[cfe-dev] missing optimization opportunity for const std::vector compared to std::array

Dennis Luehring via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 22 07:59:00 PDT 2018


its is fixed: forgot to use libc+++ at all

godbolt: -O2 -stdlib=libc++

is the answer

thx

Am 22.10.2018 um 16:53 schrieb Dennis Luehring via cfe-dev:
> so i've rembered correct - Richard Smith already fixed that with an
> patch to LibC++ years ago
>
> http://clang-developers.42468.n3.nabble.com/missing-optimization-opportunity-for-const-std-vector-compared-to-std-array-td4034587.html#none
>
>
> see post ~Jun 04, 2014; 12:16am
> http://clang-developers.42468.n3.nabble.com/missing-optimization-opportunity-for-const-std-vector-compared-to-std-array-tp4034587p4039822.html
>
> this post informed about the patch
> http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20140602/106887.html
>
> what happend to this optimization - was it removed or broken over time?
> or is it just not possible to test the behavior on gcc.godbolt.org with
> the laste clang, missing flags or something?
>
> Am 20.10.2018 um 12:00 schrieb Dennis Luehring via cfe-dev:
> > for gcc-discussion see: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58483
> >
> > is there a (good) reason not to remove the new/delete combination when
> > the result is already known?
> >
> > #include <vector>
> > #include <numeric>
> > #include <array>
> >
> > static int calc(const std::array<int,3> p_ints, const int& p_init)
> > //static int calc(const std::vector<int> p_ints, const int& p_init)
> > {
> >     return std::accumulate(p_ints.begin(), p_ints.end(), p_init);
> > }
> >
> > int main()
> > {
> >     const int result = calc({10,20,30},100);
> >     return result;
> > }
> >
> > results:
> >
> > gcc.godbolt x86-64 clang trunk, with -O2
> >
> > with std::array
> >
> > main: # @main
> > mov eax, 160
> > ret
> >
> > with std::vector
> >
> > main: # @main
> > push    rax
> > mov     edi, 12
> > call    operator new(unsigned long)
> > movabs  rcx, 85899345930
> > mov     qword ptr [rax], rcx
> > mov     dword ptr [rax + 8], 30
> > mov     rdi, rax
> > call    operator delete(void*)
> > mov     eax, 160
> > pop     rcx
> > ret
> >
> > ---
> >
> > clang reduces new/delete with std::string but vector seems to be missing
> > (buildin_new/delete missing in libc++?)
> >
> > #include <string>
> >
> > int main()
> > {
> >     return std::string("hello").size();
> > }
> >
> > down to
> >
> > main: # @main
> > mov eax, 5
> > ret
> >
> >
> > _______________________________________________
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> > cfe-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
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