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Pascal/Delphi do not use regular object files, instead they have DPU/TPU files which both contain parsed headers and object code. Those files are not compatibl

by theamk 6y ago

Pascal/Delphi do not use regular object files, instead they have DPU/TPU files which both contain parsed headers and object code. Those files are not compatible between major versions, so this probably means they are designed to make them as easy to parse as possible, and map closely to internal compiler structures. This means including a unit is way faster than re-parsing the unit's text. Additional speculation: those DPU files might contain a copy of relevant definition (like names of fields, sizes and offsets). This means if you have "program" which uses "my_unit" which uses "high_level_comms" which uses "Handle" defined in "low_level_comms", then definition of "Handle" could be copied directly into my_unit.DPU. So when you compile "program", and it includes "my_unit", all you need to read is "my_unit.DPU". You do not care about total size of "high_level_comms" and "low_level_comms" files. Unfortunately, in C/C++, when people '#include <iostream>', they can now use "std::plus", "mbrtowc" and "timer_getoverrun". Which means including high-level modules has to bring tens of thousands of lines in translation units. (Yes, impl pattern, well-designed interfaces, forward declarations can fix this, but few people bother. Stdlib certainly does not)

barrkel 6y ago
Compiled Delphi unit files include a serialisation of the symbol table. Import definitions are not included. There's an import symbol for each import that has a pointer to the definition information which is updated when the used unit is loaded. This linking is fast, and uses a combo of three things: hash code, symbol name and symbol version. Hash codes in the Delphi compiler are cached in each symbol, so each symbol linked is a hash probe and a few string compares at most. The symbol version is a hash of its (original) definition. If there's a version mismatch, Delphi knows it needs to recompile the unit. (I spent about 6 years working on the compiler at Borland / Codegear / Embarcadero.)
zozbot234 6y ago
C/C++ object files (and shared libraries) can include DWARF info that effectively gives you the same featureset as Pascal "parsed headers".
- pjmlp 6y ago
  Not everyone uses DWARF though.
pjmlp 6y ago
C++20 interface modules are just the same.

by theamk 6y ago

barrkel 6y ago

Compiled Delphi unit files include a serialisation of the symbol table. Import definitions are not included. There's an import symbol for each import that has a pointer to the definition information which is updated when the used unit is loaded. This linking is fast, and uses a combo of three things: hash code, symbol name and symbol version. Hash codes in the Delphi compiler are cached in each symbol, so each symbol linked is a hash probe and a few string compares at most. The symbol version is a hash of its (original) definition. If there's a version mismatch, Delphi knows it needs to recompile the unit. (I spent about 6 years working on the compiler at Borland / Codegear / Embarcadero.)

zozbot234 6y ago

C/C++ object files (and shared libraries) can include DWARF info that effectively gives you the same featureset as Pascal "parsed headers".

pjmlp 6y ago
Not everyone uses DWARF though.

pjmlp 6y ago

C++20 interface modules are just the same.