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.
>
> 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
>
>
