On 07/24/2017 04:03 PM, Enrico Weigelt, metux IT consult wrote:
On 24.07.2017 21:29, Steve Fink wrote:
My guess is that the compilers/linkers don't really handle heavy C++
template usage very well, and end up generating and then eliminating
massive number of duplicate template instantiations. But that's idle
speculation.
I doubt there's much that compilers can do better here.
When compiling a C++ source, it needs to compile all that's needed
within that file (header-defined functions, constructors, template-
instantiations, etc). Later, at the linking phase, it can throw out
lots of duplicated code. For statically linked binaries, it's still
quite easy, but w/ shared libraries it gets pretty complex.
Sure, I get that. But first, I'm a little skeptical that the slowdown is
merely linear in the amount of duplication; it feels worse than that.
And second, even if this is the exact problem at hand, it feels like
C++'s reliance on independent compilation units is outdated. It's great
for modularity and simplicity, but in fact C++ the language does not
work that way, and stripping out redundancies during linking is the
wrong algorithm. Not that I would assert that an alternative would be
straightforward, especially with the preprocessor involved and differing
scope lookup environments etc. But from an algorithmic standpoint,
separate compilation is simply incorrect.
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