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by comex
3407 days ago
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> It's just that the actual benefit is far from reducing link time to "O(changes in the input)" Not sure exactly what you mean by this. If you give up determinism, it can be O(changes) - except for time spent statting the input files which, at least in theory, should be possible to avoid by getting the info from the build system somehow. I can understand if LLD doesn't want to trade off determinism, but I personally think it should :) One practical problem I can think of is ensuring that the binary isn't still running when the linker tries to overwrite bits of it. Windows denies file writes in that case anyway… On Unix that's traditionally the job of ETXTBSY, which I think Linux supports, but xnu doesn't. I guess it should be possible to fake it with APFS snapshots. > In its fastest mode, LLD actually spends most of its time memcpy'ing into the output file and applying relocations. This happens after symbol resolution and does not touch the input .o files except to read the data being copied into the output file. Interesting. What is this mode? How does it work if it's not incremental and it doesn't read the symbols at all? |
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Not quite. For example, a change in the symbols in a single object file can cause different archive members to be fetched for archives later on the command line. A link can be constructed where that would be O(all inputs) changes due to a change in a single file.
Even though a practical link won't hit that pathological case, you still have to do the appropriate checking to ensure that it doesn't happen, which is an annoying transitive-closure/reachability type problem. ( If you need a refresher on archive semantics see the description here: http://llvm.org/devmtg/2016-03/Presentations/EuroLLVM%202016... Even with the ELF LLD using the windows link.exe archive semantics (which are in practice compatible with traditional unix archive semantics), the problem still remains. )
In practice, with the current archive semantics, any change to symbol resolution would likely be best served by bailing out from an incremental link in order to ensure correct output.
Note: some common things that one does during development actually do change the symbol table. E.g. printf debugging is going to add calls to printf where there were none. (and I think "better printf debugging" is one of the main use cases for faster link times). Or if you use C++ streams, then while printf-debugging you may have had `output_stream << "foo: " << foo << "\n"` where `foo` is a string, but then if you change to also output `bar` which is an int, you're still changing the symbol table of the object file (due to different overloads).
> Interesting. What is this mode? How does it work if it's not incremental and it doesn't read the symbols at all?
Compared to the default, mostly it just skips string merging, which is what the linker spends most of its time on otherwise for typical debug links (debug info contains tons of identical strings; e.g. file names of common headers). [1]
To clarify, there are two separate things:
- the fastest mode, which is mostly about skipping string merging. It's just like the default linking mode, it just skips some optional stuff that is expensive.
- the part of the linker profile that the linker spends most of its time doing in its fastest mode (memcpy + relocate); for example, I've measured this as 60% of the profile. This happens after symbol resolution and some preprocessing of the relocations.
Sorry for any confusion.
[1] The linker has "-O<n>" flags (totally different from the "-O<n>" family of flags passed to the compiler). Basically higher -O numbers (from -O0 to -O3 just like the compile, confusingly) cause the linker to do more "fancy stuff" like string deduplication, string tail merging, and identical code folding. Mostly these things just reduce binary size somewhat at a fairly significant link time cost vs "just spit out a working binary".