[cfe-dev] Clang generates absurd amount of assembly for libc++ std::vector::emplace

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Wed Jul 25 09:12:46 PDT 2018


On Wed, Jul 25, 2018 at 8:57 AM <aw1621107 at gmail.com> wrote:

> The number of lines of assembly isn't really a good proxy for the
> performance of some code - mostly due to inlining (one piece of code may be
> many more lines of assembly because it's not calling large/complicated
> external functions - or, even taken as a whole (including those external
> functions) it might still be more efficient to have longer code (because
> it's more specialized - ie: two calls to one generic function were inlined
> into two places and each one simplified/optimized a bit for those
> situations))
>
> Yeah, you’re right; that was also pointed out to me by someone on one of
> the IRC channels I lurk on. A bit more investigation on Godbolt revealed
> that the difference could be to unrolling. It was certainly a surprise to
> me, as I expected that libstdc++ and libc++ would have relatively similar
> implementations that would produce relatively similar outputs. Guess
> something about libc++’s implementation is a bit easier for Clang to
> inspect? In any case, let that be a lesson to me to be a bit more careful
> about drawing conclusions from code size
>
>
>
> That said, libc++ does have a bunch of forced inlining that's not for
> performance reasons, but for linkage reasons (to ensure that certain kinds
> of changes/updates to libc++ don't break existing compiled code/libraries).
> It's a tradeoff that not every user of libc++ needs to make & there are
> steps being taken to make that tradeoff more configurable/optional, so far
> as I understand it.
>
> Huh, that’s interesting. That isn’t what is happening here, though, right?
> I didn’t see anything that looks like that around the
> declarations/implementations of emplace() and friends
>

Yeah, probably doesn't come up for the fully dependent template things in
the standard library - but maybe some implementation details that are used
in there like allocators, etc, might have some of these features. There's a
lot of stuff in there - so hard for me to check at a glance. (though you
can see it around otehr functions in the form of _LIBCPP_INLINE_VISIBILITY)

- Dave


>
>
> On Mon, Jul 23, 2018 at 4:43 PM via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
>
> Hello all,
>
>
>
> Just a quick question to make sure I’m not missing something.
>
>
>
> This program:
>
>
>
> #include <vector>
>
> void f(std::vector<double>& vec, double val) {
>
>       vals.emplace(std::cbegin(vec), val);
>
> }
>
>
>
> When compiled with trunk Clang on Godbolt with -O3 -march=haswell
> -std=c++17 -stdlib=libstdc++, 132 lines of assembly are produced. If
> -stdlib=libc++ is used, though, 638 (!) lines of assembly are produced. A
> few of those lines are due to f() itself, but it appears the vast
> majority are due to the implementation of emplace(). As a partial
> comparison, GCC trunk produced 136 lines of assembly, and seems to have
> partially inlined emplace(), leaving 94 lines of assembly for
> _M_realloc_insert.
>
>
>
> I can sort of duplicate this on Debian sid, with libc++-dev 6.0.1-1 and
> clang++-7 (--version doesn’t appear to give a revision number,
> unfortunately?). Using libstdc++ results in 176 lines of assembly, and
> libc++ results in 803 lines of assembly (counted by wc -l).
>
>
>
> Is this something to be worried about? I’m still rather new to
> performance-related work, so I’m working from a relatively simplistic view
> of what could be affecting performance. A 4x difference in what could be a
> commonly-used function seems rather unusual to me, though.
>
>
>
> Thanks,
>
>
>
> Alex
>
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