scope of the lookup to just the main
>> +  symfile objfile, 'full' uses all objfiles in a progspace.
>> +  The default value is 'off'.
>> +
>> +set dwarf-type-signature-fallback-jit on|off
>> +  New command that is used in combination with type-signature-fallback
>> +  and is on by default.  If type signature fallback is enabled,
>> +  this restricts the fallback to objfiles that were loaded with the
>> +  JIT debug interface.
>> +
>>  * Changed commands
>>  
>>  remove-symbol-file
>> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
>> index 04719bf65e6..901152dd15e 100644
>> --- a/gdb/doc/gdb.texinfo
>> +++ b/gdb/doc/gdb.texinfo
>> @@ -42325,6 +42325,32 @@ Symbol "argc" is a complex DWARF expression:
>>  For more information on these expressions, see
>>  @uref{http://www.dwarfstd.org/, the DWARF standard}.
>>  
>> +@kindex set dwarf-type-signature-fallback
>> +@kindex show dwarf-type-signature-fallback
>> +@item set dwarf-type-signature-fallback @samp{full|main|off}
>> +@itemx show dwarf-type-signature-fallback
>> +Control the behavior of the type signature lookup for type units if a
>> +type signature is undefined within an objfile.
>> +
>> +The default is @code{off}, which means @value{GDBN} will not use other
>> +objfiles in the progspace to lookup missing type signatures. When
>> +@code{main}, @value{GDBN} will check the current main objfile for a
>> +missing signatured type. When @code{full}, all objfiles in the current
>> +progspace will be searched for the missing signatured type.
>> +
>> +@kindex set dwarf-type-signature-fallback-jit
>> +@kindex show dwarf-type-signature-fallback-jit
>> +@item set dwarf-type-signature-fallback-jit
>> +@itemx show dwarf-type-signature-fallback-jit
>> +
>> +Control whether @code{dwarf type-signature-fallback} is limited to
>> +objfiles originated from the JIT interface (@pxref{JIT Interface})
>> +or all objfiles with a missing signatured type.
>> +
>> +The default is @code{on}, which limits the type signature fallback to
>> +only objfiles from the JIT interface. When @code{off}, the type signature
>> +fallback will look for missing signatured types for all objfiles.
> 
> Please leave two spaces between sentences, otherwise the printed
> version will be typeset sub-optimally.
> 
> More generally, I think both NEWS and the text for the manual doesn't
> tell what this means in practical terms and when it would be useful to
> fiddle with these options.  I find myself wondering why and when would
> I need to help GDB look up type signature, and even what is a "type
> signature" (it is used in the manual only once, without any
> explanations).  I think at least the manual should answer these
> questions.
> 

Would this be enough, or does it need to be more detailed:

diff --git a/gdb/NEWS b/gdb/NEWS
index 53368d7463c..5274c84f168 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -262,18 +262,25 @@ disable missing-objfile-handler LOCUS HANDLER
   of the primary executable in each program space.
 
 set dwarf-type-signature-fallback (off|main|full)
-  New command which enables a type signature lookup fallback, if a
-  dwarf objfile from a JIT compilation does not provide its own type
-  units, but relies on type units from other objfiles.  The 'main'
-  setting allows to limit the scope of the lookup to just the main
-  symfile objfile, 'full' uses all objfiles in a progspace.
+  New command which enables a type signature look up fallback for DWARF objfiles.
+  If type signatures in DWARF objfile are unique, enabling this option to look
+  up type signatures from fallback DWARF objfiles.  Reusing type units from
+  fallback objfiles can be used to avoid duplicating those type units.
+
+  The 'main' setting allows to limit the scope of the lookup to just the main
+  symfile objfile to reduce the overhead of type signature lookup.  The 'full'
+  setting uses all objfiles in a progspace to find as many type signatures as
+  possible.
   The default value is 'off'.
 
 set dwarf-type-signature-fallback-jit on|off
-  New command that is used in combination with type-signature-fallback
+  New command that is used in combination with dwarf-type-signature-fallback
   and is on by default.  If type signature fallback is enabled,
   this restricts the fallback to objfiles that were loaded with the
   JIT debug interface.
+  JIT objfiles are produced at run-time and often rely on already available
+  type information.  This allows the debug info producer to skip generating type
+  units that are known to be present in other objfiles.
 
 * Changed commands
 
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 901152dd15e..9581ea915e4 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -42329,27 +42329,35 @@ For more information on these expressions, see
 @kindex show dwarf-type-signature-fallback
 @item set dwarf-type-signature-fallback @samp{full|main|off}
 @itemx show dwarf-type-signature-fallback
-Control the behavior of the type signature lookup for type units if a
-type signature is undefined within an objfile.
+Control the behavior of the type signature look up in DWARF objfiles.  Type
+Signatures in DWARF uniquely identify a type unit.  A type unit contains debug
+info for a type and can b referenced by its type signature.  Type signature
+look up allows @value{GDBN} to look up type signatures not only in objfile
+containing the refererence to a type signature, but also other type signatures
+in the progspace.  This allows to avoid duplicating type units, by looking them
+up from other DWARF objfiles.
 
 The default is @code{off}, which means @value{GDBN} will not use other
-objfiles in the progspace to lookup missing type signatures. When
-@code{main}, @value{GDBN} will check the current main objfile for a
-missing signatured type. When @code{full}, all objfiles in the current
+objfiles in the progspace to look up missing type signatures.  When
+@code{main}, @value{GDBN} will check the current main symfile objfile for a
+missing signatured type.  If the main symfile objfile contains most of the
+debug info required for debugging, this setting reduces the impact of type
+signature look up.  When @code{full}, all DWARF objfiles in the current
 progspace will be searched for the missing signatured type.
 
 @kindex set dwarf-type-signature-fallback-jit
 @kindex show dwarf-type-signature-fallback-jit
 @item set dwarf-type-signature-fallback-jit
 @itemx show dwarf-type-signature-fallback-jit
-
-Control whether @code{dwarf type-signature-fallback} is limited to
+Control whether @code{dwarf-type-signature-fallback} is limited to DWARF
 objfiles originated from the JIT interface (@pxref{JIT Interface})
-or all objfiles with a missing signatured type.
+or all DWARF objfiles with a missing signatured type.  If only objfiles loaded
+through the JIT interface rely on existing types and type units this
+setting allows to limit type signature look up to avoid undesired behavior.
 
 The default is @code{on}, which limits the type signature fallback to
-only objfiles from the JIT interface. When @code{off}, the type signature
-fallback will look for missing signatured types for all objfiles.
+only DWARF objfiles from the JIT interface.  When @code{off}, type signature
+look up is performed for missing signatured types in all DWARF objfiles.
 
 @kindex maint set dwarf max-cache-age
 @kindex maint show dwarf max-cache-age


>> +  add_setshow_enum_cmd ("dwarf-type-signature-fallback", class_support,
>> +			type_signature_fallback_enums, &type_signature_fallback,
>> +			_ ("\
>> +Set resolution of type signatures with other objfiles as fallback."), _ ("\
>> +Show resolution of type signatures with other objfiles as fallback."), _ ("\
>> +This option will only take effect if set before loading symbols.\n\
>> +off  == turn type signature fallback off\n\
>> +main == use the main symbolfile as fallback objfile\n\
>> +	for type signature resolution\n\
>> +full == use all objfiles with type units in the current progspace\n\
>> +	as fallback objfiles for type signature resolution."),
> 
> The list of possible values should probably start with "main" or
> "full", as otherwise the reference to "fallback" in "off" will not be
> understood.
> 
> Also note that you use "resolution" in the above doc strings, but
> "look up" in the manual and NEWS.  I think it's better to use the same
> terminology; "look up" sounds more accurate to me.
> 

Ok, I will change that.

> Reviewed-By: Eli Zaretskii <eliz@gnu.org>