[PATCH v1 3/6] libstdc++: Improve low-rank layout_{left, right}::stride.
Luc Grosheintz
luc.grosheintz@gmail.com
Mon Jul 28 08:12:51 GMT 2025
On 7/28/25 08:32, Tomasz Kaminski wrote:
> 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);
> }
True, purely dynamic extents, is another interesting case to optimize,
I suspect there's more potential in that case. For example the
indirection E[i] := D[k[i]] is not needed because k[i] == i.
Therefore, since the series already had several commits, I chose
to leave it for later.
>
>
> On Mon, Jul 28, 2025 at 8:02 AM Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>>
>>
>> On Sun, Jul 27, 2025 at 2:47 PM Luc Grosheintz <luc.grosheintz@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.grosheintz@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
>>>
>>>
>
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