In the __fwd_prod, __rev_prod I would also add the case for all-dynamic
extents, so we do not instantiate an array with all 1, i.e. we would have:
constexpr size_t __rank = _Extents::rank();
+ size_t __sta_prod = _RevProd<_Extents>::_S_value[__r];
+ return __extents_prod(__exts, __sta_prod, __r + 1, __rank);
if constexpr (__rank == 1)
return 1;
else if constexpr (__rank == 2)
return __i == 0 ? _M_extents.extent(1) : 1;
else if constexpr (Extents::rank_dynamic() == __rank)
return __extents_prod(__exts, __1u, __r + 1, __rank);
else
{
size_t __sta_prod = _RevProd<_Extents>::_S_value[__r];
return __extents_prod(__exts, __sta_prod, __r + 1, __rank);
}
On Mon, Jul 28, 2025 at 8:02 AM Tomasz Kaminski <tkami...@redhat.com> 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.
>
>>
>> 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
>>
>>
