On Tue, Aug 5, 2025 at 12:33 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
>
>
> On 8/5/25 09:36, Tomasz Kaminski wrote:
> > On Sun, Aug 3, 2025 at 11:07 PM Luc Grosheintz <luc.groshei...@gmail.com
> >
> > wrote:
> >
> >> The methods layout_{left,right}::mapping::stride are defined
> >> as
> >>
> >> \prod_{i = 0}^r E[i]
> >> \prod_{i = r+1}^n E[i]
> >>
> >> This is computed as the product of a precomputed static product and the
> >> product of the required dynamic extents.
> >>
> >> Disassembly shows that even for low-rank extents, i.e. rank == 1 and
> >> rank == 2, with at least one dynamic extent, the generated code loads
> >> two values; and then runs the loop over at most one element, e.g. for
> >> stride_left_d5 defined below the generated code is:
> >>
> >> 220: 48 8b 04 f5 00 00 00 mov rax,QWORD PTR [rsi*8+0x0]
> >> 227: 00
> >> 228: 31 d2 xor edx,edx
> >> 22a: 48 85 c0 test rax,rax
> >> 22d: 74 23 je 252 <stride_left_d5+0x32>
> >> 22f: 48 8b 0c f5 00 00 00 mov rcx,QWORD PTR [rsi*8+0x0]
> >> 236: 00
> >> 237: 48 c1 e1 02 shl rcx,0x2
> >> 23b: 74 13 je 250 <stride_left_d5+0x30>
> >> 23d: 48 01 f9 add rcx,rdi
> >> 240: 48 63 17 movsxd rdx,DWORD PTR [rdi]
> >> 243: 48 83 c7 04 add rdi,0x4
> >> 247: 48 0f af c2 imul rax,rdx
> >> 24b: 48 39 f9 cmp rcx,rdi
> >> 24e: 75 f0 jne 240 <stride_left_d5+0x20>
> >> 250: 89 c2 mov edx,eax
> >> 252: 89 d0 mov eax,edx
> >> 254: c3 ret
> >>
> >> If there's no dynamic extents, it simply loads the precomputed product
> >> of static extents.
> >>
> >> For rank == 1 the answer is the constant `1`; for rank == 2 it's either
> 1
> >> or
> >> extents.extent(k), with k == 0 for layout_left and k == 1 for
> >> layout_right.
> >>
> >> Consider,
> >>
> >> using Ed = std::extents<int, dyn>;
> >> int stride_left_d(const std::layout_left::mapping<Ed>& m, size_t r)
> >> { return m.stride(r); }
> >>
> >> using E3d = std::extents<int, 3, dyn>;
> >> int stride_left_3d(const std::layout_left::mapping<E3d>& m, size_t r)
> >> { return m.stride(r); }
> >>
> >> using Ed5 = std::extents<int, dyn, 5>;
> >> int stride_left_d5(const std::layout_left::mapping<Ed5>& m, size_t r)
> >> { return m.stride(r); }
> >>
> >> The optimized code for these three cases is:
> >>
> >> 0000000000000060 <stride_left_d>:
> >> 60: b8 01 00 00 00 mov eax,0x1
> >> 65: c3 ret
> >>
> >> 0000000000000090 <stride_left_3d>:
> >> 90: 48 83 fe 01 cmp rsi,0x1
> >> 94: 19 c0 sbb eax,eax
> >> 96: 83 e0 fe and eax,0xfffffffe
> >> 99: 83 c0 03 add eax,0x3
> >> 9c: c3 ret
> >>
> >> 00000000000000a0 <stride_left_d5>:
> >> a0: b8 01 00 00 00 mov eax,0x1
> >> a5: 48 85 f6 test rsi,rsi
> >> a8: 74 02 je ac <stride_left_d5+0xc>
> >> aa: 8b 07 mov eax,DWORD PTR [rdi]
> >> ac: c3 ret
> >>
> >> For rank == 1 it simply returns 1 (as expected). For rank == 2, it
> >> either implements a branchless formula, or conditionally loads one
> >> value. In all cases involving a dynamic extent this seems like it's
> >> always doing clearly less work, both in terms of computation and loads.
> >> In cases not involving a dynamic extent, it replaces loading one value
> >> with a branchless sequence of four instructions.
> >>
> >> This commit also refactors __size to no use any of the precomputed
> >> arrays. This prevents instantiating __{fwd,rev}_partial_prods for
> >> low-rank extents. This results in a further size reduction of a
> >> reference object file (described two commits prior) by 9% from 46.0kB to
> >> 41.9kB.
> >>
> >> In a prior commit we optimized __size to produce better object code by
> >> precomputing the static products. This refactor enables the optimizer to
> >> generate the same optimized code.
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >> * include/std/mdspan (__mdspan::__fwd_prod): Optimize
> >> for rank <= 2.
> >> (__mdspan::__rev_prod): Ditto.
> >> (__mdspan::__size): Refactor to use a pre-computed product, not
> >> a partial product.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> >> ---
> >> libstdc++-v3/include/std/mdspan | 32 ++++++++++++++++++++++++++------
> >> 1 file changed, 26 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index dc1b44ee35c..11a5063f60d 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -423,25 +423,45 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> constexpr typename _Extents::index_type
> >> __fwd_prod(const _Extents& __exts, size_t __r) noexcept
> >> {
> >> + constexpr size_t __rank = _Extents::rank();
> >> constexpr auto __sta_exts = __static_extents<_Extents>();
> >> - size_t __sta_prod = __fwd_partial_prods<__sta_exts>[__r];
> >> - return __extents_prod(__exts, __sta_prod, 0, __r);
> >> + if constexpr (__rank == 1)
> >> + return 1;
> >> + else if constexpr (__rank == 2)
> >> + return __r == 0 ? 1 : __exts.extent(0);
> >> + else
> >> + {
> >> + size_t __sta_prod = __fwd_partial_prods<__sta_exts>[__r];
> >> + return __extents_prod(__exts, __sta_prod, 0, __r);
> >> + }
> >> }
> >>
> >> template<typename _Extents>
> >> constexpr typename _Extents::index_type
> >> __rev_prod(const _Extents& __exts, size_t __r) noexcept
> >> {
> >> - constexpr auto __sta_exts = __static_extents<_Extents>();
> >> constexpr size_t __rank = _Extents::rank();
> >> - size_t __sta_prod = __rev_partial_prods<__sta_exts>[__r];
> >> - return __extents_prod(__exts, __sta_prod, __r + 1, __rank);
> >> + constexpr auto __sta_exts = __static_extents<_Extents>();
> >> + if constexpr (__rank == 1)
> >> + return 1;
> >> + else if constexpr (__rank == 2)
> >> + return __r == 0 ? __exts.extent(1) : 1;
> >> + else
> >> + {
> >> + size_t __sta_prod = __rev_partial_prods<__sta_exts>[__r];
> >> + return __extents_prod(__exts, __sta_prod, __r + 1, __rank);
> >> + }
> >> }
> >>
> >> template<typename _Extents>
> >> constexpr typename _Extents::index_type
> >> __size(const _Extents& __exts) noexcept
> >> - { return __fwd_prod(__exts, __exts.rank()); }
> >> + {
> >> + constexpr auto __sta_exts = __static_extents<_Extents>();
> >>
> > I am also changing this to constexpr auto&, there is no need to make a
> copy
> > of aray.
>
> The optimizer agrees: there's no need to make a copy and then correctly
> chooses to not make a copy.
>
You cannot make a constexpr span covering it, unless it is static:
#include <array>
#include <span>
int main() {
constexpr std::array<int, 5> x{};
constexpr std::span<const int> s = x; // ill-formed
}
Link here: https://godbolt.org/z/4MYoY5noq
>
> I can see why it makes sense. However, having disassembled mdspan related
> code several times, I got the feeling that this is an easy optimization for
> GCC. Therefore, it didn't register as a concern. Please let me know if
> there's something I missed (it's quite likely).
>
> The dilemma is: either it doesn't matter (then why change it); or it does
> matter which implies that it could change the generated code. If it's the
> latter I need to recheck everything, because I superstitiously believe I'm
> always "unlucky" if I "wing it" in these situations.
>
> You've mentioned a separate commit, would it make sense to put these
> changes
> into a [PATCH 9/8]? That way the source code will soon have the preferred
> form, and whatever mistakes are in the commit messages will have been my
> own
> doing.
>
> If we're doing this to optimize -O0, then I think it should be a different
> task, because there will be a lot more to fix. Personally, I've given up
> on -O0 in scientific codes, it's easily 10x slower and that then ended up
> hovering on the threshold to unacceptably slow.
>
> >
> >> + constexpr size_t __rank = _Extents::rank();
> >> + constexpr size_t __sta_prod = __static_prod<__sta_exts>(0,
> __rank);
> >> + return __extents_prod(__exts, __sta_prod, 0, __rank);
> >> + }
> >>
> >> template<typename _IndexType, size_t... _Counts>
> >> auto __build_dextents_type(integer_sequence<size_t, _Counts...>)
> >> --
> >> 2.50.0
> >>
> >>
> >
>
>
