On Mon, Jul 28, 2025 at 10:03 AM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
>
>
> On 7/28/25 08:02, Tomasz Kaminski wrote:
> > On Sun, Jul 27, 2025 at 2:47 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 pre-comupted 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.
> >>
> >> 220: 48 8b 0c f5 00 00 00 mov rcx,QWORD PTR [rsi*8+0x0]
> >> 227: 00
> >> 228: 48 8b 04 f5 00 00 00 mov rax,QWORD PTR [rsi*8+0x0]
> >> 22f: 00
> >> 230: 48 c1 e1 02 shl rcx,0x2
> >> 234: 74 1a je 250 <stride_left_d5+0x30>
> >> 236: 48 01 f9 add rcx,rdi
> >> 239: 0f 1f 80 00 00 00 00 nop DWORD PTR [rax+0x0]
> >> 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: c3 ret
> >>
> >> If there's no dynamic extents, it simply loads the precomputed product
> >> of static extents.
> >>
> >> For rank == 1 the answer is 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.
> >>
> >> For rank == 2, it trades loading one value for a branchless sequence of
> >> four instructions that don't require loading any values.
> >>
> > I will put this optimization into the __fwd_prod and __rev_pord
> functions,
> > so it will be applied for all uses. This will also avoid us creating this
> > caching
> > tables for such small ranks.
>
> The problem is that we don't have the same amount of information in
> the stride and __fwd_prod. The valid values for __r are 1, ..., rank;
> for __fwd_prod it's inclusive, while in stride it's exclusive. Therefore,
> we can do the optimization with one comparison less in stride than in
> __fwd_prod.
>
Make sense, and I am OK having this optimization there. However, out of
curiosity, if we put always_inline on the __fwd_prod, wouldn't compiler be
able
to eliminate __rank == __i. Also once we have separate call to __size, we
could
put assertions (just as a comment) that for __fwd_prod and __rev_prod __r is
required to be smaller than rank().
>
> I purposefully checked mdspan::size (part of the previous commit); and
> on optimized builds it fully unrolls the loop and does everything
> automatically, meaning it doesn't need help and we're not repeating
> the same optimization several times.
>
> As for avoiding the tables for small ranks, we can refactor _RevProd
> as follows:
>
> template<size_t... _Extents>
> struct _RevProd
> {
> size_t _S_value(size_t i)
> { return _S_data[i]; }
>
> private:
> array _S_data = consteval ...;
> }
>
> and use partial specialization to eliminate _S_data.
>
> >
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >> * include/std/mdspan (layout_left::mapping::stride): Optimize
> >> for rank <= 2.
> >> (layout_right::mapping::stride): Ditto.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> >> ---
> >> libstdc++-v3/include/std/mdspan | 14 ++++++++++++--
> >> 1 file changed, 12 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan
> >> b/libstdc++-v3/include/std/mdspan
> >> index 06ccf3e3827..f288af96cdb 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -652,7 +652,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> requires (extents_type::rank() > 0)
> >> {
> >> __glibcxx_assert(__i < extents_type::rank());
> >> - return __mdspan::__fwd_prod(_M_extents, __i);
> >> + if constexpr (extents_type::rank() == 1)
> >> + return 1;
> >> + else if constexpr (extents_type::rank() == 2)
> >> + return __i == 0 ? 1 : _M_extents.extent(0);
> >> + else
> >> + return __mdspan::__fwd_prod(_M_extents, __i);
> >> }
> >>
> >> template<typename _OExtents>
> >> @@ -797,7 +802,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> requires (extents_type::rank() > 0)
> >> {
> >> __glibcxx_assert(__i < extents_type::rank());
> >> - return __mdspan::__rev_prod(_M_extents, __i);
> >> + if constexpr (extents_type::rank() == 1)
> >> + return 1;
> >> + else if constexpr (extents_type::rank() == 2)
> >> + return __i == 0 ? _M_extents.extent(1) : 1;
> >> + else
> >> + return __mdspan::__rev_prod(_M_extents, __i);
> >> }
> >>
> >> template<typename _OExtents>
> >> --
> >> 2.50.0
> >>
> >>
> >
>
>
